Ethylene-dependent and -independent processes associated with floral organ abscission in Arabidopsis

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.

Reactive Oxygen and Related Regulatory Factors Involved in Ethylene-Induced Petal Abscission in Roses

Petal abscission affects the growth, development, and economic value of plants, but the mechanism of ethylene-ROS-induced petal abscission is not clear. Therefore, we treated roses with different treatments (MOCK, ETH, STS, and ETH + STS), and phenotypic characteristics of petal abscission, changed ratio of fresh weight, morphology of cells in AZ and the expression of RhSUC2 were analyzed. On this basis, we measured reactive oxygen species (ROS) content in petals and AZ cells of roses, and analyzed the expression levels of some genes related to ROS production and ROS scavenging. Ethylene promoted the petal abscission of rose through decreasing the fresh weight of the flower, promoting the stacking and stratification of AZ cells, and repressing the expression of RhSUC2. During this process, ethylene induced the ROS accumulation of AZ cells and petals mainly through increasing the expressions of some genes (RhRHS17, RhIDH1, RhIDH-III, RhERS, RhPBL32, RhFRS5, RhRAC5, RhRBOHD, RhRBOHC, and RhPLATZ9) related to ROS production and repressing those genes (RhCCR4, RhUBC30, RhSOD1, RhAPX6.1, and RhCATA) related to ROS scavenging. In summary, ROS and related regulatory factors involved in ethylene induced petal abscission in roses.

Transcriptomic and physiological analysis provide new insight into seed shattering mechanism in Pennisetum alopecuroides 'Liqiu'

KEY MESSAGE

Through the histological, physiological, and transcriptome-level identification of the abscission zone of Pennisetum alopecuroides 'Liqiu', we explored the structure and the genes related to seed shattering, ultimately revealing the regulatory network of seed shattering in P. alopecuroides. Pennisetum alopecuroides is one of the most representative ornamental grass species of Pennisetum genus. It has unique inflorescence, elegant appearance, and strong stress tolerance. However, the shattering of seeds not only reduces the ornamental effect, but also hinders the seed production. In order to understand the potential mechanisms of seed shattering in P. alopecuroides, we conducted morphological, histological, physiological, and transcriptomic analyses on P. alopecuroides cv. 'Liqiu'. According to histological findings, the seed shattering of 'Liqiu' was determined by the abscission zone at the base of the pedicel. Correlation analysis showed that seed shattering was significantly correlated with cellulase, lignin, auxin, gibberellin, cytokinin and jasmonic acid. Through a combination of histological and physiological analyses, we observed the accumulation of cellulase and lignin during 'Liqiu' seed abscission. We used PacBio full-length transcriptome sequencing (SMRT) combined with next-generation sequencing (NGS) transcriptome technology to improve the transcriptome data of 'Liqiu'. Transcriptomics further identified many differential genes involved in cellulase, lignin and plant hormone-related pathways. This study will provide new insights into the research on the shattering mechanism of P. alopecuroides.

Integrated analysis of the transcriptome and metabolome reveals the molecular mechanism regulating cotton boll abscission under low light intensity

BACKGROUND

Cotton boll shedding is one of the main factors adversely affecting the cotton yield. During the cotton plant growth period, low light conditions can cause cotton bolls to fall off prematurely. In this study, we clarified the regulatory effects of low light intensity on cotton boll abscission by comprehensively analyzing the transcriptome and metabolome.

RESULTS

When the fruiting branch leaves were shaded after pollination, all of the cotton bolls fell off within 5 days. Additionally, H2O2 accumulated during the formation of the abscission zone. Moreover, 10,172 differentially expressed genes (DEGs) and 81 differentially accumulated metabolites (DAMs) were identified. A KEGG pathway enrichment analysis revealed that the identified DEGs and DAMs were associated with plant hormone signal transduction and flavonoid biosynthesis pathways. The results of the transcriptome analysis suggested that the expression of ethylene (ETH) and abscisic acid (ABA) signaling-related genes was induced, which was in contrast to the decrease in the expression of most of the IAA signaling-related genes. A combined transcriptomics and metabolomics analysis revealed that flavonoids may help regulate plant organ abscission. A weighted gene co-expression network analysis detected two gene modules significantly related to abscission. The genes in these modules were mainly related to exosome, flavonoid biosynthesis, ubiquitin-mediated proteolysis, plant hormone signal transduction, photosynthesis, and cytoskeleton proteins. Furthermore, TIP1;1, UGT71C4, KMD3, TRFL6, REV, and FRA1 were identified as the hub genes in these two modules.

CONCLUSIONS

In this study, we elucidated the mechanisms underlying cotton boll abscission induced by shading on the basis of comprehensive transcriptomics and metabolomics analyses of the boll abscission process. The study findings have clarified the molecular basis of cotton boll abscission under low light intensity, and suggested that H2O2, phytohormone, and flavonoid have the potential to affect the shedding process of cotton bolls under low light stress.

The YABBY gene SHATTERING1 controls activation rather than patterning of the abscission zone in Setaria viridis

Abscission is predetermined in specialized cell layers called the abscission zone (AZ) and activated by developmental or environmental signals. In the grass family, most identified AZ genes regulate AZ anatomy, which differs among lineages. A YABBY transcription factor, SHATTERING1 (SH1), is a domestication gene regulating abscission in multiple cereals, including rice and Setaria. In rice, SH1 inhibits lignification specifically in the AZ. However, the AZ of Setaria is nonlignified throughout, raising the question of how SH1 functions in species without lignification. Crispr-Cas9 knockout mutants of SH1 were generated in Setaria viridis and characterized with histology, cell wall and auxin immunofluorescence, transmission electron microscopy, hormonal treatment and RNA-Seq analysis. The sh1 mutant lacks shattering, as expected. No differences in cell anatomy or cell wall components including lignin were observed between sh1 and the wild-type (WT) until abscission occurs. Chloroplasts degenerated in the AZ of WT before abscission, but degeneration was suppressed by auxin treatment. Auxin distribution and expression of auxin-related genes differed between WT and sh1, with the signal of an antibody to auxin detected in the sh1 chloroplast. SH1 in Setaria is required for activation of abscission through auxin signaling, which is not reported in other grass species.

