Advances in abscission signaling

Drought-induced cell wall degradation in the base of pedicel is associated with accelerated cotton square shedding

Drought significantly impacts cotton square (flower buds with bracts) shedding, directly affecting yield. To address the internal physiological mechanisms of drought affecting cotton square shedding, a polyethylene glycol-simulated drought study was conducted with Dexiamian 1 and Yuzaomian 9110 to investigate cell wall degradation changes in the base of pedicel where the detachment of cotton square takes place, and its relationship with cotton square shedding. Results revealed significant decreases in cellulose, hemicellulose, and pectin contents in the base of square pedicel, leading to cell wall degradation and consequent square shedding. Furthermore, drought stress exacerbated the hydrolysis of cellulose and pectin in the base of pedicel, although not hemicellulose, resulting in more noticeable alterations in the morphology and structure of the base of pedicel, such as more significant degradation in the epidermis, cortex, and phloem. Regarding the cellulose hydrolysis, drought mainly increased the expression of genes β-glucosidase (GhBG1) and endoglucanase (GhEG1), and the activity of β-glucosidase and endoglucanase in the base of pedicel, promoting the conversion of cellulose to cellobiose, and eventually glucose. Regarding the pectin hydrolysis, drought significantly enhanced the expression of the gene pectin methylase (GhPE1), thereby accelerating pectin hydrolysis to generate polygalacturonic acid. Additionally, drought increased the expression of genes pectin lyase (GhPL1) and polygalacturonase (GhPG1), as well as the activity of pectin lyase, which further accelerated the hydrolysis of polygalacturonic acid into galacturonic acid. These findings suggest that drought mainly promotes cellulose and pectin hydrolysis in the base of pedicel, hastening cell wall degradation and final cotton square shedding.

Transcriptome analysis unravels key pathways and hub genes related to immature fruit abscission in Camellia oleifera

Immature fruit abscission of Camellia oleifera (C. oleifera) is a common problem limiting yield increases. However, the regulatory mechanisms underlying immature fruit abscission in C. oleifera are unclear. In this study, we systematically investigated changes in the morphological, physiological, and gene expression of fruit abscission zones (FAZs) of soon-to-abscise fruits (M2). We found that fruit abscission before ripening mainly occurs during the August abscission stage of 'Huashuo'. At the beginning of this stage, the FAZs of M2 have a marked dent, and the separation layer structures are preliminarily formed. Phytohormone analysis showed that the contents of indole-3-acetic acid (IAA) and jasmonic acid (JA) in the FAZs of M2 were significantly decreased compared with the non-abscised fruits, while the content of trans-zeatin (TZR) was increased. Transcriptome analysis identified differentially expressed genes (DEGs) mainly involved in phytohormone metabolism, including ethylene, auxin, JA, and the cis-zeatin signal transduction pathway. There were also many DEGs involved in cell wall catabolism. Weighted gene co-expression network analysis (WGCNA) further suggested that the transcription factors NAC100 and ERF114 participate in the immature fruit abscission of C. oleifera. This study provides insights into the fruit abscission mechanism of C. oleifera.

The single RRM domain-containing protein SARP1 is required for establishment of the separation zone in Arabidopsis

Abscission is the shedding of plant organs in response to developmental and environmental cues. Abscission involves cell separation between two neighboring cell types, residuum cells (RECs) and secession cells (SECs) in the floral abscission zone (AZ) in Arabidopsis thaliana. However, the regulatory mechanisms behind the spatial determination that governs cell separation are largely unknown. The class I KNOTTED-like homeobox (KNOX) transcription factor BREVIPEDICELLUS (BP) negatively regulates AZ cell size and number in Arabidopsis. To identify new players participating in abscission, we performed a genetic screen by activation tagging a weak complementation line of bp-3. We identified the mutant ebp1 (enhancer of BP1) displaying delayed floral organ abscission. The ebp1 mutant showed a concaved surface in SECs and abnormally stacked cells on the top of RECs, in contrast to the precisely separated surface in the wild-type. Molecular and histological analyses revealed that the transcriptional programming during cell differentiation in the AZ is compromised in ebp1. The SECs of ebp1 have acquired REC-like properties, including cuticle formation and superoxide production. We show that SEPARATION AFFECTING RNA-BINDING PROTEIN1 (SARP1) is upregulated in ebp1 and plays a role in the establishment of the cell separation layer during floral organ abscission in Arabidopsis.

Rhytidome- and cork-type barks of holm oak, cork oak and their hybrids highlight processes leading to cork formation

BACKGROUND

The periderm is basic for land plants due to its protective role during radial growth, which is achieved by the polymers deposited in the cell walls. In most trees, like holm oak, the first periderm is frequently replaced by subsequent internal periderms yielding a heterogeneous outer bark made of a mixture of periderms and phloem tissues, known as rhytidome. Exceptionally, cork oak forms a persistent or long-lived periderm which results in a homogeneous outer bark of thick phellem cell layers known as cork. Cork oak and holm oak distribution ranges overlap to a great extent, and they often share stands, where they can hybridize and produce offspring showing a rhytidome-type bark.

RESULTS

Here we use the outer bark of cork oak, holm oak, and their natural hybrids to analyse the chemical composition, the anatomy and the transcriptome, and further understand the mechanisms underlying periderm development. We also include a unique natural hybrid individual corresponding to a backcross with cork oak that, interestingly, shows a cork-type bark. The inclusion of hybrid samples showing rhytidome-type and cork-type barks is valuable to approach cork and rhytidome development, allowing an accurate identification of candidate genes and processes. The present study underscores that abiotic stress and cell death are enhanced in rhytidome-type barks whereas lipid metabolism and cell cycle are enriched in cork-type barks. Development-related DEGs showing the highest expression, highlight cell division, cell expansion, and cell differentiation as key processes leading to cork or rhytidome-type barks.

CONCLUSION

Transcriptome results, in agreement with anatomical and chemical analyses, show that rhytidome and cork-type barks are active in periderm development, and suberin and lignin deposition. Development and cell wall-related DEGs suggest that cell division and expansion are upregulated in cork-type barks whereas cell differentiation is enhanced in rhytidome-type barks.

Species-Specific miRNAs Contribute to the Divergence between Deciduous and Evergreen Species in Ilex

MicroRNAs (miRNAs) are pivotal regulators of gene expression, playing crucial roles in plant developmental processes and environmental responses. However, the function of miRNAs in influencing deciduous traits has been little explored. Here, we utilized sRNA-seq on two deciduous species, Ilex polyneura (Hand.-Mazz.) S. Y. Hu and Ilex asprella Champ. ex Benth., along with an evergreen species, Ilex latifolia Thunb., to identify and annotate miRNAs within these species. Our analysis revealed 162 species-specific miRNAs (termed SS-miRNAs) from 120 families, underscoring the fundamental roles and potential influence of SS-miRNAs on plant phenotypic diversity and adaptation. Notably, three SS-miRNAs in I. latifolia were found to target crucial genes within the abscission signaling pathway. Analysis of cis-regulatory elements suggested a novel regulatory relationship that may contribute to the evergreen phenotype of I. latifolia by modulating the abscission process in a light-independent manner. These findings propose a potential mechanism by which SS-miRNAs can influence the conserved abscission pathway, contributing to the phenotypic divergence between deciduous and evergreen species within the genus Ilex.

Transcriptome differential expression analysis of defoliation of two different lemon varieties

'Allen Eureka' is a bud variety of Eureka lemon with excellent fruiting traits. However, it suffers from severe winter defoliation that leads to a large loss of organic nutrients and seriously affects the tree's growth and development as well as the yield of the following year, and the mechanism of its response to defoliation is still unclear. In order to investigate the molecular regulatory mechanisms of different leaf abscission periods in lemon, two lemon cultivars ('Allen Eureka' and 'Yunning No. 1') with different defoliation traits were used as materials. The petiole abscission zone (AZ) was collected at three different defoliation stages, namely, the pre-defoliation stage (CQ), the mid-defoliation stage (CZ), and the post-defoliation stage (CH). Transcriptome sequencing was performed to analyze the gene expression differences between these two cultivars. A total of 898, 4,856, and 3,126 differentially expressed genes (DEGs) were obtained in CQ, CZ, and CH, respectively, and the number of DEGs in CZ was the largest. GO analysis revealed that the DEGs between the two cultivars were mainly enriched in processes related to oxidoreductase, hydrolase, DNA binding transcription factor, and transcription regulator activity in the defoliation stages. KEGG analysis showed that the DEGs were concentrated in CZ and involved plant hormone signal transduction, phenylpropanoid biosynthesis, glutathione metabolism, and alpha-linolenic acid metabolism. The expression trends of some DEGs suggested their roles in regulating defoliation in lemon. Eight gene families were obtained by combining DEG clustering analysis and weighted gene co-expression network analysis (WGCNA), including β-glucosidase, AUX/IAA, SAUR, GH3, POD, and WRKY, suggesting that these genes may be involved in the regulation of lemon leaf abscission. The above conclusions enrich the research related to lemon leaf abscission and provide reliable data for the screening of lemon defoliation candidate genes and analysis of defoliation pathways.

