The molecular analysis of leaf senescence--a genomics approach

Unveiling the novel role of spermidine in leaf senescence: A study of eukaryotic translation factor 5A-independent and dependent mechanisms

Senescence is a crucial and highly active process in plants, optimising resource allocation and promoting phenotypic plasticity under restricted conditions. It involves global metabolic reprogramming for the organised disintegration and remobilization of resources. Polyamines (PAs) are polycationic biogenic amines prevalent in all eukaryotes and are necessary for cell survival. The commonly used PAs in plants include putrescine, spermidine, and spermine. Notably, the leaf's expression of S-adenosylmethionine decarboxylase and spermidine synthase gene family transcripts significantly changes during senescence. This suggests these genes are critical in spermidine metabolism and may condition metabolic reprogramming. One key role of spermidine in eukaryotes is to provide the 4-aminobutyl group for the posttranslational modification of lysine in eukaryotic translation factor 5A (eIF5A). This modification is catalysed by two sequential enzymatic steps leading to the activation of eIF5A by converting lysine to the unusual amino acid hypusine. Although eIF5A is well characterised to be involved in the translation of proline-rich repeat proteins and other hard-to-read motifs, the biological role of eIF5A has recently been clarified only in mammals. It could be better described at the plant functional level. The expression patterns of eIF5A isoforms and genes encoding machinery responsible for hypusination, differ between induced and developmental leaf senescence. In this paper, we summarise the existing knowledge on spermidine-dependent senescence control mechanisms in plants, raising the possibility that spermidine could be an element of a biological switch controlling the onset of a different type of senescence in an eIF5A-independent and dependent manner.

Ecophysiological and Molecular Analysis of Contrasting Genotypes for Leaf Senescence in Sunflower (Helianthus annuus L.) Under Differential Doses of N in Soil

Leaf senescence in plants is the last stage of leaf development and is characterized by a decline in photosynthetic activity, an active degeneration of cellular structures, and the recycling of accumulated nutrients to areas of active growth, such as buds, young leaves, flowers, fruits, and seeds. This process holds economic significance as it can impact yield, influencing the plant's ability to maintain an active photosynthetic system during prolonged periods, especially during the grain filling stage, which affects plant weight and oil content. It can be associated with different stresses or environmental conditions, manifesting itself widely in the context of climate change and limiting yield, especially in crops of agronomic relevance. In this work, we study the stability of two widely described sunflower (Helianthus annuus L.) genotypes belonging to the INTA Breeding Program against differential N conditions, to verify their yield stability in control conditions and under N supply. Two inbred lines were utilized, namely R453 (early senescence) and B481-6 (late senescence), with contrasting nitrogen availability in the soil but sharing the same ontogeny cycle length. It was observed that, starting from R5.5, the B481-6 genotype not only delayed senescence but also exhibited a positive response to increased nitrogen availability in the soil. This response included an increase in intercepted radiation, resulting in a statistically significant enhancement in grain yield. Conversely, the R453 genotype did not show significant differences under varying nitrogen availability and exhibited a tendency to decrease grain yield when nitrogen availability was increased. The response to nitrogen can vary depending on the specific genotype.

An osmesl mutant delayed rice leaf senescence through inhibiting cell death by OsBI-1

Leaf senescence following heading in rice is subject to rigorous regulation, with many of the underlying control mechanisms remaining largely unknown. In this study, we identified a novel gene, OsMESL, which exerts a positive regulatory effect on leaf senescence in rice. The T-DNA insertion mutant known as osmesl and RNA interference plants displayed a phenotype characterized by stay-green after heading. Genetic analysis indicated that the mutant phenotype could be rescued through complementation, while the overexpression of OsMESL accelerated leaf senescence after heading, underscoring OsMESL's positive regulatory role in rice leaf senescence. Subsequent investigations revealed that OsMESL modulates the process of cell death by influencing the stability of its interacting protein, the cell death suppressor OsBI-1, thereby governing leaf senescence. Furthermore, the leaves of the osmesl mutant exhibited a delayed reduction in photosynthesis, along with increased grain length and 1000-grain weight. In conclusion, we identified OsMESL as a novel positive regulator of leaf senescence in rice, which likely participates in leaf senescence through the mediation of cell death by OsBI-1, resulting in the phenotype of stay-green in the osmesl mutant after heading.

The genetically programmed rhythmic alteration of diurnal gene expression in the aged Arabidopsis leaves

The circadian clock regulates the daily pattern of temporal gene expression. In Arabidopsis, aging is associated with a shortening of the endogenous period of circadian rhythms under circadian conditions. However, the functional link between the circadian clock and aging under diurnal conditions and its physiological relevance remain elusive. In this study, we investigate and characterize the effect of aging on the waveforms of rhythmic gene expression patterns under light/dark cycles. Our analysis revealed that the diurnal rhythmic patterns of core clock genes undergo significant rhythmic alteration with phase shift and change of waveforms in aged plants compared to younger plants. Transcriptomic analysis indicated that this age-dependent rhythmic alteration occurs not only in core clock genes but also globally. Due to the rhythmic alteration patterns of the diurnal rhythmic gene expression, aged plants experience subjectively a shorter day and longer night. We also observed that genetic mutants of core clock component genes exhibited broadly yet distinctively altered changes in diurnal rhythmic gene expression patterns as aging progresses. Collectively, our findings support that age-dependent rhythmic alteration of diurnal gene expression rhythms reprograms the timetable of daily gene expression, leading to the physiological changes required for plant senescence.