LcERF10 functions as a positive regulator of litchi fruitlet abscission

Phytohormone ethylene is well-known in positive modulation of plant organ abscission. However, the molecular mechanism underlying ethylene-induced abscission remains largely unknown. Here, we identified an ethylene-responsive factor, LcERF10, as a key regulatory gene in litchi fruitlet abscission. LcERF10 was strongly induced in the fruitlet abscission zone (FAZ) during the ethylene-activated abscission. Silencing of LcERF10 in litchi weakened the cytosolic alkalization of the FAZ and reduced fruitlet abscission. Moreover, LcERF10 directly bound the promoter and repressed the expression of LcNHX7, a Na+/H+ exchanger that was down-regulated in FAZ following the ethylene-activated abscission and up-regulated after LcERF10 silencing. Additionally, ectopic expression of LcERF10 in Arabidopsis promoted the cytosolic alkalization of the floral organ AZ and accelerated the floral organ abscission. Collectively, our results suggest that the transcription factor LcERF10 plays a positive role in litchi fruitlet abscission.

How far can omics go in unveiling the mechanisms of floral senescence?

Floral senescence is of fundamental interest in understanding plant developmental regulation, it is of ecological and agricultural interest in relation to seed production, and is of key importance to the production of cut flowers. The biochemical changes occurring are well-studied and involve macromolecular breakdown and remobilisation of nutrients to developing seeds or other young organs in the plant. However, the initiation and regulation of the process and inter-organ communication remain to be fully elucidated. Although ethylene emission, which becomes autocatalytic, is a key regulator in some species, in other species it appears not to be as important. Other plant growth regulators such as cytokinins, however, seem to be important in floral senescence across both ethylene sensitive and insensitive species. Other plant growth regulators are also likely involved. Omics approaches have provided a wealth of data especially in ornamental species where genome data is lacking. Two families of transcription factors: NAC and WRKY emerge as major regulators, and omics information has been critical in understanding their functions. Future progress would greatly benefit from a single model species for understanding floral senescence; however, this is challenging due to the diversity of regulatory mechanisms. Combining omics data sets can be powerful in understanding different layers of regulation, but in vitro biochemical and or genetic analysis through transgenics or mutants is still needed to fully verify mechanisms and interactions between regulators.

The AtERF19 gene regulates meristem activity and flower organ size in plants

Ethylene-responsive factors (ERFs) have diverse functions in the regulation of various plant developmental processes. Here, we demonstrate the dual role of an Arabidopsis ERF gene, AtERF19, in regulating reproductive meristem activity and flower organ size through the regulation of genes involved in CLAVATA-WUSCHEL (CLV-WUS) and auxin signaling, respectively. We found that AtERF19 stimulated the formation of flower primordia and controlled the number of flowers produced by activating WUS and was negatively regulated by CLV3. 35S::AtERF19 expression resulted in significantly more flowers, whereas 35S::AtERF19 + SRDX dominant-negative mutants produced fewer flowers. In addition, AtERF19 also functioned to control flower organ size by promoting the division/expansion of the cells through activating Small Auxin Up RNA Gene 32 (SAUR32), which positively regulated MYB21/24 in the auxin signaling pathway. 35S::AtERF19 and 35S::SAUR32 resulted in similarly larger flowers, whereas 35S::AtERF19 + SRDX and 35S::SAUR32-RNAi mutants produced smaller flowers than the wild type. The functions of AtERF19 were confirmed by the production of similarly more and larger flowers in 35S::AtERF19 transgenic tobacco (Nicotiana benthamiana) and in transgenic Arabidopsis which ectopically expressed the orchid gene (Nicotiana benthamiana) PaERF19 than in wild-type plants. The finding that AtERF19 regulates genes involved in both CLV-WUS and auxin signaling during flower development significantly expands the current knowledge of the multifunctional evolution of ERF genes in plants. The results presented in this work indicate a dual role for the transcription factor AtERF19 in controlling the number of flowers produced and flower organ size through the regulation of genes involved in CLV-WUS and auxin signaling, respectively. Our findings expand the knowledge of the roles of ERF genes in the regulation of reproductive development.

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.

The RhLOL1-RhILR3 module mediates cytokinin-induced petal abscission in rose

In many plant species, petal abscission can be considered the final step of petal senescence. Cytokinins (CKs) are powerful suppressors of petal senescence; however, their role in petal abscission is ambiguous. Here, we observed that, in rose (Rosa hybrida), biologically active CK is accumulated during petal abscission and acts as an accelerator of the abscission process. Using a combination of reverse genetics, and molecular and biochemical techniques, we explored the roles of a LESION SIMULATING DISEASE1 (LSD1) family member RhLOL1 interacting with a bHLH transcription factor RhILR3 in CK-induced petal abscission. Silencing RhLOL1 delays rose petal abscission, while the overexpression of its ortholog SlLOL1 in tomato (Solanum lycopersicum) promotes pedicel abscission, indicating the conserved function of LOL1 in activating plant floral organ abscission. In addition, we identify a bHLH transcription factor, RhILR3, that interacts with RhLOL1. We show that RhILR3 binds to the promoters of the auxin signaling repressor auxin/indole-3-acetic acid (Aux/IAA) genes to inhibit their expression; however, the interaction of RhLOL1 with RhILR3 activates the expression of the Aux/IAA genes including RhIAA4-1. Silencing RhIAA4-1 delays rose petal abscission. Our results thus reveal a RhLOL1-RhILR3 regulatory module involved in CK-induced petal abscission via the regulation of the expression of the Aux/IAA genes.

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.

MSD2-mediated ROS metabolism fine-tunes the timing of floral organ abscission in Arabidopsis

The timely removal of end-of-purpose flowering organs is as essential for reproduction and plant survival as timely flowering. Despite much progress in understanding the molecular mechanisms of floral organ abscission, little is known about how various environmental factors are integrated into developmental programmes that determine the timing of abscission. Here, we investigated whether reactive oxygen species (ROS), mediators of various stress-related signalling pathways, are involved in determining the timing of abscission and, if so, how they are integrated with the developmental pathway in Arabidopsis thaliana. MSD2, encoding a secretory manganese superoxide dismutase, was preferentially expressed in the abscission zone of flowers, and floral organ abscission was accelerated by the accumulation of ROS in msd2 mutants. The expression of the genes encoding the receptor-like kinase HAESA (HAE) and its cognate peptide ligand INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), the key signalling components of abscission, was accelerated in msd2 mutants, suggesting that MSD2 acts upstream of IDA-HAE. Further transcriptome and pharmacological analyses revealed that abscisic acid and nitric oxide facilitate abscission by regulating the expression of IDA and HAE during MSD2-mediated signalling. These results suggest that MSD2-dependent ROS metabolism is an important regulatory point integrating environmental stimuli into the developmental programme leading to abscission.