Genome-wide analysis of the mulberry (Morus abla L.) GH9 gene family and the functional characterization of MaGH9B6 during the development of the abscission zone

Plant glycoside hydrolase family 9 genes (GH9s) are widely distributed in plants and involved in a variety of cellular and physiological processes. In the current study, nine GH9 genes were identified in the mulberry and were divided into two subfamilies based on the phylogenetic analysis. Conserved motifs and gene structure analysis suggested that the evolution of the two subfamilies is relatively conserved and the glycoside hydrolase domain almost occupy the entire coding region of the GH9s gene. Only segmental duplication has played a role in the expansion of gene family. Collinearity analysis showed that mulberry GH9s had the closest relationship with poplar GH9s. MaGH9B1, MaGH9B6, MaGH9B5, and MaGH9B3 were detected to have transcript accumulation in the stalk of easy-to drop mature fruit drop, suggesting that these could play a role in mulberry fruit drop. Multiple cis-acting elements related to plant hormones and abiotic stress responses were found in the mulberry GH9 promoter regions and showed different activities under exogenous abscisic acid (ABA) and 2,4- dichlorophenoxyacetic acid (2,4-D) stresses. We found that the lignin content in the fruit stalk decreased with the formation of the abscission zone (AZ), which could indirectly reflect the formation process of the AZ. These results provide a theoretical basis for further research on the role of GH9s in mulberry abscission.

Identification of PavHB16 gene in Prunus avium and validation of its function in Arabidopsis thaliana

Sweet cherry (Prunus avium L.) is one of the most economically important fruits in the world. However, severe fruit abscission has brought significant challenges to the cherry industry. To better understand the molecular regulation mechanisms underlying excessive fruit abscission in sweet cherry, the fruit abscission characteristics, the anatomical characteristics of the abscission zone (AZ), as well as a homeodomain-Leucine Zipper gene family member PavHB16 function were analyzed. The results showed that the sweet cherry exhibited two fruit abscission peak stages, with the "Brooks" cultivar demonstrating the highest fruit-dropping rate (97.14%). During these two fruit abscission peak stages, both the retention pedicel and the abscising pedicel formed AZs. but the AZ in the abscising pedicel was more pronounced. In addition, a transcription factor, PavHB16, was identified from sweet cherry. The evolutionary analysis showed that there was high homology between PavHB16 and AtHB12 in Arabidopsis. Moreover, the PavHB16 protein was localized in the nucleus. Overexpression of PavHB16 in Arabidopsis accelerated petal shedding. In the PavHB16-overexpressed lines, the AZ cells in the pedicel became smaller and denser, and the expression of genes involved in cell wall remodeling, such as cellulase 3 gene (AtCEL3), polygalacturonase 1 (AtPG1), and expandin 24(AtEXPA24) were upregulated. The results suggest that PavHB16 may promote the expression of genes related to cell wall remodeling, ultimately facilitating fruit abscission. In summary, this study cloned the sweet cherry PavHB16 gene and confirmed its function in regulating sweet cherry fruit abscission, which provided new data for further study on the fruit abscission mechanism.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01443-8.

Genomic loci associated with leaf abscission contribute to machine picking and environmental adaptability in upland cotton (Gossypium hirsutum L.)

INTRODUCTION

Defoliation by applying defoliants before machine picking is an important agricultural practice that enhances harvesting efficiency and leads to increased raw cotton purity. However, the fundamental characteristics of leaf abscission and the underlying genetic basis in cotton are not clearly understood.

OBJECTIVES

In this study, we aimed to (1) reveal the phenotypic variations in cotton leaf abscission, (2) discover the whole-genome differentiation sweeps and genetic loci related to defoliation, (3) identify and verify the functions of key candidate genes associated with defoliation, and (4) explore the relationship between haplotype frequency of loci and environmental adaptability.

METHODS

Four defoliation-related traits of 383 re-sequenced Gossypium hirsutum accessions were investigated in four environments. The genome-wide association study (GWAS), linkage disequilibrium (LD) interval genotyping and functional identification were conducted. Finally, the haplotype variation related to environmental adaptability and defoliation traits was revealed.

RESULTS

Our findings revealed the fundamental phenotypic variations of defoliation traits in cotton. We showed that defoliant significantly increased the defoliation rate without incurring yield and fiber quality penalties. The strong correlations between defoliation traits and growth period traits were observed. A genome-wide association study of defoliation traits identified 174 significant SNPs. Two loci (RDR7 on A02 and RDR13 on A13) that significantly associated with the relative defoliation rate were described, and key candidate genes GhLRR and GhCYCD3;1, encoding a leucine-rich repeat (LRR) family protein and D3-type cell cyclin 1 protein respectively, were functional verified by expression pattern analysis and gene silencing. We found that combining of two favorable haplotypes (HapRDR7 and HapRDR13) improved sensitivity to defoliant. The favorable haplotype frequency generally increased in high latitudes in China, enabling adaptation to the local environment.

CONCLUSION

Our findings lay an important foundation for the potentially broad application of leveraging key genetic loci in breeding machine-pickable cotton.

Genome-wide identification of GH9 gene family and the assessment of its role during fruit abscission zone formation in Vaccinium ashei

KEY MESSAGE

The genomic location and stage-specific expression pattern of GH9 genes reveal their critical roles during fruit abscission zone formation in Vaccinium ashei. Glycosyl hydrolase family 9 (GH9) cellulases play a crucial role in both cellulose synthesis and hydrolysis during plant growth and development. Despite this importance, there is currently no study on the involvement of GH9-encoding genes, specifically VaGH9s, in abscission zone formation of rabbiteye blueberries (Vaccinium ashei). In this study, we identified a total of 61 VaGH9s in the genome, which can be classified into 3 subclasses based on conserved motifs and domains, gene structures, and phylogenetic analyses. Our synteny analysis revealed that VaGH9s are more closely related to the GH9s of Populus L. than to those of Arabidopsis, Vitis vinifera, and Citrus sinensis. In silico structural analysis predicted that most of VaGH9s are hydrophilic, and localized in cell membrane and/or cell wall, and the variable sets of cis-acting regulatory elements and functional diversity with four categories of stress response, hormone regulation, growth and development, and transcription factor-related elements are present in the promoter sequence of VaGH9s genes. Transcriptomic analysis showed that there were 22 differentially expressed VaGH9s in fruit abscission zone tissue at the veraison stage, and the expression of VaGH9B2 and VaGH9C10 was continuously increased during fruit maturation, which were in parallel with the increasing levels of cellulase activity and oxidative stress indicators, suggesting that they are involved in the separation stage of fruit abscission in Vaccinium ashei. Our work identified 22 VaGH9s potentially involved in different stages of fruit abscission and would aid further investigation into the molecular regulation of abscission in rabbiteye blueberries fruit.

Historical Selection, Adaptation Signatures, and Ambiguity of Introgressions in Wheat

Wheat was one of the crops domesticated in the Fertile Crescent region approximately 10,000 years ago. Despite undergoing recent polyploidization, hull-to-free-thresh transition events, and domestication bottlenecks, wheat is now grown in over 130 countries and accounts for a quarter of the world's cereal production. The main reason for its widespread success is its broad genetic diversity that allows it to thrive in different environments. To trace historical selection and hybridization signatures, genome scans were performed on two datasets: approximately 113K SNPs from 921 predominantly bread wheat accessions and approximately 110K SNPs from about 400 wheat accessions representing all ploidy levels. To identify environmental factors associated with the loci, a genome-environment association (GEA) was also performed. The genome scans on both datasets identified a highly differentiated region on chromosome 4A where accessions in the first dataset were dichotomized into a group (n = 691), comprising nearly all cultivars, wild emmer, and most landraces, and a second group (n = 230), dominated by landraces and spelt accessions. The grouping of cultivars is likely linked to their potential ancestor, bread wheat cv. Norin-10. The 4A region harbored important genes involved in adaptations to environmental conditions. The GEA detected loci associated with latitude and temperature. The genetic signatures detected in this study provide insight into the historical selection and hybridization events in the wheat genome that shaped its current genetic structure and facilitated its success in a wide spectrum of environmental conditions. The genome scans and GEA approaches applied in this study can help in screening the germplasm housed in gene banks for breeding, and for conservation purposes.