Incomplete filling in the basal region of maize endosperm: timing of development of starch synthesis and cell vitality

Starch synthesis in maize endosperm adheres to the basipetal sequence from the apex downwards. However, the mechanism underlying nonuniformity among regions of the endosperm in starch accumulation and its significance is poorly understood. Here, we examined the spatiotemporal transcriptomes and starch accumulation dynamics in apical (AE), middle (ME), and basal (BE) regions of endosperm throughout the filling stage. Results demonstrated that the BE had lower levels of gene transcripts and enzymes facilitating starch synthesis, corresponding to incomplete starch storage at maturity, compared with AE and ME. Contrarily, the BE showed abundant gene expression for genetic processing and slow progress in physiological development (quantified by an index calculated from the expression values of development progress marker genes), revealing a sustained cell vitality of the BE. Further analysis demonstrated a significant parabolic correlation between starch synthesis and physiological development. An in-depth examination showed that the BE had more active signaling pathways of IAA and ABA than the AE throughout the filling stage, while ethylene showed the opposite pattern. Besides, SNF1-related protein kinase1 (SnRK1) activity, a regulator for starch synthesis modulated by trehalose-6-phosphate (T6P) signaling, was kept at a lower level in the BE than the AE and ME, corresponding to the distinct gene expression in the T6P pathway in starch synthesis regulation. Collectively, the findings support an improved understanding of the timing of starch synthesis and cell vitality in regions of the endosperm during development, and potential regulation from hormone signaling and T6P/SnRK1 signaling.

Casein kinase 1 AELs promote senescence by enhancing ethylene biosynthesis through phosphorylating WRKY22 transcription factor

Leaf senescence is a complex process regulated by developmental and environmental factors, and plays a pivotal role in the development and life cycle of higher plants. Casein kinase 1 (CK1) is a highly conserved serine/threonine protein kinase in eukaryotes and functions in various cellular processes including cell proliferation, light signaling and hormone effects of plants. However, the biological function of CK1 in plant senescence remains unclear. Through systemic genetic and biochemical studies, we here characterized the function of Arabidopsis EL1-like (AEL), a CK1, in promoting leaf senescence by stimulating ethylene biosynthesis through phosphorylating transcription factor WRKY22. Seedlings lacking or overexpressing AELs presented delayed or accelerated leaf senescence, respectively. AELs interact with and phosphorylate WRKY22 at Thr57, Thr60 and Ser69 residues to enhance whose transactivation activity. Being consistent, increased or suppressed phosphorylation of WRKY22 resulted in the promoted or delayed leaf senescence. WRKY22 directly binds to promoter region and stimulates the transcription of 1-amino-cyclopropane-1-carboxylate synthase 7 gene to promote ethylene level and hence leaf senescence. Our studies demonstrated the crucial role of AEL-mediated phosphorylation in regulating ethylene biosynthesis and promoting leaf senescence by enhancing WRKY22 transactivation activity, which helps to elucidate the fine-controlled ethylene biosynthesis and regulatory network of leaf senescence.

Unveiling the role of epigenetics in leaf senescence: a comparative study to identify different epigenetic regulations of senescence types in barley leaves

BACKGROUND

Developmental leaf senescence (DLS) is an irreversible process followed by cell death. Dark-induced leaf senescence (DILS) is a reversible process that allows adaptations to changing environmental conditions. As a result of exposure to adverse environmental changes, plants have developed mechanisms that enable them to survive. One of these is the redirection of metabolism into the senescence pathway. The plant seeks to optimise resource allocation. Our research aims to demonstrate how epigenetic machinery regulates leaf senescence, including its irreversibility.

RESULTS

In silico analyses allowed the complex identification and characterisation of 117 genes involved in epigenetic processes in barley. These genes include those responsible for DNA methylation, post-translational histone modifications, and ATP-dependent chromatin remodelling complexes. We then performed RNAseq analysis after DILS and DLS to evaluate their expression in senescence-dependent leaf metabolism. Principal component analysis revealed that evaluated gene expression in developmental senescence was similar to controls, while induced senescence displayed a distinct profile. Western blot experiments revealed that senescence engages senescence-specific histone modification. During DILS and DLS, the methylation of histone proteins H3K4me3 and H3K9me2 increased. H3K9ac acetylation levels significantly decreased during DILS and remained unchanged during DLS.

CONCLUSIONS

The study identified different epigenetic regulations of senescence types in barley leaves. These findings are valuable for exploring epigenetic regulation of senescence-related molecular mechanisms, particularly in response to premature, induced leaf senescence. Based on the results, we suggest the presence of an epigenetically regulated molecular switch between cell survival and cell death in DILS, highlighting an epigenetically driven cell survival metabolic response.