EPIP as an abscission promoting agent in the phytohormonal pathway

Understanding the mechanisms underlying the activation of the abscission zone (AZ) responsible for organ separation from plant body in crop species will help improve their yielding and economic importance. Special attention has been given recently to the role of the INFLORESCENCE DEFICIENT IN ABSCISSION protein, particularly its functional fragment, EPIP peptide. Its stimulatory effect on abscission in different crops has been demonstrated. Recently we described the role of EPIP in the redox, lipid, and pectin-related events taking place in AZ of Lupinus luteus flowers, which undergo massive abscission in natural conditions. To further examine EPIP contribution in AZ functioning, here, we analyze its impact on the ultrastructural changes, synthesis of two hormonal abscission stimulators - abscisic acid (ABA) and ethylene (ET), and the appearance of phosphoproteins. As our results show, the response of flower AZ to exogenous EPIP involves the induction of distinct modifications related to the one hand with upregulation of cell activity but on the other hand degradation processes and possible autophagy. Furthermore, the EPIP stimulated biosynthesis pathways of ABA and ET precisely in AZ cells. In addition, progressive phosphorylation of proteins has been observed under EPIP influence. The highly accumulated ones were identified as those, related to primary metabolism and reactive oxygen species homeostasis, and their role in abscission has been discussed. To summarizing, the presented detailed description of EPIP action in AZ cells in combination with our previous data offers new insights into its regulatory function and provides opportunities to counteract excessive flower abscission in lupine.

The Oncidium Ethylene Synthesis Gene Oncidium 1-Aminocyclopropane-1 Carboxylic Acid Synthase 12 and Ethylene Receptor Gene Oncidium ETR1 Affect GA-DELLA and Jasmonic Acid Signaling in Regulating Flowering Time, Anther Dehiscence, and Flower Senescence in Arabidopsis

In plants, the key enzyme in ethylene biosynthesis is 1-aminocyclopropane-1 carboxylic acid (ACC) synthase (ACS), which catalyzes S-adenosyl-L-methionine (SAM) to ACC, the precursor of ethylene. Ethylene binds to its receptors, such as ethylene response 1 (ETR1), to switch on ethylene signal transduction. To understand the function of ACS and ETR1 in orchids, Oncidium ACC synthase 12 (OnACS12) and Oncidium ETR1 (OnETR1) from Oncidium Gower Ramsey were functionally analyzed in Arabidopsis. 35S::OnACS12 caused late flowering and anther indehiscence phenotypes due to its effect on GA-DELLA signaling pathways. 35S::OnACS12 repressed GA biosynthesis genes (CPS, KS, and GA3ox1), which caused the upregulation of DELLA [GA-INSENSITIVE (GAI), RGA-LIKE1 (RGL1), and RGL2] expression. The increase in DELLAs not only suppressed LEAFY (LFY) expression and caused late flowering but also repressed the jasmonic acid (JA) biosynthesis gene DAD1 and caused anther indehiscence by downregulating the endothecium-thickening-related genes MYB26, NST1, and NST2. The ectopic expression of an OnETR1 dominant-negative mutation (OnETR1-C65Y) caused both ethylene and JA insensitivity in Arabidopsis. 35S::OnETR1-C65Y delayed flower/leaf senescence by suppressing downstream genes in ethylene signaling, including EDF1-4 and ERF1, and in JA signaling, including MYC2 and WRKY33. JA signaling repression also resulted in indehiscent anthers via the downregulation of MYB26, NST1, NST2, and MYB85. These results not only provide new insight into the functions of ACS and ETR1 orthologs but also uncover their functional interactions with other hormone signaling pathways, such as GA-DELLA and JA, in plants.

Increasing yield on dry fields: molecular pathways with growing potential

Drought stress constitutes one of the major constraints to agriculture all over the world, and its devastating effect is only expected to increase in the following years due to climate change. Concurrently, the increasing food demand in a steadily growing population requires a proportional increase in yield and crop production. In the past, research aimed to increase plant resilience to severe drought stress. However, this often resulted in stunted growth and reduced yield under favorable conditions or moderate drought. Nowadays, drought tolerance research aims to maintain plant growth and yield under drought conditions. Overall, recently deployed strategies to engineer drought tolerance in the lab can be classified into a 'growth-centered' strategy, which focuses on keeping growth unaffected by the drought stress, and a 'drought resilience without growth penalty' strategy, in which the main aim is still to boost drought resilience, while limiting the side effects on plant growth. In this review, we put the scope on these two strategies and some molecular players that were successfully engineered to generate drought-tolerant plants: abscisic acid, brassinosteroids, cytokinins, ethylene, ROS scavenging genes, strigolactones, and aquaporins. We discuss how these pathways participate in growth and stress response regulation under drought. Finally, we present an overview of the current insights and future perspectives in the development of new strategies to improve drought tolerance in the field.

Xyloglucan endotransglucosylase/hydrolase genes LcXTH4/7/19 are involved in fruitlet abscission and are activated by LcEIL2/3 in litchi

Organ abscission in plants requires the hydrolysis of cell wall components, mainly including celluloses, pectins, and xyloglucans. However, how the genes that encode those hydrolytic enzymes are regulated and their function in abscission remains unclear. Previously we revealed that two cellulase genes LcCEL2/8 and two polygalacturonase genes LcPG1/2 were responsible for the degradation of celluloses and pectins, respectively, during fruitlet abscission in litchi. Here, we further identified three xyloglucan endotransglucosylase/hydrolase genes (LcXTH4, LcXTH7, LcXTH19) that are also involved in this process. Nineteen LcXTHs, named LcXTH1-19, were identified in the litchi genome. Transcriptome data and qRT-PCR confirmed that LcXTH4/7/19 were significantly induced at the abscission zone (AZ) during fruitlet abscission in litchi. The GUS reporter driven by each promoter of LcXTH4/7/19 was specifically expressed at the floral abscission zone of Arabidopsis, and importantly ectopic expression of LcXTH19 in Arabidopsis resulted in precocious floral organ abscission. Moreover, electrophoretic mobility shift assay (EMSA) and dual-luciferase reporter analysis showed that the expression of LcXTH4/7/19 could be directly activated by two ETHYLENE INSENSITIVE 3-like (EIL) transcription factors LcEIL2/3. Collectively, we propose that LcXTH4/7/19 are involved in fruitlet abscission, and LcEIL2/3-mediated transcriptional regulation of diverse cell wall hydrolytic genes is responsible for this process in litchi.