Transcriptomic and metabolomic analyses reveal molecular mechanisms associated with the natural abscission of blue honeysuckle (Lonicera caerulea L.) ripe fruits

Blue honeysuckle (Lonicera caerulea L.) is rich in phenolic compounds and has an extremely high nutritional value. Fruit abscission in the ripe period significantly impacts production and economic benefits. However, the mechanism associated with the abscission of blue honeysuckle fruit remains largely unknown. The easy-abscission cultivar 'HSY' and the hard-abscission cultivar 'Berel' were selected as plant materials. Anatomical changes of the 'HSY' fruit abscission zone (FAZ) during the abscission mainly included cell expansion, detachment, and collapse. Active changes in cell wall-degrading enzyme activity between 39 days postanthesis (DPA) and 55 DPA in 'HSY' FAZ, but not in 'Berel', suggest a critical role for cell-wall-degrading enzymes in regulating abscission. Transcriptome and metabolome analyses revealed that the genes and metabolites responding to abscission mainly act on pathways such as plant hormone signal transduction, starch and sucrose metabolism, pentose and glucuronate interconversions, and phenylpropanoid biosynthesis. The regulatory pathways of fruit abscission are mainly summarized into two parts: phytohormone synthesis and signal transduction, FAZ cell wall metabolism. In this study, 46 key genes related to plant hormone response, 45 key genes involved in FAZ cell wall metabolism, and 73 transcription factors were screened. Quantitative real-time PCR (qRT-PCR) assessed the expression pattern of 12 selected candidate genes, demonstrating the accuracy of the transcriptome data and elucidating the expression patterns of key candidate genes during growth and development. This study will provide an essential resource for understanding the molecular regulatory mechanism of fruit abscission in the blue honeysuckle.

Comparative Physiological and Transcriptomic Mechanisms of Defoliation in Cotton in Response to Thidiazuron versus Ethephon

Thidiazuron (TDZ) is a widely used chemical defoliant in cotton and can stimulate the production of ethylene in leaves, which is believed to be the key factor in inducing leaf abscission. Ethephon (Eth) can also stimulate ethylene production in leaves, but it is less effective in promoting leaf shedding. In this study, the enzyme-linked immunosorbent assays (ELISA) and RNA-seq were used to determine specific changes at hormonal levels as well as transcriptomic mechanisms induced by TDZ compared with Eth. The TDZ significantly reduced the levels of auxin and cytokinin in cotton leaves, but no considerable changes were observed for Eth. In addition, TDZ specifically increased the levels of brassinosteroids and jasmonic acid in the leaves. A total of 13 764 differentially expressed genes that specifically responded to TDZ were identified by RNA-seq. The analysis of KEGG functional categories suggested that the synthesis, metabolism, and signal transduction of auxin, cytokinin, and brassinosteroid were all involved in the TDZ-induced abscission of cotton leaves. Eight auxin transport genes (GhPIN1-c_D, GhPIN3_D, GhPIN8_A, GhABCB19-b_A, GhABCB19-b_D, GhABCB2-b_D, GhLAX6_A, and GhLAX7_D) specifically responded to TDZ. The pro35S::GhPIN3a::YFP transgenic plants showed lower defoliation than the wild type treated with TDZ, and YFP fluorescence in leaves was almost extinguished after treatment with TDZ rather than Eth. This provides direct evidence that GhPIN3a is involved in the leaf abscission induced by TDZ. We found that 959 transcription factors (TFs) specifically responded to TDZ, and a co-expression network analysis (WGCNA) showed five hub TFs (GhNAC72, GhWRKY51, GhWRKY70, GhWRKY50, and GhHSF24) during chemical defoliation with TDZ. Our work sheds light on the molecular basis of TDZ-induced leaf abscission in cotton.

Variability and limits of nitrogen and phosphorus resorption during foliar senescence

Foliar nutrient resorption (NuR) plays a key role in ecosystem functioning and plant nutrient economy. Most of this recycling occurs during the senescence of leaves and is actively addressed by cells. Here, we discuss the importance of cell biochemistry, physiology, and subcellular anatomy to condition the outcome of NuR at the cellular level and to explain the existence of limits to NuR. Nutrients are transferred from the leaf in simple metabolites that can be loaded into the phloem. Proteolysis is the main mechanism for mobilization of N, whereas P mobilization requires the involvement of different catabolic pathways, making the dynamics of P in leaves more variable than those of N before, during, and after foliar senescence. The biochemistry and fate of organelles during senescence impose constraints that limit NuR. The efficiency of NuR decreases, especially in evergreen species, as soil fertility increases, which is attributed to the relative costs of nutrient acquisition from soil decreasing with increasing soil nutrient availability, while the energetic costs of NuR from senescing leaves remain constant. NuR is genetically determined, with substantial interspecific variability, and is environmentally regulated in space and time, with nutrient availability being a key driver of intraspecific variability in NuR.

Leaf Senescence of the Seagrass Cymodocea nodosa in Cádiz Bay, Southern Spain

Leaf decay in seagrasses is enhanced in some seasons since large green senescent beach-cast seagrass leaves are frequently recorded during autumn and winter seasons. Here, we explore if senescence is operating in seagrass leaf decay or if hydrodynamic stress is responsible for the seasonal leaf abscission. A seasonal study on the temperate seagrass Cymodocea nodosa was carried out in four locations with contrasting hydrodynamic regimes. The morphological, biomechanical and material properties of C. nodosa were measured. The force required to break the ligule was always lower than that required to break the blade. This could be considered an adaptive strategy to reduce acute drag forces and thus lessen the chance of plant uprooting. The absolute force needed to dislodge the blade at the ligule level varied with season and location, with the lowest forces recorded in autumn. This may indicate that senescence is operating in this species. On the other hand, the minimum estimated failure velocities for leaf abscission were also recorded in autumn. Consequently, this may cause the premature shedding of leaves in this season before the senescence process has finished and can probably explain the occurrence of green beach-cast seagrass leaves usually found during autumn and winter.

Transcriptome and targeted hormone metabolome reveal the molecular mechanisms of flower abscission in camellia

INTRODUCTION

Camellia is among the most ornamentally valuable flowers and plants worldwide. Flower abscission typically causes significant financial losses by the horticultural landscape. Previous research has revealed that phytohormones, transcription factors, and other genes involved in floral development regulate the maintenance and mortality of flowers.

METHODS

In this study, for the first time, the transcriptomes and targeted hormone metabolomics of three developmental stages of the receptacles of two distinct camellia strains (CF: abscission strain, CHF: nonabscission strain) were analyzed to determine their roles in regulating blossom abscission in camellia.

RESULTS

ABA content was shown to be considerably upregulated throughout all phases of CF development, as were the genes implicated in the ABA production pathway and their downstream counterparts. Highly expressed genes in CF were involved in galactose metabolism, phenylpropanoid biosynthesis, amino and nucleotide sugar metabolism, pentose and glucuronate interconversions, and MAPK. Among others, highly expressed genes in CHF are associated with fructose and mannose metabolism, alpha-linolenic acid metabolism, biosynthesis of secondary metabolites, starch and sucrose metabolism, and cutin, suberin, and wax biosynthesis. A vast variety of stress response-related pathways and redox-related activities were also shown to be active in CHF. In contrast, CF dramatically activated pathways associated with lignin production, keratinogenesis, cell wall biogenesis, and ABA response. A comparative transcriptomic study of the CF and CHF pathways revealed that the downstream response pathways of hormones, including CTK, BR, IAA, ethylene, and GA, were very active in CF, indicating a significant amount of signal transduction and transcriptional regulation by CF. In addition, members of the transcription factor family, such as MYB, bHLH, MADS, and WD40, may regulate flower abscission.

DISCUSSION

A comparative transcriptome analysis of two distinct strains of camellia receptacles elucidates the molecular processes and regulatory characteristics of flower abscission and provides direction for the targeted improvement and breeding of camellia.

Low Temperature Inhibits the Defoliation Efficiency of Thidiazuron in Cotton by Regulating Plant Hormone Synthesis and the Signaling Pathway

Thidiazuron (TDZ) is the main defoliant used in production to promote leaf abscission for machine-picked cotton. Under low temperatures, the defoliation rate of cotton treated with TDZ decreases and the time of defoliation is delayed, but there is little information about this mechanism. In this study, RNA-seq and physiological analysis are performed to reveal the transcriptome profiling and change in endogenous phytohormones upon TDZ treatment in abscission zones (AZs) under different temperatures (daily mean temperatures: 25 °C and 15 °C). Genes differentially expressed in AZs between TDZ treatment and control under different temperatures were subjected to gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses to compare the enriched GO terms and KEGG pathways between the two temperature conditions. The results show that, compared with the corresponding control group, TDZ induces many differentially expressed genes (DEGs) in AZs, and the results of the GO and KEGG analyses show that the plant hormone signaling transduction pathway is significantly regulated by TDZ. However, under low temperature, TDZ induced less DEGs, and the enriched GO terms and KEGG pathways were different with those under normal temperature condition. Many genes in the plant hormone signal transduction pathway could not be induced by TDZ under low temperature conditions. In particular, the upregulated ethylene-signaling genes and downregulated auxin-signaling genes in AZs treated with TDZ were significantly affected by low temperatures. Furthermore, the expression of ethylene and auxin synthesis genes and their content in AZs treated with TDZ were also regulated by low temperature conditions. The upregulated cell wall hydrolase genes induced by TDZ were inhibited by low temperatures. However, the inhibition of low temperature on genes in AZs treated with TDZ was relieved with the extension of the treatment time. Together, these results indicate that the responses of ethylene and auxin synthesis and the signaling pathway to TDZ are inhibited by low temperatures, which could not induce the expression of cell wall hydrolase genes, and then inhibit the separation of AZ cells and the abscission of cotton leaves. This result provides new insights into the mechanism of defoliation induced by TDZ under low temperature conditions.