The AtMINPP Gene, Encoding a Multiple Inositol Polyphosphate Phosphatase, Coordinates a Novel Crosstalk between Phytic Acid Metabolism and Ethylene Signal Transduction in Leaf Senescence

Plant senescence is a highly coordinated process that is intricately regulated by numerous endogenous and environmental signals. The involvement of phytic acid in various cell signaling and plant processes has been recognized, but the specific roles of phytic acid metabolism in Arabidopsis leaf senescence remain unclear. Here, we demonstrate that in Arabidopsis thaliana the multiple inositol phosphate phosphatase (AtMINPP) gene, encoding an enzyme with phytase activity, plays a crucial role in regulating leaf senescence by coordinating the ethylene signal transduction pathway. Through overexpressing AtMINPP (AtMINPP-OE), we observed early leaf senescence and reduced chlorophyll contents. Conversely, a loss-of-function heterozygous mutant (atminpp/+) exhibited the opposite phenotype. Correspondingly, the expression of senescence-associated genes (SAGs) was significantly upregulated in AtMINPP-OE but markedly decreased in atminpp/+. Yeast one-hybrid and chromatin immunoprecipitation assays indicated that the EIN3 transcription factor directly binds to the promoter of AtMINPP. Genetic analysis further revealed that AtMINPP-OE could accelerate the senescence of ein3-1eil1-3 mutants. These findings elucidate the mechanism by which AtMINPP regulates ethylene-induced leaf senescence in Arabidopsis, providing insights into the genetic manipulation of leaf senescence and plant growth.

Delayed senescence and crop performance under stress: always a functional couple?

Exposure to abiotic stresses accelerates leaf senescence in most crop plant species, thereby reducing photosynthesis and other assimilatory processes. In some cases, genotypes with delayed leaf senescence (i.e. 'stay-green') show stress resistance, particularly in cases of water deficit, and this has led to the proposal that senescence delay improves crop performance under some abiotic stresses. In this review, we summarize the evidence for increased resistance to abiotic stress, mostly water deficit, in genotypes with delayed senescence, and specifically focus on the physiological mechanisms and agronomic conditions under which the stay-green trait may ameliorate grain yield under stress.

Genome-wide expression analysis of vegetative organs during developmental and herbicide-induced whole plant senescence in Arabidopsis thaliana

BACKGROUND

Whole plant senescence represents the final stage in the life cycle of annual plants, characterized by the decomposition of aging organs and transfer of nutrients to seeds, thereby ensuring the survival of next generation. However, the transcriptomic profile of vegetative organs during this death process remains to be fully elucidated, especially regarding the distinctions between natural programmed death and artificial sudden death induced by herbicide.

RESULTS

Differential genes expression analysis using RNA-seq in leaves and roots of Arabidopsis thaliana revealed that natural senescence commenced in leaves at 45-52 days after planting, followed by roots initiated at 52-60 days. Additionally, both organs exhibited similarities with artificially induced senescence by glyphosate. Transcription factors Rap2.6L and WKRY75 appeared to serve as central mediators of regulatory changes during natural senescence, as indicated by co-expression networks. Furthermore, the upregulation of RRTF1, exclusively observed during natural death, suggested its role as a regulator of jasmonic acid and reactive oxygen species (ROS) responses, potentially triggering nitrogen recycling in leaves, such as the glutamate dehydrogenase (GDH) shunt. Root senescence was characterized by the activation of AMT2;1 and GLN1;3, facilitating ammonium availability for root-to-shoot translocation, likely under the regulation of PDF2.1.

CONCLUSIONS

Our study offers valuable insights into the transcriptomic interplay between phytohormones and ROS during whole plant senescence. We observed distinct regulatory networks governing nitrogen utilization in leaf and root senescence processes. Furthermore, the efficient allocation of energy from vegetative organs to seeds emerges as a critical determinant of population sustainability of annual Arabidopsis.

Group C MAP kinases phosphorylate MBD10 to regulate ABA-induced leaf senescence in Arabidopsis

Abscisic acid (ABA) is a phytohormone that promotes leaf senescence in response to environmental stress. We previously identified methyl CpG-binding domain 10 (MBD10) as a phosphoprotein that becomes differentially phosphorylated after ABA treatment in Arabidopsis. ABA-induced leaf senescence was delayed in mbd10 knockout plants but accelerated in MBD10-overexpressing plants, suggesting that MBD10 positively regulates ABA-induced leaf senescence. ABA-induced phosphorylation of MBD10 occurs in planta on Thr-89, and our results demonstrated that Thr-89 phosphorylation is essential for MBD10's function in leaf senescence. The in vivo phosphorylation of Thr-89 in MBD10 was significantly downregulated in a quadruple mutant of group C MAPKs (mpk1/2/7/14), and group C MAPKs directly phosphorylated MBD10 in vitro. Furthermore, mpk1/2/7/14 showed a similar phenotype as seen in mbd10 for ABA-induced leaf senescence, suggesting that group C MAPKs are the cognate kinases of MBD10 for Thr-89. Because group C MAPKs have been reported to function downstream of SnRK2s, our results indicate that group C MAPKs and MBD10 constitute a regulatory pathway for ABA-induced leaf senescence.