Auxin response and transport during induction of pedicel abscission in tomato

Auxin plays a central role in control of organ abscission, and it is thought that changes in the auxin gradient across the abscission zone are the primary determinant of the onset of abscission. The nature of this gradient, whether in concentration, flow, or perhaps in the response system has not conclusively been determined. We employed a DR5::GUS auxin response reporter system to examine the temporal and spatial distribution of the auxin response activity in response to developmental and environmental cues during pedicel abscission in tomato. In pedicels of young and fully open flowers, auxin response, as indicated by GUS activity, was predominantly detected in the vascular tissues and was almost entirely confined to the abscission zone (AZ) and to the distal portion of the pedicel, with a striking reduction in the proximal tissues below the AZ-a 'step', rather than a gradient. Following pollination and during early fruit development, auxin response increased substantially throughout the pedicel. Changes in GUS activity following treatments that caused pedicel abscission (flower removal, high temperature, darkness, ethylene, or N-1-naphthylphthalamic acid (NPA) treatment) were relatively minor, with reduced auxin response in the AZ and some reduction above and below it. Expression of genes encoding some auxin efflux carriers (PIN) and influx carriers (AUX⁄LAX) was substantially reduced in the abscission zone of NPA-treated pedicels, and in pedicels stimulated to abscise by flower removal. Our results suggest that changes in auxin flow distribution through the abscission zone are likely more important than the auxin response system in the regulation of abscission.

LcERF2 modulates cell wall metabolism by directly targeting a UDP-glucose-4-epimerase gene to regulate pedicel development and fruit abscission of litchi

Elucidating the biochemical and molecular basis of premature abscission in fruit crops should help develop strategies to enhance fruit set and yield. Here, we report that LcERF2 contributes to differential abscission rates and responses to ethylene in Litchi chinensis (litchi). Reduced LcERF2 expression in litchi was observed to reduce fruit abscission, concurrent with enhanced pedicel growth and increased levels of hexoses, particularly galactose, as well as pectin abundance in the cell wall. Ecoptic expression of LcERF2 in Arabidopsis thaliana caused enhanced petal abscission, together with retarded plant growth and reduced pedicel galactose and pectin contents. Transcriptome analysis indicated that LcERF2 modulates the expression of genes involved in cell wall modification. Yeast one-hybrid, dual-luciferase reporter and electrophoretic mobility shift assays all demonstrated that a UDP-glucose-4-epimerase gene (LcUGE) was the direct downstream target of LcERF2. This result was further supported by a significant reduction in the expression of the A. thaliana homolog AtUGE2-4 in response to LcERF2 overexpression. Significantly reduced pedicel diameter and enhanced litchi fruit abscission were observed in response to LcUGE silencing. We conclude that LcERF2 mediates fruit abscission by orchestrating cell wall metabolism, and thus pedicel growth, in part by repressing the expression of LcUGE.

Silencing of FOREVER YOUNG FLOWER-Like Genes from Phalaenopsis Orchids Promotes Flower Senescence and Abscission

Ectopic expression of FOREVER YOUNG FLOWER (FYF) delays floral senescence and abscission in transgenic Arabidopsis. To analyze the FYF function in Phalaenopsis orchids, two FYF-like genes (PaFYF1/2) were identified. PaFYF1/2 were highly expressed in young Phalaenopsis flowers, and their expression decreased significantly afterward until flower senescence. This pattern was strongly correlated with the process of flower senescence and revealed that PaFYF1/2 function to suppress senescence/abscission during early flower development. Interestingly, in flowers, PaFYF1 was consistently expressed less in petals than in lips/sepals, whereas PaFYF2 was expressed relatively evenly in all flower organs. This difference suggests a regulatory modification of the functions of PaFYF1 and PaFYF2 during Phalaenopsis flower evolution. Delayed flower senescence and abscission, which were unaffected by ethylene treatment, were observed in 35S::PaFYF1/2 and 35S::PaFYF1/2 + SRDX transgenic Arabidopsis plants due to the downregulation of the ethylene signaling and abscission-associated genes EDF1-4, IDA and BOP1/2. These results suggest a possible repressor role for Phalaenopsis PaFYF1/2 in controlling floral senescence/abscission by suppressing ethylene signaling and abscission-associated genes. To further validate the function of PaFYF1/2, PaFYF1/2-VIGS (virus-induced gene silencing) Phalaenopsis were generated and analyzed. Promotion of senescence and abscission was observed in PaFYF1/2-VIGS Phalaenopsis flowers by the upregulation of PeEDF1/2, PeSAG39 and PeBOP1/2 expression, the early occurrence of greening according to their increased chlorophyll content and the reduction in water content in flower organs. Our results support that PaFYF1/2 function as transcriptional repressors to prohibit flower senescence and abscission in Phalaenopsis.

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.

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.

Auxin Regulates Sucrose Transport to Repress Petal Abscission in Rose (Rosa hybrida)

Developmental transitions in plants require adequate carbon resources, and organ abscission often occurs due to competition for carbohydrates/assimilates. Physiological studies have indicated that organ abscission may be activated by Suc deprivation; however, an underlying regulatory mechanism that links Suc transport to organ shedding has yet to be identified. Here, we report that transport of Suc and the phytohormone auxin to petals through the phloem of the abscission zone (AZ) decreases during petal abscission in rose (Rosa hybrida), and that auxin regulates Suc transport into the petals. Expression of the Suc transporter RhSUC2 decreased in the AZ during rose petal abscission. Similarly, silencing of RhSUC2 reduced the Suc content in the petals and promotes petal abscission. We established that the auxin signaling protein RhARF7 binds to the promoter of RhSUC2, and that silencing of RhARF7 reduces petal Suc contents and promotes petal abscission. Overexpression of RhSUC2 in the petal AZ restored accelerated petal abscission caused by RhARF7 silencing. Moreover, treatment of rose petals with auxin and Suc delayed ethylene-induced abscission, whereas silencing of RhARF7 and RhSUC2 accelerated ethylene-induced petal abscission. Our results demonstrate that auxin modulates Suc transport during petal abscission, and that this process is regulated by a RhARF7-RhSUC2 module in the AZ.