Metabolome and transcriptome integration reveals insights into the process of delayed petal abscission in rose by STS

The abscission of plant organs plays an important role in ensuring the normal life activities. Rose is one of the most important ornamental plants, and its premature abscission of petal has seriously affected the quality and commercial value. Silver Thiosulfate (STS) is an ethylene inhibitor, which is often used preservative to delay the senescence of fresh cut flowers. To understand the regulatory mechanism of petal abscission in rose by STS, integrative analysis of the metabolome and transcriptome profiles was performed in abscission zone (AZ) tissues of rose under different treatments (MOCK, STS, ETH, STS+ETH). The results showed that STS significantly delayed the petal abscission in phenotype and reduced the activity of two enzymes (pectinase and cellulase) associated with cell wall degradation in physiological level. STS affected the contents of five metabolites (shikonin, jasmonic acid, gluconolactone, stachyose and D-Erythrose 4-phosphate), and involved changes in the expression of 39 differentially expressed genes (DEGs) associated with these five metabolites. Five DEGs (LOC112192149, LOC112196726, LOC112189737, LOC112188495, and LOC112188936) were probably directly associated with the biosynthesis of shikonin, jasmonic acid, and D-Erythrose 4-phosphate. Meanwhile, the effect of STS on the abscission process significantly involved in the pentose phosphate pathway and amino acid biosynthesis pathway. In addition, STS had a greater effect on the transcription factors, phytohormone related DEGs represented by auxin and ethylene, DEGs related to disease resistance and amino acid, etc. Above all, STS negatively influences petal abscission of rose, these results maybe provide a reference for subsequent studies on petal abscission of rose by STS.

Poplar leaf abscission through induced chlorophyll breakdown by Mg-dechelatase

Chlorophyll breakdown is observed during senescence. The first step in chlorophyll breakdown is the removal of central Mg by Mg-dechelatase. This reaction is the rate-limiting step in the chlorophyll breakdown pathway. We evaluated the effect of induced chlorophyll breakdown on abscission through the removal of Mg by Mg-dechelatase. Poplar transformants carrying the dexamethasone-inducible Mg-dechelatase gene were prepared using the Arabidopsis Stay-Green1 cDNA. When leaves were treated with dexamethasone, chlorophyll was degraded, photosynthetic capacity was reduced, and an abscission zone was formed, resulting in leaf abscission. In addition, ethylene, which plays an important role during senescence, was produced in this process. Thus, chlorophyll breakdown induces the phenotype in the same way as commonly observed during leaf senescence. This study suggests a physiological role of chlorophyll breakdown in the leaf abscission of deciduous trees. Furthermore, this study shows that the dexamethasone-inducible gene expression system is an available option for deciduous tree studies.

Floral organ abscission in Arabidopsis requires the combined activities of three TALE homeodomain transcription factors

Floral organ abscission is a separation process in which sepals, petals, and stamens detach from the plant at abscission zones. Here, we investigated the collective role of three amino-acid-loop-extension (TALE) homeobox genes ARABIDOPSIS THALIANA HOMEOBOX GENE1 (ATH1), KNAT6 (for KNOTTED LIKE from Arabidopsis thaliana) and KNAT2, which form a module that patterns boundaries under the regulation of BLADE-ON-PETIOLE 1 and 2 (BOP1/2) co-activators. These TALE homeodomain transcription factors were shown to maintain boundaries in the flower, functioning as a unit to coordinate the growth, patterning, and activity of abscission zones. Together with BOP1 and BOP2, ATH1 and its partners KNAT6 and KNAT2 collectively contribute to the differentiation of lignified and separation layers of the abscission zone. The genetic interactions of BOP1/2 and ATH1 with INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) were also explored. We showed that BOP1/2 co-activators and ATH1 converge with the IDA signalling pathway to promote KNAT6 and KNAT2 expression in the abscission zone and cell separation. ATH1 acts as a central regulator in floral organ abscission as it controls the expression of other TALE genes in abscission zone cells.

Evolving role of synthetic cytokinin 6-benzyl adenine for drought stress tolerance in soybean (Glycine max L. Merr.)

The enhanced growth and productivity of soybeans during the past decades were possible due to the application of agrichemicals such as bio-fertilizers, chemical fertilizers, and the use of high yielding, as well as disease resistant transgenic and non-transgenic varieties. Agrichemicals applied as seed primers, plant protectants, and growth regulators, however, had a diminutive significance on growth and productivity improvements across the globe. The utilization of plant growth regulators (PGRs) for vegetative growth, reproduction and yield quality improvements remains unexplored, particularly, the use of cytokinins such as 6-benzyl adenine (6-BAP) to improve soybean response to abiotic stresses. Therefore, an understanding of the role of 6-BAP in the mediation of an array of adaptive responses that provide plants with the ability to withstand abiotic stresses must be thoroughly investigated. Such mitigative effects will play a critical role in encouraging exogenous application of plant hormones like 6-BAP as a mechanism for overcoming drought stress related effects in soybean. This paper discusses the evolving role of synthetic cytokinin 6-bezyl adenine in horticulture, especially the implications of its exogenous applications in soybean to confer tolerance to drought stress.

A multifaceted comparison between the fruit-abscission and fruit-retention cultivars in ornamental crabapple

The ornamental crabapple is a multipurpose landscaping tree that bears brilliant fruit throughout the winter. However, whether or not its fruit persists after maturation is specifically correlated to cultivar characteristics. In this work, we screened two different types that display fruit-retention ("Donald Wyman," "Red Jewel," and "Sugar Tyme") and fruit-abscission ("Radiant" and "Flame") in Northern China across the whole winter using multi-year successional records. Fruit-abscission was determined predominantly by the abscission zone established at the base of the pedicel, regardless of fruit size and pedicel length, according to the results of the comparative research. The primary physiological rationale was the accumulation of hydrolases activity (pectinesterase, cellulase, polygalacturonase, and β-glucosidase). Comparative transcriptomics further identified a number of upregulated DEGs involved in the synthesis pathways of canonical phytohormones, such as ethylene, jasmonic acid, abscisic acid, and cytokinin, as well as 12 transcription factors linked in downstream signaling in fruit-abscission cultivars. Finally, a model incorporating multi-layered modulation was proposed for the fruit abscission of ornamental crabapple. This study will serve as the foundation for the development of fruit-viewing crabapples that have an extended ornamental lifetime.

Hydrogen sulfide inhibits the abscission of tomato pedicel through reconstruction of a basipetal auxin gradient

Abscission is an important developmental process and an essential agricultural trait. Auxin and ethylene are two phytohormones with important roles in the complex, but still elusive signaling network of abscission. Here, we found that hydrogen sulfide (H₂S), a newly identified gasotransmitter, inhibits the initiation of tomato pedicel abscission. The underlying mechanism was explored through transcriptome profile analysis in various pedicel tissues with or without H₂S treatment in the early abscission stage. The data suggested that H₂S strongly influences the global transcription of pedicel tissues, exerts differential expression regulation along the pedicel, and markedly influences both the auxin and ethylene signaling pathways. Computational analysis revealed that H₂S reconstructs a basipetal auxin gradient along the pedicel at 4 h after treatment; this finding was further substantiated by the GUS-staining results of DR5::GUS pedicels. The inhibitory effect of H₂S to the ethylene signaling pathway might be an indirect action. Moreover, the subtilisin-like proteinase family members involved in the release of peptide signal molecules are critical components of the abscission signaling network downstream of auxin and ethylene.

Thidiazuron Promotes Leaf Abscission by Regulating the Crosstalk Complexities between Ethylene, Auxin, and Cytokinin in Cotton

Thidiazuron (TDZ) is widely used as a defoliant to induce leaf abscission in cotton. However, the underlying molecular mechanism is still unclear. In this study, RNA-seq and enzyme-linked immunosorbent assays (ELISA) were performed to reveal the dynamic transcriptome profiling and the change of endogenous phytohormones upon TDZ treatment in leaf, petiole, and abscission zone (AZ). We found that TDZ induced the gene expression of ethylene biosynthesis and signal, and promoted ethylene accumulation earlier in leaf than that in AZ. While TDZ down-regulated indole-3-acetic acid (IAA) biosynthesis genes mainly in leaf and IAA signal and transport genes. Furthermore, the IAA content reduced more sharply in the leaf than that in AZ to change the auxin gradient for abscission. TDZ suppressed CTK biosynthesis genes and induced CTK metabolic genes to reduce the IPA accumulation for the reduction of ethylene sensitivity. Furthermore, TDZ regulated the gene expression of abscisic acid (ABA) biosynthesis and signal and induced ABA accumulation between 12-48 h, which could up-regulate ABA response factor genes and inhibit IAA transporter genes. Our data suggest that TDZ orchestrates metabolism and signal of ethylene, auxin, and cytokinin, and also the transport of auxin in leaf, petiole, and AZ, to control leaf abscission.