PHR1 involved in the regulation of low phosphate-induced leaf senescence by modulating phosphorus homeostasis in Arabidopsis

Phosphorus (P) is a crucial macronutrient for plant growth, development, and reproduction. The effects of low P (LP) stress on leaf senescence and the role of PHR1 in LP-induced leaf senescence are still unknown. Here, we report that PHR1 plays a crucial role in LP-induced leaf senescence, showing delayed leaf senescence in phr1 mutant and accelerated leaf senescence in 35S:PHR1 transgenic Arabidopsis under LP stress. The transcriptional profiles indicate that 763 differentially expressed SAGs (DE-SAGs) were upregulated and 134 DE-SAGs were downregulated by LP stress. Of the 405 DE-SAGs regulated by PHR1, 27 DE-SAGs were involved in P metabolism and transport. PHR1 could bind to the promoters of six DE-SAGs (RNS1, PAP17, SAG113, NPC5, PLDζ2, and Pht1;5), and modulate them in LP-induced senescing leaves. The analysis of RNA content, phospholipase activity, acid phosphatase activity, total P and phosphate content also revealed that PHR1 promotes P liberation from senescing leaves and transport to young tissues under LP stress. Our results indicated that PHR1 is one of the crucial modulators for P recycling and redistribution under LP stress, and the drastic decline of P level is at least one of the causes of early senescence in P-deficient leaves.

Physico-Chemical Responses of Alstroemeria spp. cv. Rebecca to the presence of Salicylic Acid and Sucrose in vase solution during postharvest life

BACKGROUND

The primary challenge in the cut flower industry, specifically in the postharvest phase, is the short vase life of flowers. This issue, along with early leaf yellowing and perianth abscission, significantly diminishes the economic value of flowers due to their accelerated senescence. To tackle this, we conducted a factorial experiment on Alstroemeria cv. Rebecca, utilizing a completely randomized design with three replications. In this experiment the effects of varying concentrations of Salicylic acid (SA) (0, 1.5, and 3 mM) and sucrose (SU) (0% and 3%) were investigated on the postharvest quality of leaves and florets, with systematic evaluations every three days throughout their vase life.

RESULTS

This experiment revealed that the specific treatment combination of 1.5 mM SA + 3% SU (T5) markedly improved various parameters, such as vase life, total chlorophyll content, membrane stability index, relative fresh weight, and water uptake of cut flowers. In our analysis, we observed that this preservative solution not only extended the vase life and enhanced water uptake but also effectively preserved total chlorophyll, mitigated the loss of fresh weight, and reduced membrane deterioration in petals. Additionally, our results showed an increase in the activities of catalase (CAT) and peroxidase (POD) enzymes, as well as total protein content, alongside a decrease in malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels. Moreover, this study noted a decrease in microbial populations in solutions containing different concentrations of salicylic acid.

CONCLUSIONS

Our research demonstrated that alstroemeria flowers maintained in a solution with 1.5 mM SA + 3% SU exhibited a significantly prolonged vase life of up to 21 days, in contrast to the 15 days observed in control flowers kept in water. These results are highly beneficial for manufacturers in the cut flower industry, as they provide a viable method to substantially extend the vase life of cut flowers. Such an enhancement in flower longevity can lead to increased market value and customer satisfaction. Furthermore, the reduction in flower senescence and decay rates can contribute to decreased waste and greater efficiency in cut flower distribution and sales, offering a substantial advantage to manufacturers in this competitive market. The extended vase life and reduced senescence observed in alstroemeria flowers treated with 1.5 mM SA and 3% SU are attributed to SA's role in enhancing endogenous defense responses and sucrose's function as an energy source, collectively improving water uptake, and delaying the natural decay process.

Hydrogen Peroxide Promotes Tomato Leaf Senescence by Regulating Antioxidant System and Hydrogen Sulfide Metabolism

Hydrogen peroxide (H2O2) is relatively stable among ROS (reactive oxygen species) and could act as a signal in plant cells. In the present work, detached tomato leaves were treated with exogenous H2O2 at 10 mmol/L for 8 h to study the mechanism of how H2O2 regulates leaf senescence. The data indicated that H2O2 treatment significantly accelerated the degradation of chlorophyll and led to the upregulation of the expression of leaf senescence-related genes (NYC1, PAO, PPH, SGR1, SAG12 and SAG15) during leaf senescence. H2O2 treatment also induced the accumulation of H2O2 and malondialdehyde (MDA), decreased POD and SOD enzyme activities and inhibited H2S production by reducing the expression of LCD1/2 and DCD1/2. A correlation analysis indicated that H2O2 was significantly and negatively correlated with chlorophyll, the expression of leaf senescence-related genes, and LCD1/2 and DCD1/2. The principal component analysis (PCA) results show that H2S showed the highest load value followed by O2•-, H2O2, DCD1, SAG15, etc. Therefore, these findings provide a basis for studying the role of H2O2 in regulating detached tomato leaf senescence and demonstrated that H2O2 plays a positive role in the senescence of detached leaves by repressing antioxidant enzymes and H2S production.

Genetic architecture of source-sink-regulated senescence in maize

Source and sink interactions play a critical but mechanistically poorly understood role in the regulation of senescence. To disentangle the genetic and molecular mechanisms underlying source-sink-regulated senescence (SSRS), we performed a phenotypic, transcriptomic, and systems genetics analysis of senescence induced by the lack of a strong sink in maize (Zea mays). Comparative analysis of genotypes with contrasting SSRS phenotypes revealed that feedback inhibition of photosynthesis, a surge in reactive oxygen species, and the resulting endoplasmic reticulum (ER) stress were the earliest outcomes of weakened sink demand. Multienvironmental evaluation of a biparental population and a diversity panel identified 12 quantitative trait loci and 24 candidate genes, respectively, underlying SSRS. Combining the natural diversity and coexpression networks analyses identified 7 high-confidence candidate genes involved in proteolysis, photosynthesis, stress response, and protein folding. The role of a cathepsin B like protease 4 (ccp4), a candidate gene supported by systems genetic analysis, was validated by analysis of natural alleles in maize and heterologous analyses in Arabidopsis (Arabidopsis thaliana). Analysis of natural alleles suggested that a 700-bp polymorphic promoter region harboring multiple ABA-responsive elements is responsible for differential transcriptional regulation of ccp4 by ABA and the resulting variation in SSRS phenotype. We propose a model for SSRS wherein feedback inhibition of photosynthesis, ABA signaling, and oxidative stress converge to induce ER stress manifested as programed cell death and senescence. These findings provide a deeper understanding of signals emerging from loss of sink strength and offer opportunities to modify these signals to alter senescence program and enhance crop productivity.