Petal abscission in fragrant roses is associated with large scale differential regulation of the abscission zone transcriptome

Flowers of fragrant roses such as Rosa bourboniana are ethylene-sensitive and undergo rapid petal abscission while hybrid roses show reduced ethylene sensitivity and delayed abscission. To understand the molecular mechanism underlying these differences, a comparative transcriptome of petal abscission zones (AZ) of 0 h and 8 h ethylene-treated flowers from R. bourboniana was performed. Differential regulation of 3700 genes (1518 up, 2182 down) representing 8.5% of the AZ transcriptome was observed between 0 and 8 h ethylene-treated R. bourboniana petal AZ. Abscission was associated with large scale up-regulation of the ethylene pathway but prominent suppression of the JA, auxin and light-regulated pathways. Regulatory genes encoding kinases/phosphatases/F-box proteins and transcription factors formed the major group undergoing differential regulation besides genes for transporters, wall modification, defense and phenylpropanoid pathways. Further comparisons with ethylene-treated petals of R. bourboniana and 8 h ethylene-treated AZ (R. hybrida) identified a core set of 255 genes uniquely regulated by ethylene in R. bourboniana AZ. Almost 23% of these encoded regulatory proteins largely conserved with Arabidopsis AZ components. Most of these were up-regulated while an entire set of photosystem genes was prominently down-regulated. The studies provide important information on regulation of petal abscission in roses.

LcEIL2/3 are involved in fruitlet abscission via activating genes related to ethylene biosynthesis and cell wall remodeling in litchi

Fruit crops are subject to precocious fruit abscission, during which the phytohormone ethylene (ET) acts as a major positive regulator. However, the molecular basis of ET-induced fruit abscission remains poorly understood. Here, we show that two ETHYLENE INSENSITIVE 3-like (EIL) homologs in litchi, LcEIL2 and LcEIL3, play a role in ET-activated fruitlet abscission. LcEIL2/3 were significantly upregulated in the fruit abscission zone (AZ) during the ET-induced fruitlet abscission in litchi. The presence of LcEIL2/3 in wild-type Arabidopsis and ein3 eil1 mutants can accelerate the floral organ abscission. Moreover, the electrophoretic mobility shift assay and dual luciferase reporter analysis illustrated that LcEIL2/3 directly interacted with the gene promoters to activate the expression of cell wall remodeling genes LcCEL2/8 and LcPG1/2, and ET biosynthetic genes LcACS1/4/7 and LcACO2/3. Furthermore, we showed that LcPG1/2 were expressed in the floral abscission zone of Arabidopsis, and constitutive expression of LcPG2 in Arabidopsis promoted the floral organ abscission. In conclusion, we propose that LcEIL2/3 are involved in ET-induced fruitlet abscission via controlling expression of genes related to ET biosynthesis and cell wall remodeling in litchi.

The N1-Methyladenosine Methylome of Petunia mRNA

N1-methyladenosine is a unique type of base methylation in that it blocks Watson-Crick base pairing and introduces a positive charge. m1A is prevalent in yeast and mammalian mRNA and plays a functional role. However, little is known about the abundance, dynamics, and topology of this modification in plant mRNA. Dot blotting and liquid chromatography tandem mass spectrometry analyses revealed a dynamic pattern of m1A mRNA modification in various tissues and at different developmental stages in petunia (Petunia hybrida), a model system for plant growth and development. We performed transcriptome-wide profiling of m1A in petunia mRNA by m1A mRNA immunoprecipitation followed by a deep-sequencing approach (m1A-seq, using an m1A-specific antibody). m1A-seq analysis identified 4,993 m1A peaks in 3,231 genes expressed in petunia corollas; there were 251 m1A peaks in which A residues were partly replaced by thymine and/or reverse transcription stopped at an adenine site. m1A was enriched in coding sequences, with single peaks located immediately after start codons. Ethylene treatment upregulated 400 m1A peaks in 375 mRNAs and downregulated 603 m1A peaks in 530 mRNAs in petunia corollas; 975 m1A peaks in mRNA were only detected in corollas treated with air and 430 were only detected in corollas treated with ethylene. Silencing of petunia tRNA-specific methyltransferase 61A (PhTRMT61A) reduced the m1A level in mRNA in vivo and in vitro. In addition, PhTRMT61A silencing caused abnormal leaf development, and the PhTRMT61A protein was localized to the nucleus. Thus, m1A in mRNA is an important epitranscriptome marker and plays a role in plant growth and development.

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.

Comparative Proteomics Profiling Illuminates the Fruitlet Abscission Mechanism of Sweet Cherry as Induced by Embryo Abortion

Sweet cherry (Prunus avium L.) is a delicious nutrient-rich fruit widely cultivated in countries such as China, America, Chile, and Italy. However, the yield often drops severely due to the frequently-abnormal fruitlet abscission, and few studies on the metabolism during its ripening process at the proteomic level have been executed so far. To get a better understanding regarding the sweet cherry abscission mechanism, proteomic analysis between the abscising carpopodium and non-abscising carpopodium of sweet cherry was accomplished using a newly developed Liquid chromatography-mass spectrometry/mass spectrometry with Tandem Mass Tag (TMT-LC-MS/MS) methodology. The embryo viability experiments showed that the vigor of the abscission embryos was significantly lower than that of retention embryo. The activity of cell wall degrading enzymes in abscising carpopodium was significantly higher than that in non-abscising carpopodium. The anatomy results suggested that cells in the abscission zone were small and separated. In total, 6280 proteins were identified, among which 5681 were quantified. It has been observed that differentially accumulated proteins (DAPs) influenced several biological functions and various subcellular localizations. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that plenty of metabolic pathways were notably enriched, particularly those involved in phytohormone biosynthesis, cell wall metabolism, and cytoskeletal metabolism, including 1-aminocyclopropane-1-carboxylate oxidase proteins which promote ethylene synthesis, and proteins promoting cell wall degradation, such as endoglucanases, pectinase, and polygalacturonase. Differential expression of proteins concerning phytohormone biosynthesis might activate the shedding regulation signals. Up-regulation of several cell wall degradation-related proteins possibly regulated the shedding of plant organs. Variations of the phytohormone biosynthesis and cell wall degradation-related proteins were explored during the abscission process. Furthermore, changes in cytoskeleton-associated proteins might contribute to the abscission of carpopodium. The current work represented the first study using comparative proteomics between abscising carpopodium and non-abscising carpopodium. These results indicated that embryo abortion might lead to phytohormone synthesis disorder, which effected signal transduction pathways, and hereby controlled genes involved in cell wall degradation and then caused the abscission of fruitlet. Overall, our data may give an intrinsic explanation of the variations in metabolism during the abscission of carpopodium.

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.

Integrated Analysis of Small RNA, Transcriptome and Degradome Sequencing Provides New Insights into Floral Development and Abscission in Yellow Lupine (Lupinus luteus L.)