Jasmonate-Dependent Response of the Flower Abscission Zone Cells to Drought in Yellow Lupine

Lipid membranes, as primary places of the perception of environmental stimuli, are a source of various oxygenated polyunsaturated fatty acids-oxylipins-functioning as modulators of many signal transduction pathways, e.g., phytohormonal. Among exogenous factors acting on plant cells, special attention is given to drought, especially in highly sensitive crop species, such as yellow lupine. Here, we used this species to analyze the contribution of lipid-related enzymes and lipid-derived plant hormones in drought-evoked events taking place in a specialized group of cells-the flower abscission zone (AZ)-which is responsible for organ detachment from the plant body. We revealed that water deficits in the soil causes lipid peroxidation in these cells and the upregulation of phospholipase D, lipoxygenase, and, concomitantly, jasmonic acid (JA) strongly accumulates in AZ tissue. Furthermore, we followed key steps in JA conjugation and signaling under stressful conditions by monitoring the level and tissue localization of enzyme providing JA derivatives (JASMONATE RESISTANT1) and the JA receptor (CORONATINE INSENSITIVE1). Collectively, drought-triggered AZ activation during the process of flower abscission is closely associated with the lipid modifications, leading to the formation of JA, its conjugation, and induction of signaling pathways.

A Possible Mode of Action of Methyl Jasmonate to Induce the Secondary Abscission Zone in Stems of Bryophyllum calycinum: Relevance to Plant Hormone Dynamics

Plants can react to environmental stresses through the abscission of infected, damaged, or senescent organs. A possible mode of action of methyl jasmonate (JA-Me) to induce the formation of the secondary abscission zone (SAZ) in the stems of Bryophyllum calycinum was investigated concerning plant hormone dynamics. Internode segments were prepared mainly from the second or third internode from the top of plants with active elongation. JA-Me applied to the middle of internode segments induced the SAZ formation above and below the treatment after 5-7 days. At 6 to 7 days after JA-Me treatment, the above and below internode pieces adjacent to the SAZ were excised and subjected to comprehensive analyses of plant hormones. The endogenous levels of auxin-related compounds between both sides adjacent to the SAZ were quite different. No differences were observed in the level of jasmonic acid (JA), but the contents of 12-oxo-phytodienoic acid (OPDA), a precursor of JA, and N-jasmonyl-leucine (JA-Leu) substantially decreased on the JA-Me side. Almost no effects of JA-Me on the dynamics of other plant hormones (cytokinins, abscisic acid, and gibberellins) were observed. Similar JA-Me effects on plant hormones and morphology were observed in the last internode of the decapitated growing plants. These suggest that the application of JA-Me induces the SAZ in the internode of B. calycinum by affecting endogenous levels of auxin- and jasmonate-related compounds.

Comparative Analysis of the Transcriptomes of Persisting and Abscised Fruitlets: Insights into Plant Hormone and Carbohydrate Metabolism Regulated Self-Thinning of Pecan Fruitlets during the Early Stage

Pecan is one of the most popular nut species in the world. The fruit drop rate of the pecan ‘Pawnee’ is more than 57%, with four fruit drop stages, which is very serious. In this study, we conducted transcriptomic profiling of persisting and abscised fruitlets in early fruit development by RNA-seq. A total of 11,976 differentially expressed genes (DEGs) were identified, 3012 upregulated and 8964 downregulated, in a comparison of abscised vs. persisting fruitlets at 35 days after anthesis (DAA). Our transcriptomic data suggest that gene subsets encoding elements involving the biosynthesis, metabolism, perception, signal transduction, and crosstalk of the plant hormones abscisic acid (ABA), auxin, cytokinin, ethylene, and gibberellin (GA) and plant growth regulators jasmonates, salicylic acid, and brassinosteroids were differentially expressed. In addition, the majority of transcriptionally activated genes involved in hormone signaling (except for ethylene and salicylic acid signaling) were downregulated in abscised fruitlets. The differential expression of transcripts coding for enzymes involved in sucrose, glucose, trehalose, starch, galactose, and galactinol metabolism shows that sucrose, galactinol, and glucose synthesis and starch content were reduced as starch biosynthesis was blocked, and retrogradation and degradation intensified. These results suggest that the abscised pecan fruitlets stopped growing and developing for some time before dropping, further indicating that their sugar supply was reduced or stopped. The transcriptome characterization described in this paper contributes to unravelling the molecular mechanisms and pathways involved in the physiological abscission of pecan fruits.

IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like peptides and HAE (HAESA)-like receptors regulate corolla abscission in Nicotiana benthamiana flowers

BACKGROUND

Abscission is an active, organized, and highly coordinated cell separation process enabling the detachment of aerial organs through the modification of cell-to-cell adhesion and breakdown of cell walls at specific sites on the plant body known as abscission zones. In Arabidopsis thaliana, abscission of floral organs and cauline leaves is regulated by the interaction of the hormonal peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), a pair of redundant receptor-like protein kinases, HAESA (HAE) and HAESA-LIKE2 (HSL2), and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptors. However, the functionality of this abscission signaling module has not yet been demonstrated in other plant species.

RESULTS

The expression of the pair of NbenIDA1 homeologs and the receptor NbenHAE.1 was supressed at the base of the corolla tube by the inoculation of two virus-induced gene silencing (VIGS) constructs in Nicotiana benthamiana. These gene suppression events arrested corolla abscission but did not produce any obvious effect on plant growth. VIGS plants retained a higher number of corollas attached to the flowers than control plants, an observation related to a greater corolla breakstrength. The arrest of corolla abscission was associated with the preservation of the parenchyma tissue at the base of the corolla tube that, in contrast, was virtually collapsed in normal corollas. In contrast, the inoculation of a viral vector construct that increased the expression of NbenIDA1A at the base of the corolla tube negatively affected the growth of the inoculated plants accelerating the timing of both corolla senescence and abscission. However, the heterologous ectopic overexpression of citrus CitIDA3 and Arabidopsis AtIDA in N. benthamiana did not alter the standard plant phenotype suggesting that the proteolytic processing machinery was unable to yield active peptides.

CONCLUSION

Here, we demonstrate that the pair of NbenIDA1 homeologs encoding small peptides of the IDA-like family and the receptor NbenHAE.1 control cellular breakdown at the base of the corolla tube awhere an adventitious AZ should be formed and, therefore, corolla abscission in N. benthamiana flowers. Altogether, our results provide the first evidence supporting the notion that the IDA-HAE/HSL2 signaling module is conserved in angiosperms.

Bioinformatics and Expression Analysis of IDA-Like Genes Reveal Their Potential Functions in Flower Abscission and Stress Response in Tobacco (Nicotiana tabacum L.)

The inflorescence deficient in abscission-like (IDL) genes have been shown to play critical roles in floral organ abscission, lateral root formation and various stress responses in Arabidopsis. The IDL gene family has been characterized in a number of plant species, while limited information is available about IDL genes of tobacco. In the current study, 15 NtIDL members were identified in the tobacco genome, and were classified into six groups together with IDL members from other species. Evolution analysis suggested that the NtIDL members form group VI might have originated from duplication events. Notably, NtIDL06 shared high similarities with AtIDA in the EPIP sequence, and its encoding gene was highly expressed in the abscission zone of flowers at late developmental stages, implying that NtIDL06 might regulate tobacco flower abscission. In addition, the results from cis-elements analysis of promoters and expression after stress treatments suggested that NtIDL members might be involved in various stress responses of tobacco. The results from this study provide information for further functional analysis related to flower abscission and stress responses of NtIDL genes.

Characterization of Two Ethephon-Induced IDA-Like Genes from Mango, and Elucidation of Their Involvement in Regulating Organ Abscission

In mango (Mangifera indica L.), fruitlet abscission limits productivity. The INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) peptide acts as a key component controlling abscission events in Arabidopsis. IDA-like peptides may assume similar roles in fruit trees. In this study, we isolated two mango IDA-like encoding-genes, MiIDA1 and MiIDA2. We used mango fruitlet-bearing explants and fruitlet-bearing trees, in which fruitlets abscission was induced using ethephon. We monitored the expression profiles of the two MiIDA-like genes in control and treated fruitlet abscission zones (AZs). In both systems, qRT-PCR showed that, within 24 h, both MiIDA-like genes were induced by ethephon, and that changes in their expression profiles were associated with upregulation of different ethylene signaling-related and cell-wall modifying genes. Furthermore, ectopic expression of both genes in Arabidopsis promoted floral-organ abscission, and was accompanied by an early increase in the cytosolic pH of floral AZ cells-a phenomenon known to be linked with abscission, and by activation of cell separation in vestigial AZs. Finally, overexpression of both genes in an Atida mutant restored its abscission ability. Our results suggest roles for MiIDA1 and MiIDA2 in affecting mango fruitlet abscission. Based on our results, we propose new possible modes of action for IDA-like proteins in regulating organ abscission.