BcNAC056 Interacts with BcWRKY1 to Regulate Leaf Senescence in Pak Choi

Senescence is the final stage of leaf development. For leafy vegetables such as pak choi, leaf senescence is adverse to yield due to the harvest period shortening. However, the regulatory mechanisms of leaf senescence are largely unknown in leafy vegetables. Here, we isolated and characterized a NAC gene, BcNAC056, in pak choi [Brassica campestris (syn. Brassica rapa) ssp. chinensis cv. 49caixin]. BcNAC056-GFP was located in the nucleus at the subcellular level, and BcNAC056 was responsive to leaf senescence and different hormones at the transcriptional level. Heterologous overexpression of BcNAC056 in Arabidopsis promoted leaf senescence, accompanied by the increased expression of senescence-associated genes (SAGs), whereas virus-induced gene silencing-based silencing in pak choi delayed leaf senescence. The following transcriptome analysis showed that heterologous overexpression of BcNAC056 enhanced some AtSAG transcripts in Arabidopsis. Electrophoretic mobility shift assay (EMSA) and dual-luciferase (LUC) reporter assay revealed that BcNAC056 activated SAG12 by directly binding to the promoter. In addition, with the LUC reporter and transient overexpression assays, we proposed that BcNAC056-BcWRKY1 interaction promoted the activation of BcSAG12. Taken together, our findings revealed a new regulatory mechanism of leaf senescence in pak choi.

Salicylic acid metabolism and signalling coordinate senescence initiation in aspen in nature

Deciduous trees exhibit a spectacular phenomenon of autumn senescence driven by the seasonality of their growth environment, yet there is no consensus which external or internal cues trigger it. Senescence starts at different times in European aspen (Populus tremula L.) genotypes grown in same location. By integrating omics studies, we demonstrate that aspen genotypes utilize similar transcriptional cascades and metabolic cues to initiate senescence, but at different times during autumn. The timing of autumn senescence initiation appeared to be controlled by two consecutive "switches"; 1) first the environmental variation induced the rewiring of the transcriptional network, stress signalling pathways and metabolic perturbations and 2) the start of senescence process was defined by the ability of the genotype to activate and sustain stress tolerance mechanisms mediated by salicylic acid. We propose that salicylic acid represses the onset of leaf senescence in stressful natural conditions, rather than promoting it as often observed in annual plants.

The RhWRKY33a-RhPLATZ9 regulatory module delays petal senescence by suppressing rapid reactive oxygen species accumulation in rose flowers

Redox homeostasis in plant cells is critical for maintaining normal growth and development because reactive oxygen species (ROS) can function as signaling molecules or toxic compounds. However, how plants fine-tune redox homeostasis during natural or stress-induced senescence remains unclear. Cut roses (Rosa hybrida), an economically important ornamental product worldwide, often undergo stress-induced precocious senescence at the post-harvest bud stage. Here, we identified RhPLATZ9, an age- and dehydration-induced PLATZ (plant AT-rich sequence and zinc-binding) protein, and determined that it functions as a transcriptional repressor in rose flowers during senescence. We also showed that RhWRKY33a regulates RhPLATZ9 expression during flower senescence. RhPLATZ9-silenced flowers and RhWRKY33a-silenced flowers showed accelerated senescence, with higher ROS contents than the control. By contrast, overexpression of RhWRKY33a or RhPLATZ9 delayed flower senescence, and overexpression in rose calli showed lower ROS accumulation than the control. RNA-sequencing analysis revealed that apoplastic NADPH oxidase genes (RhRbohs) were enriched among the upregulated differentially expressed genes in RhPLATZ9-silenced flowers compared to wild-type flowers. Yeast one-hybrid assays, electrophoretic mobility shift assays, dual luciferase assays and chromatin immunoprecipitation quantitative PCR confirmed that the RhRbohD gene is a direct target of RhPLATZ9. These findings suggest that the RhWRKY33a-RhPLATZ9-RhRbohD regulatory module acts as a brake to help maintain ROS homeostasis in petals and thus antagonize age- and stress-induced precocious senescence in rose flowers.