The floral development in an important legume crop yellow lupine (Lupinus luteus L., Taper cv.) is often affected by the abscission of flowers leading to significant economic losses. Small non-coding RNAs (sncRNAs), which have a proven effect on almost all developmental processes in other plants, might be of key players in a complex net of molecular interactions regulating flower development and abscission. This study represents the first comprehensive sncRNA identification and analysis of small RNA, transcriptome and degradome sequencing data in lupine flowers to elucidate their role in the regulation of lupine generative development. As shedding in lupine primarily concerns flowers formed at the upper part of the inflorescence, we analyzed samples from extreme parts of raceme separately and conducted an additional analysis of pedicels from abscising and non-abscising flowers where abscission zone forms. A total of 394 known and 28 novel miRNAs and 316 phased siRNAs were identified. In flowers at different stages of development 59 miRNAs displayed differential expression (DE) and 46 DE miRNAs were found while comparing the upper and lower flowers. Identified tasiR-ARFs were DE in developing flowers and were strongly expressed in flower pedicels. The DEmiR-targeted genes were preferentially enriched in the functional categories related to carbohydrate metabolism and plant hormone transduction pathways. This study not only contributes to the current understanding of how lupine flowers develop or undergo abscission but also holds potential for research aimed at crop improvement.

Rosa hybrida RhERF1 and RhERF4 mediate ethylene- and auxin-regulated petal abscission by influencing pectin degradation

The timing of plant organ abscission is modulated by the balance of two hormones, ethylene and auxin, while the mechanism of organ shedding depends on the loss of middle lamella pectin in the abscission zone (AZ). However, the mechanisms involved in sensing the balance of auxin and ethylene and that affect pectin degradation during abscission are not well understood. In this study, we identified two members of the APETALA2/ethylene-responsive factor (AP2/ERF) transcription factor family in rose (Rosa hybrida), RhERF1 and RhERF4 which play a role in petal abscission. The expression of RhERF1 and RhERF4 was influenced by ethylene and auxin, respectively. Reduced expression of RhERF1 or RhERF4 was observed to accelerate petal abscission. Global expression analysis and real-time PCR assays revealed that RhERF1 and RhERF4 modulate the expression of genes encoding pectin-metabolizing enzymes. A reduction in the abundance of pectin epitopes was detected in the AZs of RhERF1 and RhERF4-silenced plants by immunofluorescence microscopy analysis. In addition, RhERF1 and RhERF4 were shown to bind to the promoter of the pectin-metabolizing gene β-GALACTOSIDASE 1 (RhBGLA1), and reduced expression of RhBGLA1 delayed petal abscission. We conclude that during petal abscission, RhERF1 and RhERF4 integrate and coordinate ethylene and auxin signals to modulate pectin metabolism, in part by regulating the expression of RhBGLA1.

The Yes and No of the Ethylene Involvement in Abscission

Abscission has significant implications in agriculture and several efforts have been addressed by researchers to understand its regulatory steps in both model and crop species. Among the main players in abscission, ethylene has exhibited some fascinating features, in that it was shown to be involved at different stages of abscission induction and, in some cases, with interesting roles also within the abscising organ at the very early stages of the process. This review summarizes the current knowledge about the role of ethylene both at the level of the abscission zone and within the shedding organ, pointing out the missing pieces of the very complicated puzzle of the abscission process in the different species.

Transcriptional Regulation of Abscission Zones

Precise and timely regulation of organ separation from the parent plant (abscission) is consequential to improvement of crop productivity as it influences both the timing of harvest and fruit quality. Abscission is tightly associated with plant fitness as unwanted organs (petals, sepals, filaments) are shed after fertilization while seeds, fruits, and leaves are cast off as means of reproductive success or in response to abiotic/biotic stresses. Floral organ abscission in Arabidopsis has been a useful model to elucidate the molecular mechanisms that underlie the separation processes, and multiple abscission signals associated with the activation and downstream pathways have been uncovered. Concomitantly, large-scale analyses of omics studies in diverse abscission systems of various plants have added valuable insights into the abscission process. The results suggest that there are common molecular events linked to the biosynthesis of a new extracellular matrix as well as cell wall disassembly. Comparative analysis between Arabidopsis and soybean abscission systems has revealed shared and yet disparate regulatory modules that affect the separation processes. In this review, we discuss our current understanding of the transcriptional regulation of abscission in several different plants that has improved on the previously proposed four-phased model of organ separation.

Crosstalk between cytokinin and ethylene signaling pathways regulates leaf abscission in cotton in response to chemical defoliants

Abscission is a process that allows plants to shed tissues or organs via cell separation, and occurs throughout the life cycle. Removal of leaves through the use of chemical defoliants is very important for mechanical harvesting of cotton (Gossypium hirsutum). However, our knowledge of the molecular mechanisms of the defoliation response involved is limited. In this study, RNA-seq was conducted in order to profile the differentially expressed genes (DEGs) between cultivars X50 (sensitive to chemical defoliants) and X33 (relatively insensitive) at different time points after treatment with thidiazuron and ethephon (TE). A total of 2434 DEGs were identified between the two cultivars across the different time-points. Functional categories according to GO and KEGG analyses revealed that plant hormone signal transduction and zeatin biosynthesis were involved in the response to TE. Cytokinin oxidase/dehydrogenase (CKX) genes and ethylene-related genes were up-regulated following TE treatment, and were associated with increased level of ethylene, especially in cultivar X50. Down-regulation of GhCKX3 resulted in delayed defoliation and a reduced ethylene response. The results show that crosstalk between cytokinin and ethylene regulates cotton defoliation, and provide new insights into the molecular mechanisms underlying the mode of action of defoliants in cotton.

Re-evaluation of the ethylene-dependent and -independent pathways in the regulation of floral and organ abscission

Abscission is a developmental process with important implications for agricultural practices. Ethylene has long been considered as a key regulator of the abscission process. The existence of an ethylene-independent abscission pathway, controlled by the complex of INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) peptide and the HAESA (HAE) and HAESA-like2 (HSL2) kinases, has been proposed, based mainly on observations that organ abscission in ethylene-insensitive mutants was delayed but not inhibited. A recent review on plant organ abscission signaling highlighted the IDA-HAE-HSL2 components as the regulators of organ abscission, while the role of auxin and ethylene in this process was hardly addressed. After a careful analysis of the relevant abscission literature, we propose that the IDA-HAE-HSL2 pathway is essential for the final stages of organ abscission, while ethylene plays a major role in its initiation and progression. We discuss the view that the IDA-HAE-HSL2 pathway is ethylene independent, and present recent evidence showing that ethylene activates the IDA-HAE-HSL2 complex. We conclude that the ability of an organ to abscise is tightly linked to cell turgidity in the abscission zone, and suggest that lack of cell turgidity might contribute to the failure of floral organ abscission in the ida mutants.