EPIP-Evoked Modifications of Redox, Lipid, and Pectin Homeostasis in the Abscission Zone of Lupine Flowers

Yellow lupine is a great model for abscission-related research given that excessive flower abortion reduces its yield. It has been previously shown that the EPIP peptide, a fragment of LlIDL (INFLORESCENCE DEFICIENT IN ABSCISSION) amino-acid sequence, is a sufficient molecule to induce flower abortion, however, the question remains: What are the exact changes evoked by this peptide locally in abscission zone (AZ) cells? Therefore, we used EPIP peptide to monitor specific modifications accompanied by early steps of flower abscission directly in the AZ. EPIP stimulates the downstream elements of the pathway-HAESA and MITOGEN-ACTIVATED PROTEIN KINASE6 and induces cellular symptoms indicating AZ activation. The EPIP treatment disrupts redox homeostasis, involving the accumulation of H2O2 and upregulation of the enzymatic antioxidant system including superoxide dismutase, catalase, and ascorbate peroxidase. A weakening of the cell wall structure in response to EPIP is reflected by pectin demethylation, while a changing pattern of fatty acids and acyl lipids composition suggests a modification of lipid metabolism. Notably, the formation of a signaling molecule-phosphatidic acid is induced locally in EPIP-treated AZ. Collectively, all these changes indicate the switching of several metabolic and signaling pathways directly in the AZ in response to EPIP, which inevitably leads to flower abscission.

Multi-scale comparative transcriptome analysis reveals key genes and metabolic reprogramming processes associated with oil palm fruit abscission

BACKGROUND

Fruit abscission depends on cell separation that occurs within specialized cell layers that constitute an abscission zone (AZ). To determine the mechanisms of fleshy fruit abscission of the monocot oil palm (Elaeis guineensis Jacq.) compared with other abscission systems, we performed multi-scale comparative transcriptome analyses on fruit targeting the developing primary AZ and adjacent tissues.

RESULTS

Combining between-tissue developmental comparisons with exogenous ethylene treatments, and naturally occurring abscission in the field, RNAseq analysis revealed a robust core set of 168 genes with differentially regulated expression, spatially associated with the ripe fruit AZ, and temporally restricted to the abscission timing. The expression of a set of candidate genes was validated by qRT-PCR in the fruit AZ of a natural oil palm variant with blocked fruit abscission, which provides evidence for their functions during abscission. Our results substantiate the conservation of gene function between dicot dry fruit dehiscence and monocot fleshy fruit abscission. The study also revealed major metabolic transitions occur in the AZ during abscission, including key senescence marker genes and transcriptional regulators, in addition to genes involved in nutrient recycling and reallocation, alternative routes for energy supply and adaptation to oxidative stress.

CONCLUSIONS

The study provides the first reference transcriptome of a monocot fleshy fruit abscission zone and provides insight into the mechanisms underlying abscission by identifying key genes with functional roles and processes, including metabolic transitions, cell wall modifications, signalling, stress adaptations and transcriptional regulation, that occur during ripe fruit abscission of the monocot oil palm. The transcriptome data comprises an original reference and resource useful towards understanding the evolutionary basis of this fundamental plant process.

Functional characterization of PETIOLULE-LIKE PULVINUS (PLP) gene in abscission zone development in Medicago truncatula and its application to genetic improvement of alfalfa

Alfalfa (Medicago sativa L.) is one of the most important forage crops throughout the world. Maximizing leaf retention during the haymaking process is critical for achieving superior hay quality and maintaining biomass yield. Leaf abscission process affects leaf retention. Previous studies have largely focused on the molecular mechanisms of floral organ, pedicel and seed abscission but scarcely touched on leaf and petiole abscission. This study focuses on leaf and petiole abscission in the model legume Medicago truncatula and its closely related commercial species alfalfa. By analysing the petiolule-like pulvinus (plp) mutant in M. truncatula at phenotypic level (breakstrength and shaking assays), microscopic level (scanning electron microscopy and cross-sectional analyses) and molecular level (expression level and expression pattern analyses), we discovered that the loss of function of PLP leads to an absence of abscission zone (AZ) formation and PLP plays an important role in leaflet and petiole AZ differentiation. Microarray analysis indicated that PLP affects abscission process through modulating genes involved in hormonal homeostasis, cell wall remodelling and degradation. Detailed analyses led us to propose a functional model of PLP in regulating leaflet and petiole abscission. Furthermore, we cloned the PLP gene (MsPLP) from alfalfa and produced RNAi transgenic alfalfa plants to down-regulate the endogenous MsPLP. Down-regulation of MsPLP results in altered pulvinus structure with increased leaflet breakstrength, thus offering a new approach to decrease leaf loss during alfalfa haymaking process.

Ethylene, not ABA, is closely linked to the recovery of gas exchange after drought in four Caragana species

Drought is a cyclical phenomenon in natural environments. During dehydration, stomatal closure is mainly regulated by abscisic acid (ABA) dynamics that limit transpiration in seed plants, but following rehydration, the mechanism of gas exchange recovery is still not clear. In this study, leaf water potential (ψ_leaf ), stomatal conductance (g_s ), leaf hydraulic conductance (K_leaf ), foliar ABA level, ethylene emission rate in response to dehydration and rehydration were investigated in four Caragana species with isohydric (Caragana spinosa and C. pruinosa) and anisohydric (C. intermedia and C. microphylla) traits. Two isohydric species with ABA-induced stomatal closure exhibited more sensitive g_s and K_leaf to decreasing ψ_leaf than two anisohydric species which exhibited a switch from ABA to water potential-driven stomatal closure during dehydration. Following rehydration, the recovery of gas exchange was not associated with a decrease in ABA level but was strongly limited by the degradation of the ethylene emission rate in all species. Furthermore, two anisohydric species with low drought-induced ethylene production exhibited more rapid recovery in gas exchange upon rehydration. Our results indicated that ethylene is a key factor regulating the drought-recovery ability in terms of gas exchange, which may shape species adaptation to drought and potential species distribution.

Evaluation of Cotton (Gossypium hirsutum L.) Leaf Abscission Sensitivity Triggered by Thidiazuron through Membership Function Value

Chemical defoliation is an essential agricultural practice in cotton production for mechanic harvesting. Thidiazuron (TDZ) is the active ingredient of the chemical defoliant used on cotton. So far, few studies havefocused on the method of identifying the sensitivity of cotton cultivars to TDZ. Therefore, a greenhouse soil culture experiment was performed by using two widely cultivatedupland cotton cultivars CRI 49 and CRI 12 treated with seven different concentrations (0, 100, 200, 300, 400, 500, and 1000 mg L⁻¹) of TDZ at the seedling stage to establish a screening system. Principal component analysis and the membership function value (MFV) method was used to analyze the physiological and phenotypic characters, including abscission rate, amino acids content, net photosynthetic rate (Pn), etc. Finally, we developed a mathematical evaluation model, selected 100 mg L⁻¹ TDZ as the optimal concentration and identified reliable characters net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr) to evaluate cotton leaf abscission sensitivity. These results also confirmed that CRI 12 was more sensitive to TDZ than CRI 49. This is the first time using a mathematical evaluation method to evaluate the cotton leaf abscission sensitivity triggered by TDZ at the seedling stage and the results were also confirmed in the field experiment. Furthermore, it will be valuable that MFV method is applied to stress sensitivity evaluation in other crop species under stress environment.

Reshaping of the Arabidopsis thaliana Proteome Landscape and Co-regulation of Proteins in Development and Immunity

Proteome remodeling is a fundamental adaptive response, and proteins in complexes and functionally related proteins are often co-expressed. Using a deep sampling strategy we define core proteomes of Arabidopsis thaliana tissues with around 10 000 proteins per tissue, and absolutely quantify (copy numbers per cell) nearly 16 000 proteins throughout the plant lifecycle. A proteome-wide survey of global post-translational modification revealed amino acid exchanges pointing to potential conservation of translational infidelity in eukaryotes. Correlation analysis of protein abundance uncovered potentially new tissue- and age-specific roles of entire signaling modules regulating transcription in photosynthesis, seed development, and senescence and abscission. Among others, the data suggest a potential function of RD26 and other NAC transcription factors in seed development related to desiccation tolerance as well as a possible function of cysteine-rich receptor-like kinases (CRKs) as ROS sensors in senescence. All of the components of ribosome biogenesis factor (RBF) complexes were found to be co-expressed in a tissue- and age-specific manner, indicating functional promiscuity in the assembly of these less-studied protein complexes in Arabidopsis.Furthermore, we characterized detailed proteome remodeling in basal immunity by treating Arabidopsis seeldings with flg22. Through simultaneously monitoring phytohormone and transcript changes upon flg22 treatment, we obtained strong evidence of suppression of jasmonate (JA) and JA-isoleucine (JA-Ile) levels by deconjugation and hydroxylation by IAA-ALA RESISTANT3 (IAR3) and JASMONATE-INDUCED OXYGENASE 2 (JOX2), respectively, under the control of JASMONATE INSENSITIVE 1 (MYC2), suggesting an unrecognized role of a new JA regulatory switch in pattern-triggered immunity. Taken together, the datasets generated in this study present extensive coverage of the Arabidopsis proteome in various biological scenarios, providing a rich resource available to the whole plant science community.