Ethylene-responsive SbWRKY50 suppresses leaf senescence by inhibition of chlorophyll degradation in sorghum

The onset of leaf de-greening and senescence is governed by a complex regulatory network including environmental cues and internal factors such as transcription factors (TFs) and phytohormones, in which ethylene (ET) is one key inducer. However, the detailed mechanism of ET signalling for senescence regulation is still largely unknown. Here, we found that the WRKY TF SbWRKY50 from Sorghum bicolor L., a direct target of the key component ETHYLENE INSENSITIVE 3 in ET signalling, functioned for leaf senescence repression. The clustered regularly interspaced short palindromic repeats/CRISPR-associated protein9-edited SbWRKY50 mutant (SbWRKY5O-KO) of sorghum displayed precocious senescent phenotypes, while SbWRKY50 overexpression delayed age-dependent and dark-induced senescence in sorghum. SbWRKY50 negatively regulated chlorophyll degradation through direct binding to the promoters of several chlorophyll catabolic genes. In addition, SbWRKY50 recruited the Polycomb repressive complex 1 through direct interaction with SbBMI1A, to induce histone 2A mono-ubiquitination accumulation on the chlorophyll catabolic genes for epigenetic silencing and thus delayed leaf senescence. Especially, SbWRKY50 can suppress early steps of chlorophyll catabolic pathway via directly repressing SbNYC1 (NON-YELLOW COLORING 1). Other senescence-related hormones could also influence leaf senescence through repression of SbWRKY50. Hence, our work shows that SbWRKY50 is an essential regulator downstream of ET and SbWRKY50 also responds to other phytohormones for senescence regulation in sorghum.

CmERF5-CmRAP2.3 transcriptional cascade positively regulates waterlogging tolerance in Chrysanthemum morifolium

Waterlogging stress affects plant growth by limiting root respiration and reducing yield and economic value. Therefore, identifying genes involved in regulating waterlogging stress is vital. This study reports the ethylene-responsive VII transcription factor (CmRAP2.3) in the chrysanthemum. Subcellular localization and transactivation assay analyses revealed that CmRAP2.3 was localized in the nucleus and possessed transactivation activity. Overexpression of CmRAP2.3 in chrysanthemum was found to enhance waterlogging tolerance by decreasing reactive oxygen species (ROS) levels. Furthermore, we found that the transcription factor CmERF5 binds to GCC-like motifs in the CmRAP2.3 promoter region and activates CmRAP2.3 expression. Additionally, CmERF5 overexpression maintained a low ROS level and improved chrysanthemum waterlogging tolerance. Taken together, this study shows a molecular mechanism by which CmERF5 transcriptionally activates CmRAP2.3 to reduce waterlogging stress via the ROS pathway in the chrysanthemum.

N6-methyladenosine (m6A) RNA modification as a metabolic switch between plant cell survival and death in leaf senescence

Crop losses caused by climate change and various (a)biotic stressors negatively affect agriculture and crop production. Therefore, it is vital to develop a proper understanding of the complex response(s) to (a)biotic stresses and delineate them for each crop plant as a means to enable translational research. In plants, the improvement of crop quality by m6A editing is believed to be a promising strategy. As a reaction to environmental changes, m6A modification showed a high degree of sensitivity and complexity. We investigated differences in gene medleys between dark-induced leaf senescence (DILS) and developmental leaf senescence in barley, including inter alia RNA modifications active in DILS. The identified upregulated genes in DILS include RNA methyltransferases of different RNA types, embracing enzymes modifying mRNA, tRNA, and rRNA. We have defined a decisive moment in the DILS model which determines the point of no return, but the mechanism of its control is yet to be uncovered. This indicates the possibility of an unknown additional switch between cell survival and cell death. Discoveries of m6A RNA modification changes in certain RNA species in different stages of leaf senescence may uncover the role of such modifications in metabolic reprogramming. Nonetheless, there is no such data about the process of leaf senescence in plants. In this scope, the prospect of finding connections between the process of senescence and m6A modification of RNA in plants seems to be compelling.

Dynamic landscape of long noncoding RNAs during leaf aging in Arabidopsis

Leaf senescence, the last stage of leaf development, is essential for whole-plant fitness as it marks the relocation of nutrients from senescing leaves to reproductive or other developing organs. Temporally coordinated physiological and functional changes along leaf aging are fine-tuned by a highly regulated genetic program involving multi-layered regulatory mechanisms. Long noncoding RNAs (lncRNAs) are newly emerging as hidden players in many biological processes; however, their contribution to leaf senescence has been largely unknown. Here, we performed comprehensive analyses of RNA-seq data representing all developmental stages of leaves to determine the genome-wide lncRNA landscape along leaf aging. A total of 771 lncRNAs, including 232 unannotated lncRNAs, were identified. Time-course analysis revealed 446 among 771 developmental age-related lncRNAs (AR-lncRNAs). Intriguingly, the expression of AR-lncRNAs was regulated more dynamically in senescing leaves than in growing leaves, revealing the relevant contribution of these lncRNAs to leaf senescence. Further analyses enabled us to infer the function of lncRNAs, based on their interacting miRNA or mRNA partners. We considered functionally diverse lncRNAs including antisense lncRNAs (which regulate overlapping protein-coding genes), competitive endogenous RNAs (ceRNAs; which regulate paired mRNAs using miRNAs as anchors), and mRNA-interacting lncRNAs (which affect the stability of mRNAs). Furthermore, we experimentally validated the senescence regulatory function of three novel AR-lncRNAs including one antisense lncRNA and two mRNA-interacting lncRNAs through molecular and phenotypic analyses. Our study provides a valuable resource of AR-lncRNAs and potential regulatory networks that link the function of coding mRNA and AR-lncRNAs. Together, our results reveal AR-lncRNAs as important elements in the leaf senescence process.

The Relationships among "STAY-GREEN" Trait, Post-Anthesis Assimilate Remobilization, and Grain Yield in Rice (Oryza sativa L.)