Differential and reciprocal regulation of ethylene pathway genes regulates petal abscission in fragrant and non-fragrant roses

The fragrant rose, Rosa bourboniana, is highly sensitive to ethylene and shows rapid petal abscission (within 16-18 h) while the non-fragrant hybrid rose, R. hybrida, shows delayed abscission (50-52 h) due to reduced ethylene sensitivity. To understand the molecular basis governing these differences, all components of the ethylene pathway (biosynthesis/ receptor/signalling) were studied for expression during abscission. Transcript accumulation of most ethylene biosynthesis genes (ACS/ACO families) increased rapidly in petal abscission zones of R. bourboniana within 4-8 h of ethylene treatment. The expression of most receptor and signalling genes encoding CTRs, EIN2 and EIN3/EIL homologues also followed similar kinetics. Under natural field conditions where abscission takes longer, there was a temporal delay in transcript accumulation of most ethylene pathway genes while some biosynthesis genes (showing reduced ethylene sensitivity) were more strongly up-regulated by abscission cues. In contrast, in R. hybrida where even ethylene-induced abscission is considerably delayed, transcript accumulation of most ethylene biosynthesis and signalling genes was, surprisingly, reduced by ethylene and showed an opposite regulation compared to R. bourboniana. The results suggest that differential and reciprocal regulation of ethylene pathway is one of the major reasons for differences in petal abscission and vase-life between Rosa bourboniana and R. hybrida.

Quantitative Analysis of Floral Organ Abscission in Arabidopsis Via a Petal Breakstrength Assay

Petal breakstrength (pBS) is a method to study floral organ abscission by quantitating the force required to pull a petal from the receptacle. However, it is only well established in some labs and used in a subset of abscission studies. Here, we describe the mechanism and operation of the pBS meter, as well as detailed measurement and further data analysis. We show that it is a powerful tool to detect early or delayed floral organ abscission in mutant or transgenic plants, which is not easily detected by phenotypic investigation.

Gibberellic acid affects the functioning of the flower abscission zone in Lupinus luteus via cooperation with the ethylene precursor independently of abscisic acid

The abscission of plant organs is a phytohormone-controlled process. Our study provides new insight into the involvement of gibberellic acid (GA₃) in the functioning of the flower abscission zone (AZ) in yellow lupine (Lupinus luteus L.). Physiological studies demonstrated that GA₃ stimulated flower abortion. Additionally, this phytohormone was abundantly presented in the AZ cells of naturally abscised flowers, especially in vascular bundles. Interesting interactions among GA₃ and other modulators of flower separation were also investigated. GA₃ accumulated after treatment with the ethylene (ET) precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Abscisic acid (ABA) treatment did not cause such an effect. Furthermore, the expression of the newly identified LlGA20ox1 and LlGA2ox1 genes encoding 2-oxoglutarate-dependent dioxygenases fluctuated after ACC or ABA treatment which confirmed the existence of regulatory crosstalk. GA₃ appears to cooperate with the ET precursor in the regulation of AZ function in L. luteus flowers; however, the presented mechanism is ABA-independent.

Advances in abscission signaling

Abscission is a process in plants for shedding unwanted organs such as leaves, flowers, fruits, or floral organs. Shedding of leaves in the fall is the most visually obvious display of abscission in nature. The very shape plants take is forged by the processes of growth and abscission. Mankind manipulates abscission in modern agriculture to do things such as prevent pre-harvest fruit drop prior to mechanical harvesting in orchards. Abscission occurs specifically at abscission zones that are laid down as the organ that will one day abscise is developed. A sophisticated signaling network initiates abscission when it is time to shed the unwanted organ. In this article, we review recent advances in understanding the signaling mechanisms that activate abscission. Physiological advances and roles for hormones in abscission are also addressed. Finally, we discuss current avenues for basic abscission research and potentially lucrative future directions for its application to modern agriculture.

Visualizing Morphological Changes of Abscission Zone Cells in Arabidopsis by Scanning Electron Microscope

Scanning electron microscope (SEM) is a type of electron microscope which produces detailed images of surface structures. It has been widely used in plants and animals to study cellular structures. Here, we describe a detailed protocol to prepare samples of floral abscission zones (AZs) for SEM, as well as further image analysis. We show that it is a powerful tool to detect morphologic changes at the cellular level during the course of abscission in wild-type plants and to establish the details of phenotypic alteration in abscission mutants.

A Profusion of Molecular Scissors for Pectins: Classification, Expression, and Functions of Plant Polygalacturonases

In plants, the construction, differentiation, maturation, and degradation of the cell wall are essential for development. Pectins, which are major constituents of primary cell walls in eudicots, function in multiple developmental processes through their synthesis, modification, and degradation. Several pectin modifying enzymes regulate pectin degradation via different modes of action. Polygalacturonases (PGs), which function in the last step of pectin degradation, are a crucial class of pectin-modifying enzymes. Based on differences in their hydrolyzing activities, PGs can be divided into three main types: exo-PGs, endo-PGs, and rhamno-PGs. Their functions were initially investigated based on the expression patterns of PG genes and measurements of total PG activity in organs. In most plant species, PGs are encoded by a large, multigene family. However, due to the lack of genome sequencing data in early studies, the number of identified PG genes was initially limited. Little was initially known about the evolution and expression patterns of PG family members in different species. Furthermore, the functions of PGs in cell dynamics and developmental processes, as well as the regulatory pathways that govern these functions, are far from fully understood. In this review, we focus on how recent studies have begun to fill in these blanks. On the basis of identified PG family members in multiple species, we review their structural characteristics, classification, and molecular evolution in terms of plant phylogenetics. We also highlight the diverse expression patterns and biological functions of PGs during various developmental processes, as well as their mechanisms of action in cell dynamic processes. How PG functions are potentially regulated by hormones, transcription factors, environmental factors, pH and Ca²⁺ is discussed, indicating directions for future research into PG function and regulation.