Spatio-temporal immunolocalization of extensin protein and hemicellulose polysaccharides during olive fruit abscission

Immunocytochemical and molecular analyses reveal that the disassembly of the cell wall may be mediated by changes in the level and subcellular location of extensin protein and hemicelluloses during olive-fruit abscission. Although cell-wall modification is believed to underlie the changes in organ abscission, information concerning the changes in cell-wall proteins and hemicellulose polysaccharides is still limited. The aim of this work was to analyze the spatio-temporal patterns of the distribution of different extensin proteins and hemicelluloses in the abscission zone (AZ) during natural ripe-fruit abscission in olive (Olea europaea L.). In this study, we employed immunogold labeling in the ripe-fruit AZ during olive AZ cell separation, using an expanded set of monoclonal antibodies that recognize different types of hemicelluloses (LM11, LM15, and LM21), callose (anti-(1,3)-β-D-glucan) and extensin (JIM19) epitopes, and transmission electron microscopy imaging. Our data demonstrate that AZ cell separation was accompanied by a loss of the JIM19 extensin epitopes and a reduction in the detection of the LM15 xyloglucan epitopes in AZ cell walls, whereas AZ cells were found to be enriched with respect to the xylan and callose levels of the cell wall during olive ripe-fruit abscission. By contrast, AZ cell-wall polysaccharide remodeling did not involve mannans. Moreover, in ripe-fruit AZ, quantitative RT-PCR analysis revealed that OeEXT1, OeEXT2, OeXTH9, and OeXTH13 genes were downregulated during abscission, whereas the expression of OeXTH1, OeXTH5, and OeXTH14 genes increased during abscission. Taken together, the results indicate that AZ cell-wall dynamics during olive ripe-fruit abscission involves extensin protein and hemicellulose modifications, as well as related expressed genes. This is the first study available demonstrating temporal degradation of extensin protein and hemicelluloses in the AZ at the subcellular level.

KNOX protein KNAT1 regulates fruitlet abscission in litchi by repressing ethylene biosynthetic genes

Abscission is triggered by multiple environmental and developmental cues, including endogenous plant hormones. KNOTTED-LIKE HOMEOBOX (KNOX) transcription factors (TFs) play an important role in controlling abscission in plants. However, the underlying molecular mechanism of KNOX TFs in abscission is largely unknown. Here, we identified LcKNAT1, a KNOTTED-LIKE FROM ARABIDOPSIS THALIANA1 (KNAT1)-like protein from litchi, which regulates abscission by modulating ethylene biosynthesis. LcKNAT1 is expressed in the fruit abscission zone and its expression decreases during fruitlet abscission. Furthermore, the expression of the ethylene biosynthetic genes LcACS1, LcACS7, and LcACO2 increases in the fruit abscission zone, in parallel with the emission of ethylene in fruitlets. In vitro and in vivo assays revealed that LcKNAT1 inhibits the expression of LcACS/ACO genes by directly binding to their promoters. Moreover, ectopic expression of LcKNAT1 represses flower abscission in tomatoes. Transgenic plants expressing LcKNAT1 also showed consistently decreased expression of ACS/ACO genes. Collectively, these results indicate that LcKNAT1 represses abscission via the negative regulation of ethylene biosynthesis.

Crosstalk between Hydrogen Sulfide and Other Signal Molecules Regulates Plant Growth and Development

Hydrogen sulfide (H₂S), once recognized only as a poisonous gas, is now considered the third endogenous gaseous transmitter, along with nitric oxide (NO) and carbon monoxide (CO). Multiple lines of emerging evidence suggest that H₂S plays positive roles in plant growth and development when at appropriate concentrations, including seed germination, root development, photosynthesis, stomatal movement, and organ abscission under both normal and stress conditions. H₂S influences these processes by altering gene expression and enzyme activities, as well as regulating the contents of some secondary metabolites. In its regulatory roles, H₂S always interacts with either plant hormones, other gasotransmitters, or ionic signals, such as abscisic acid (ABA), ethylene, auxin, CO, NO, and Ca²⁺. Remarkably, H₂S also contributes to the post-translational modification of proteins to affect protein activities, structures, and sub-cellular localization. Here, we review the functions of H₂S at different stages of plant development, focusing on the S-sulfhydration of proteins mediated by H₂S and the crosstalk between H₂S and other signaling molecules.

Mechanism and Regulation of Silique Dehiscence, Which Affects Oil Seed Production

Silique dehiscence is an important physiological process during natural growth that enables mature seeds to be released from plants, which then undergo reproduction and ensure the survival of future generations. In agricultural production, the time and degree of silique dehiscence affect the harvest time and processing of crops. Premature silique dehiscence leads to seeds being shed before harvest, resulting in substantial reductions to yields. Conversely, late silique dehiscence is not conducive to harvesting, and grain weight and oil content will be reduced due to the respiratory needs of seeds. In this paper, the mechanisms and regulation of silique dehiscence, and its application in agricultural production is reviewed.

Formation of the Secondary Abscission Zone Induced by the Interaction of Methyl Jasmonate and Auxin in Bryophyllum calycinum: Relevance to Auxin Status and Histology

The interaction of methyl jasmonate (JA-Me) and indole-3-acetic acid (IAA) to induce the formation of the secondary abscission zone in the middle of internode segments of Bryophyllum calycinum was investigated in relation to auxin status and histology. When IAA at 0.1% (w/w, in lanolin) was applied to the segments, the formation of the secondary abscission zone at a few mm above the treatment in the apical direction was observed. On the contrary, IAA at 0.5% (w/w, in lanolin) did not induce the formation of the secondary abscission zone. JA-Me at 0.5% (w/w, in lanolin) applied to the middle of internode segments kept in the normal (natural) or inverted positions also induced the formation of the secondary abscission zone below and above parts of the treatment. IAA at 0.5% applied to the cut surface of the upper part of the segments completely prevented the formation of the secondary abscission zone induced by JA-Me. Simultaneous application of IAA 0.5% with JA-Me 0.5% in the middle part of the internode segments induced the formation of the secondary abscission zone at 10 mm to 12 mm above the treatment. Histological analyses indicated that the formation of the secondary abscission zone was characterized by the presence of newly synthesized cell plates that resulted from periclinal cell division within one layer of mother cells in stems. The effects of IAA (0.1%) and JA-Me (0.5%) on the formation of the secondary abscission zone were histologically similar. Comprehensive analyses of plant hormones revealed that the balance of the endogenous levels of IAA in both sides adjacent to the abscission zone was significantly disturbed when the secondary abscission formation was induced by the application of IAA. These results strongly suggest that an auxin gradient is important in the formation of the secondary abscission zone in the internode segments of B. calycinum, and IAA gradient results from polar IAA transport from the application site. IAA is important in the regulation of formation of the secondary abscission zone induced by JA-Me. Further possible mechanisms of the formation of the secondary abscission zone in the internode segments of B. calycinum are also discussed in the interaction of JA-Me and IAA.

Chemical Defoliant Promotes Leaf Abscission by Altering ROS Metabolism and Photosynthetic Efficiency in Gossypium hirsutum

Chemical defoliation is an important part of cotton mechanical harvesting, which can effectively reduce the impurity content. Thidiazuron (TDZ) is the most used chemical defoliant on cotton. To better clarify the mechanism of TDZ promoting cotton leaf abscission, a greenhouse experiment was conducted on two cotton cultivars (CRI 12 and CRI 49) by using 100 mg L⁻¹ TDZ at the eight-true-leaf stage. Results showed that TDZ significantly promoted the formation of leaf abscission zone and leaf abscission. Although the antioxidant enzyme activities were improved, the reactive oxygen species and malondialdehyde (MDA) contents of TDZ increased significantly compared with CK (water). The photosynthesis system was destroyed as net photosynthesis (Pn), transpiration rate (Tr), and stomatal conductance (Gs) decreased dramatically by TDZ. Furthermore, comparative RNA-seq analysis of the leaves showed that all of the photosynthetic related genes were downregulated and the oxidation-reduction process participated in leaf shedding caused by TDZ. Consequently, a hypothesis involving possible cross-talk between ROS metabolism and photosynthesis jointly regulating cotton leaf abscission is proposed. Our findings not only provide important insights into leaf shedding-associated changes induced by TDZ in cotton, but also highlight the possibility that the ROS and photosynthesis may play a critical role in the organ shedding process in other crops.

Peptide signaling for drought-induced tomato flower drop

The premature abscission of flowers and fruits limits crop yield under environmental stress. Drought-induced flower drop in tomato plants was found to be regulated by phytosulfokine (PSK), a peptide hormone previously known for its growth-promoting and immune-modulating activities. PSK formation in response to drought stress depends on phytaspase 2, a subtilisin-like protease of the phytaspase subtype that generates the peptide hormone by aspartate-specific processing of the PSK precursor in the tomato flower pedicel. The mature peptide acts in the abscission zone where it induces expression of cell wall hydrolases that execute the abscission process. Our results provide insight into the molecular control of abscission as regulated by proteolytic processing to generate a small plant peptide hormone.