The mobilization and translocation of carbohydrates and mineral nutrients from vegetative plant parts to grains are pivotal for grain filling, often involving a whole plant senescence process. Loss of greenness is a hallmark of leaf senescence. However, the relationship between crop yield and senescence has been controversial for many years. Here, in this study, the overexpression and RNA interference lines of gene of OsNYC3 (Non-Yellow Coloring 3), a chlorophyll catabolism gene, were investigated. Furthermore, exogenous phytohormones were applied, and a treatment of alternate wetting and moderate drying (AWMD) was introduced to regulate the processes of leaf senescence. The results indicated that the delayed senescence of the "STAY-GREEN" trait of rice is undesirable for the process of grain filling, and it would cause a lower ratio of grain filling and lower grain weight of inferior grains, because of unused assimilates in the stems and leaves. Through the overexpression of OsNYC3, application of exogenous chemicals of abscisic acid (ABA), and water management of AWMD, leaf photosynthesis was less influenced, a high ratio of carbohydrate assimilates was partitioned to grains other than leaves and stems as labeled by 13C, grain filling was improved, especially for inferior spikelets, and activities of starch-synthesizing enzymes were enhanced. However, application of ethephon not only accelerated leaf senescence, but also caused seed abortion and grain weight reduction. Thus, plant senescence needs to be finely adjusted in order to make a contribution to crop productivity.

Dynamics of nitration during dark-induced leaf senescence in Arabidopsis reveals proteins modified by tryptophan nitration

Nitric oxide (NO) is a critical molecule that links plant development with stress responses. Herein, new insights into the role of NO metabolism during leaf senescence in Arabidopsis are presented. A gradual decrease in NO emission accompanied dark-induced leaf senescence (DILS), and a transient wave of peroxynitrite (ONOO-) formation was detected by day 3 of DILS. The boosted ONOO- did not promote tryptophan (Trp) nitration, while the pool of 6-nitroTrp-containing proteins was depleted as senescence progressed. Immunoprecipitation combined with mass spectrometry was used to identify 63 and 4 characteristic 6-nitroTrp-containing proteins in control and individually darkened leaves, respectively. The potential in vivo targets of Trp nitration were mainly related to protein biosynthesis and carbohydrate metabolism. In contrast, nitration of tyrosine-containing proteins was intensified 2-fold on day 3 of DILS. Also, nitrative modification of RNA and DNA increased significantly on days 3 and 7 of DILS, respectively. Taken together, ONOO- can be considered a novel pro-senescence regulator that fine-tunes the redox environment for selective bio-target nitration. Thus, DILS-triggered nitrative changes at RNA and protein levels promote developmental shifts during the plant's lifespan and temporal adjustment in plant metabolism under suboptimal environmental conditions.

Transcriptome analysis of mulberry (Morus alba L.) leaves to identify differentially expressed genes associated with post-harvest shelf-life elongation

Present study deals with molecular expression patterns responsible for post-harvest shelf-life extension of mulberry leaves. Quantitative profiling showed retention of primary metabolite and accumulation of stress markers in NS7 and CO7 respectively. The leaf mRNA profiles was sequenced using the Illumina platform to identify DEGs. A total of 3413 DEGs were identified between the treatments. Annotation with Arabidopsis database has identified 1022 DEGs unigenes. STRING generated protein-protein interaction, identified 1013 DEGs nodes with p < 1.0e-16. KEGG classifier has identified genes and their participating biological processes. MCODE and BiNGO detected sub-networking and ontological enrichment, respectively at p ≤ 0.05. Genes associated with chloroplast architecture, photosynthesis, detoxifying ROS and RCS, and innate-immune response were significantly up-regulated, responsible for extending shelf-life in NS7. Loss of storage sucrose, enhanced activity of senescence-related hormones, accumulation of xenobiotics, and development of osmotic stress inside tissue system was the probable reason for tissue deterioration in CO7. qPCR validation of DEGs was in good agreement with RNA sequencing results, indicating the reliability of the sequencing platform. Present outcome provides a molecular insight regarding involvement of genes in self-life extension, which might help the sericulture industry to overcome their pre-existing problems related to landless farmers and larval feeding during monsoon.

Melatonin mediates reactive oxygen species homeostasis via SlCV to regulate leaf senescence in tomato plants

Melatonin (MT) functions in removing reactive oxygen species (ROS) and delaying plant senescence, thereby acting as an antioxidant; however, the molecular mechanism underlying the specific action of MT is unclear. Herein, we used the mutant plants carrying the MT decomposition gene melatonin 3-hydroxylase (M3H) in tomato to elucidate the specific mechanism of action of MT. SlM3H-OE accelerated senescence by decreasing the content of endogenous MT in plants. SlM3H is a senescence-related gene that positively regulates aging. MT inhibited the expression of the senescence-related gene SlCV to scavenge ROS, induced stable chloroplast structure, and delayed leaf senescence. Simultaneously, MT weakened the interaction between SlCV and SlPsbO/SlCAT3, reduced ROS production in photosystem II, and promoted ROS elimination. In conclusion, MT regulates ROS homeostasis and delays leaf aging in tomato plants through SlCV expression modulation.