Leaf shedding as an anti-bacterial defense in Arabidopsis cauline leaves

Plants utilize an innate immune system to protect themselves from disease. While many molecular components of plant innate immunity resemble the innate immunity of animals, plants also have evolved a number of truly unique defense mechanisms, particularly at the physiological level. Plant's flexible developmental program allows them the unique ability to simply produce new organs as needed, affording them the ability to replace damaged organs. Here we develop a system to study pathogen-triggered leaf abscission in Arabidopsis. Cauline leaves infected with the bacterial pathogen Pseudomonas syringae abscise as part of the defense mechanism. Pseudomonas syringae lacking a functional type III secretion system fail to elicit an abscission response, suggesting that the abscission response is a novel form of immunity triggered by effectors. HAESA/HAESA-like 2, INFLORESCENCE DEFICIENT IN ABSCISSION, and NEVERSHED are all required for pathogen-triggered abscission to occur. Additionally phytoalexin deficient 4, enhanced disease susceptibility 1, salicylic acid induction-deficient 2, and senescence-associated gene 101 plants with mutations in genes necessary for bacterial defense and salicylic acid signaling, and NahG transgenic plants with low levels of salicylic acid fail to abscise cauline leaves normally. Bacteria that physically contact abscission zones trigger a strong abscission response; however, long-distance signals are also sent from distal infected tissue to the abscission zone, alerting the abscission zone of looming danger. We propose a threshold model regulating cauline leaf defense where minor infections are handled by limiting bacterial growth, but when an infection is deemed out of control, cauline leaves are shed. Together with previous results, our findings suggest that salicylic acid may regulate both pathogen- and drought-triggered leaf abscission.

Ethylene Role in Plant Growth, Development and Senescence: Interaction with Other Phytohormones

The complex juvenile/maturity transition during a plant's life cycle includes growth, reproduction, and senescence of its fundamental organs: leaves, flowers, and fruits. Growth and senescence of leaves, flowers, and fruits involve several genetic networks where the phytohormone ethylene plays a key role, together with other hormones, integrating different signals and allowing the onset of conditions favorable for stage progression, reproductive success and organ longevity. Changes in ethylene level, its perception, and the hormonal crosstalk directly or indirectly regulate the lifespan of plants. The present review focused on ethylene's role in the development and senescence processes in leaves, flowers and fruits, paying special attention to the complex networks of ethylene crosstalk with other hormones. Moreover, aspects with limited information have been highlighted for future research, extending our understanding on the importance of ethylene during growth and senescence and boosting future research with the aim to improve the qualitative and quantitative traits of crops.

The Solanum lycopersicum auxin response factor SlARF2 participates in regulating lateral root formation and flower organ senescence

ARF2 as apleiotropic developmental regulator has been reported in Arabidopsis thaliana and tomato (Solanum lycopersicum). The present study showed SlARF2 transcripts in all tomato plant tissues but with higher accumulation in flowers. During bud-anthesis stages, SlARF2 transcripts showed a dynamic expression pattern in sepal, stamen, ovary and petal. Hormone treatment analysis suggested that SlARF2 transcript accumulation was positively regulated by auxin and gibberellic acid, and negatively regulated by ethylene in tomato seedlings. Phenotypes and molecular analyses of SlARF2-upregulated transgenic tomato indicated that SlARF2 regulated tomato lateral root formation and flower organ senescence may be partially mediated by regulating the gene expression of auxin and ethylene response factors. The data enlarges the functional characterization of SlARF2 in tomato, and broadens our understanding of auxin signaling in regulating plant growth and development.

Transcriptome profiling of developmental leaf senescence in sorghum (Sorghum bicolor)

This piece of the submission is being sent via mail. Leaf senescence is essential for the nutrient economy of crops and is executed by so-called senescence-associated genes (SAGs). Here we explored the monocot C₄ model crop Sorghum bicolor for a holistic picture of SAG profiles by RNA-seq. Leaf samples were collected at four stages during developmental senescence, and in total, 3396 SAGs were identified, predominantly enriched in GO categories of metabolic processes and catalytic activities. These genes were enriched in 13 KEGG pathways, wherein flavonoid and phenylpropanoid biosynthesis and phenylalanine metabolism were overrepresented. Seven regions on Chromosomes 1, 4, 5 and 7 contained SAG 'hotspots' of duplicated genes or members of cupin superfamily involved in manganese ion binding and nutrient reservoir activity. Forty-eight expression clusters were identified, and the candidate orthologues of the known important senescence transcription factors such as ORE1, EIN3 and WRKY53 showed "SAG" expression patterns, implicating their possible roles in regulating sorghum leaf senescence. Comparison of developmental senescence with salt- and dark- induced senescence allowed for the identification of 507 common SAGs, 1996 developmental specific SAGs as well as 176 potential markers for monitoring senescence in sorghum. Taken together, these data provide valuable resources for comparative genomics analyses of leaf senescence and potential targets for the manipulation of genetic improvement of Sorghum bicolor.

The Arabidopsis transcription factor ABIG1 relays ABA signaled growth inhibition and drought induced senescence

Drought inhibits plant growth and can also induce premature senescence. Here we identify a transcription factor, ABA INSENSITIVE GROWTH 1 (ABIG1) required for abscisic acid (ABA) mediated growth inhibition, but not for stomatal closure. ABIG1 mRNA levels are increased both in response to drought and in response to ABA treatment. When treated with ABA, abig1 mutants remain greener and produce more leaves than comparable wild-type plants. When challenged with drought, abig1 mutants have fewer yellow, senesced leaves than wild-type. Induction of ABIG1 transcription mimics ABA treatment and regulates a set of genes implicated in stress responses. We propose a model in which drought acts through ABA to increase ABIG1 transcription which in turn restricts new shoot growth and promotes leaf senescence. The results have implications for plant breeding: the existence of a mutant that is both ABA resistant and drought resistant points to new strategies for isolating drought resistant genetic varieties.

Spatial and temporal transcriptome changes occurring during flower opening and senescence of the ephemeral hibiscus flower, Hibiscus rosa-sinensis

Flowers are complex systems whose vegetative and sexual structures initiate and die in a synchronous manner. The rapidity of this process varies widely in flowers, with some lasting for months while others such as Hibiscus rosa-sinensis survive for only a day. The genetic regulation underlying these differences is unclear. To identify key genes and pathways that coordinate floral organ senescence of ephemeral flowers, we identified transcripts in H. rosa-sinensis floral organs by 454 sequencing. During development, 2053 transcripts increased and 2135 decreased significantly in abundance. The senescence of the flower was associated with increased abundance of many hydrolytic genes, including aspartic and cysteine proteases, vacuolar processing enzymes, and nucleases. Pathway analysis suggested that transcripts altering significantly in abundance were enriched in functions related to cell wall-, aquaporin-, light/circadian clock-, autophagy-, and calcium-related genes. Finding enrichment in light/circadian clock-related genes fits well with the observation that hibiscus floral development is highly synchronized with light and the hypothesis that ageing/senescence of the flower is orchestrated by a molecular clock. Further study of these genes will provide novel insight into how the molecular clock is able to regulate the timing of programmed cell death in tissues.