Cell Wall Composition and Ultrastructural Immunolocalization of Pectin and Arabinogalactan Protein during Olea europaea L. Fruit Abscission

Cell wall modification is integral to many plant developmental processes where cells need to separate, such as abscission. However, changes in cell wall composition during natural fruit abscission are poorly understood. In olive (Olea europaea L.), some cultivars such as 'Picual' undergo massive natural fruit abscission after fruit ripening. This study investigates the differences in cell wall polysaccharide composition and the localization of pectins and arabinogalactan protein (AGP) in the abscission zone (AZ) during cell separation to understand fruit abscission control in 'Picual' olive. To this end, immunogold labeling employing a suite of monoclonal antibodies to cell wall components (JIM13, LM5, LM6, LM19 and LM20) was investigated in olive fruit AZ. Cell wall polysaccharide extraction revealed that the AZ cell separation is related to the de-esterification and degradation of pectic polysaccharides. Moreover, ultrastructural localization showed that both esterified and unesterified homogalacturonans (HGs) localize mainly in the AZ cell walls, including the middle lamella and tricellular junction zones. Our results indicate that unesterified HGs are likely to contribute to cell separation in the olive fruit AZ. Similarly, immunogold labeling demonstrated a decrease in both galactose-rich and arabinose-rich pectins in AZ cell walls during ripe fruit abscission. In addition, AGPs were localized in the cell wall, plasma membrane and cytoplasm of AZ cells with lower levels of AGPs during ripe fruit abscission. This detailed temporal profile of the cell wall polysaccharide composition, and the pectins and AGP immunolocalization in the olive fruit AZ, offers new insights into cell wall remodeling during ripe fruit abscission.

Hydrogen sulfide inhibits ethylene-induced petiole abscission in tomato (Solanum lycopersicum L.)

Abscission is a dynamic physiological process that is ubiquitous in plants and can also be an essential agronomic trait in crops, thus attracting attention from plant growers and breeders. In general, the process of plant organ abscission can be divided into four steps, among which the step to obtain the competence to respond to abscission signals (step 2) is the most complex; however, the molecular mechanism underlying this process remains unclear. In this study, we found that hydrogen sulfide (H2S) inhibited the abscission of the tomato petiole in a dose-dependent manner, and the abscission of the petiole was accelerated when an H2S scavenger was applied. Further enzymatic activity and gene expression analyses showed that H2S suppressed the activity of enzymes capable of modifying the cell wall by inhibiting the usual upregulation of the transcription of the corresponding genes during the abscission process but not by affecting the activities of these enzymes by direct posttranslational modification. H2S treatment upregulated the expression levels of SlIAA3 and SlIAA4 but downregulated the transcription of ILR-L3 and ILR-L4 in the earlier stages of the abscission process, indicating that H2S probably functioned in the second step of the abscission process by preventing the abscission zone cells from obtaining the competence to respond to abscission signals by modulating the content of the bioactive-free auxin in these cells. Moreover, similar H2S inhibitory effects were also demonstrated in the process of floral organ abscission and anther dehiscence in other plant species, suggesting a ubiquitous role for H2S in cell separation processes.

Differential expression of IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like genes in Nicotiana benthamiana during corolla abscission, stem growth and water stress

BACKGROUND

IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like signaling peptides and the associated HAE (HAESA)-like family of receptor kinases were originally reported in the model plant Arabidopsis thaliana (Arabidopsis) to be deeply involved in the regulation of abscission. Actually, IDA peptides, as cell-to-cell communication elements, appear to be implicated in many developmental processes that rely on cell separation events, and even in the responses to abiotic stresses. However, the knowledge related to the molecular machinery regulating abscission in economically important crops is scarce. In this work, we determined the conservation and phylogeny of the IDA-like and HAE-like gene families in relevant species of the Solanaceae family and analyzed the expression of these genes in the allopolyploid Nicotiana benthamiana, in order to identify members involved in abscission, stem growth and in the response to drought conditions.

RESULTS

The phylogenetic relationships among the IDA-like members of the Solanaceae studied, grouped the two pairs of NbenIDA1 and NbenIDA2 protein homeologs with the Arabidopsis prepropeptides related to abscission. Analysis of promoter regions searching for regulatory elements showed that these two pairs of homeologs contained both hormonal and drought response elements, although NbenIDA2A lacked the hormonal regulatory elements. Expression analyses showed that the pair of NbenIDA1 homeologs were upregulated during corolla abscission. NbenIDA1 and NbenIDA2 pairs showed tissue differential expression under water stress conditions, since NbenIDA1 homeologs were highly expressed in stressed leaves while NbenIDA2 homeologs, especially NbenIDA2B, were highly expressed in stressed roots. In non-stressed active growing plants, nodes and internodes were the tissues with the highest expression levels of all members of the IDA-like family and their putative HAE-like receptors.

CONCLUSION

Our results suggest that the pair of NbenIDA1 homeologs are involved in the natural process of corolla abscission while both pairs of NbenIDA1 and NbenIDA2 homeologs are implicated in the response to water stress. The data also suggest that IDA peptides may be important during stem growth and development. These results provide additional evidence that the functional module formed by IDA peptides and its receptor kinases, as defined in Arabidopsis, may also be conserved in Solanaceae.

Differential expression of IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like genes in Nicotiana benthamiana during corolla abscission, stem growth and water stress

BACKGROUND

IDA (INFLORESCENCE DEFICIENT IN ABSCISSION)-like signaling peptides and the associated HAE (HAESA)-like family of receptor kinases were originally reported in the model plant Arabidopsis thaliana (Arabidopsis) to be deeply involved in the regulation of abscission. Actually, IDA peptides, as cell-to-cell communication elements, appear to be implicated in many developmental processes that rely on cell separation events, and even in the responses to abiotic stresses. However, the knowledge related to the molecular machinery regulating abscission in economically important crops is scarce. In this work, we determined the conservation and phylogeny of the IDA-like and HAE-like gene families in relevant species of the Solanaceae family and analyzed the expression of these genes in the allopolyploid Nicotiana benthamiana, in order to identify members involved in abscission, stem growth and in the response to drought conditions.

RESULTS

The phylogenetic relationships among the IDA-like members of the Solanaceae studied, grouped the two pairs of NbenIDA1 and NbenIDA2 protein homeologs with the Arabidopsis prepropeptides related to abscission. Analysis of promoter regions searching for regulatory elements showed that these two pairs of homeologs contained both hormonal and drought response elements, although NbenIDA2A lacked the hormonal regulatory elements. Expression analyses showed that the pair of NbenIDA1 homeologs were upregulated during corolla abscission. NbenIDA1 and NbenIDA2 pairs showed tissue differential expression under water stress conditions, since NbenIDA1 homeologs were highly expressed in stressed leaves while NbenIDA2 homeologs, especially NbenIDA2B, were highly expressed in stressed roots. In non-stressed active growing plants, nodes and internodes were the tissues with the highest expression levels of all members of the IDA-like family and their putative HAE-like receptors.

CONCLUSION

Our results suggest that the pair of NbenIDA1 homeologs are involved in the natural process of corolla abscission while both pairs of NbenIDA1 and NbenIDA2 homeologs are implicated in the response to water stress. The data also suggest that IDA peptides may be important during stem growth and development. These results provide additional evidence that the functional module formed by IDA peptides and its receptor kinases, as defined in Arabidopsis, may also be conserved in Solanaceae.

More than cell wall hydrolysis: orchestration of cellular dynamics for organ separation

Plants' ability to cope with the ever-changing environment is one of the hallmarks that distinguishes plants from animals. Plants stationed in one place have evolved to remodel their architecture in response to the environmental factors by continuously creating new organ systems and removing existing organs through abscission. Herein, I provide insights into developmental plasticity of plants, focusing on the exit strategy (abscission). When plants start developing organs, the elimination tactics are also established in the form of abscission zones (AZ), that is, specialized cell layers for organ separation. Herein, recent advances in understanding the spatial regulatory mechanism of AZ in terms of cellular dynamics, coordination, and reconfiguration of the physical barrier of the cell wall to achieve precise abscission are discussed.

SOBIR1/EVR prevents precocious initiation of fiber differentiation during wood development through a mechanism involving BP and ERECTA

In plants, secondary growth results in radial expansion of stems and roots, generating large amounts of biomass in the form of wood. Using genome-wide association studies (GWAS)-guided reverse genetics in Arabidopsis thaliana, we discovered SOBIR1/EVR, previously known to control plant immunoresponses and abscission, as a regulator of secondary growth. We present anatomical, genetic, and molecular evidence indicating that SOBIR1/EVR prevents the precocious differentiation of xylem fiber, a key cell type for wood development. SOBIR1/EVR acts through a mechanism that involves BREVIPEDICELLUS (BP) and ERECTA (ER), 2 proteins previously known to regulate xylem fiber development. We demonstrate that BP binds SOBIR1/EVR promoter and that SOBIR1/EVR expression is enhanced in bp mutants, suggesting a direct, negative regulation of BP over SOBIR1/EVR expression. We show that SOBIR1/EVR physically interacts with ER and that defects caused by the sobir1/evr mutation are aggravated by mutating ER, indicating that SOBIR1/EVR and ERECTA act together in the control of the precocious formation of xylem fiber development.