Know when and how to die: gaining insights into the molecular regulation of leaf senescence

Senescence is the ultimate phase in the life cycle of leaves which is crucial for recycling of nutrients to maintain plant fitness and reproductive success. The earliest visible manifestation of leaf senescence is their yellowing, which usually commences with the breakdown of chlorophyll. The degradation process involves a gradual and highly coordinated disassembly of macromolecules resulting in the accumulation of nutrients, which are subsequently mobilized from the senescing leaves to the developing organs. Leaf senescence progresses under overly tight genetic and molecular control involving a well-orchestrated and intricate network of regulators that coordinate spatio-temporally with the influence of both internal and external cues. Owing to the advancements in omics technologies, the availability of mutant resources, scalability of molecular analyses methodologies and the advanced capacity to integrate multidimensional data, our understanding of the genetic and molecular basis of leaf ageing has greatly expanded. The review provides a compilation of the multitier regulation of senescence process and the interrelation between the environment and the terminal phase of leaf development. The knowledge gained would benefit in devising the strategies for manipulation of leaf senescence process to improve crop quality and productivity.

MdNAC4 Interacts With MdAPRR2 to Regulate Nitrogen Deficiency-Induced Leaf Senescence in Apple (Malus domestica)

Nitrogen (N) is one of the important macronutrients in plants, and N deficiency induces leaf senescence. However, the molecular mechanism underlying how N deficiency affects leaf senescence is unclear. Here, we report an apple NAC TF, MdNAC4, that participates in N deficiency-induced leaf senescence. The senescence phenotype of apple leaves overexpressing MdNAC4 was enhanced after N deficiency. Consistently, the chlorophyll content of transgenic leaves was significantly lower than that in the WT control leaves, the expression of chlorophyll catabolism-related genes (MdNYC1, MdPAO, and MdSGR1) was significantly higher than that in the WT controls, and the expression of chlorophyll synthesis-related genes (MdHEMA, MdCHLI, and MdCHLM) was significantly lower than that in the WT control leaves. Furthermore, MdNAC4 was found to directly activate the transcription of the chlorophyll catabolism-related genes MdNYC1 and MdPAO. Additionally, MdNAC4 was proven to interact with MdAPRR2 proteins both in vitro and in vivo, and overexpression of MdAPRR2 seemed to delay N deficiency-induced leaf senescence. Correspondingly, the chlorophyll loss of MdAPRR2-overexpressing (MdAPRR2-OE) lines was significantly lower than in WT control plants. Although downregulated, the expression of the chlorophyll synthesis-related genes MdHEMA, MdCHLI, and MdCHLM in the transgenic plants was more than twice that in the WT control plants. Taken together, our results enrich the regulatory network of leaf senescence induced by N deficiency through the interaction between MdNAC4 and MdAPRR2.

Ethylene responsive factor34 mediates stress-induced leaf senescence by regulating salt stress-responsive genes

Leaf senescence proceeds with age but is modulated by various environmental stresses and hormones. Salt stress is one of the most well-known environmental stresses that accelerate leaf senescence. However, the molecular mechanisms that integrate salt stress signalling with leaf senescence programmes remain elusive. In this study, we characterised the role of ETHYLENE RESPONSIVE FACTOR34 (ERF34), an Arabidopsis APETALA2 (AP2)/ERF family transcription factor, in leaf senescence. ERF34 was differentially expressed under various leaf senescence-inducing conditions, and negatively regulated leaf senescence induced by age, dark, and salt stress. ERF34 also promoted salt stress tolerance at different stages of the plant life cycle such as seed germination and vegetative growth. Transcriptome analysis revealed that the overexpression of ERF34 increased the transcript levels of salt stress-responsive genes including COLD-REGULATED15A (COR15A), EARLY RESPONSIVE TO DEHYDRATION10 (ERD10), and RESPONSIVE TO DESICCATION29A (RD29A). Moreover, ERF34 directly bound to ERD10 and RD29A promoters and activated their expression. Our findings indicate that ERF34 plays a key role in the convergence of the salt stress response with the leaf senescence programmes, and is a potential candidate for crop improvement, particularly by enhancing salt stress tolerance.

Genetic mechanisms of aging in plants: What can we learn from them?

Plants hold all records in longevity. Their aging is a complex process. In the presented review, we analyzed published data on various aspects of plant aging with focus on any inferences that could shed a light on aging in animals and help to fight it in human. Plant aging can be caused by many factors, such as telomere depletion, genomic instability, loss of proteostasis, changes in intercellular interaction, desynchronosis, autophagy misregulation, epigenetic changes and others. Plants have developed a number of mechanisms to increase lifespan. Among these mechanisms are gene duplication ("genetic backup"), the active work of telomerases, abundance of meristematic cells, capacity of maintaining the meristems permanently active and continuous activity of phytohormones. Plant aging usually occurs throughout the whole perennial life, but could be also seasonal senescence. Study of causes for seasonal aging can also help to uncover the mechanisms of plant longevity. The influence of different factors such as microbiome communities, glycation, alternative oxidase activity, mitochondrial dysfunction on plant longevity was also reviewed. Adaptive mechanisms of long-lived plants are considered. Further comparative study of the mechanisms underlying longevity of plants is necessary. This will allow us to reach a potentially new level of understanding of the aging process of plants.

Function of Protein Kinases in Leaf Senescence of Plants

Leaf senescence is an evolutionarily acquired process and it is critical for plant fitness. During senescence, macromolecules and nutrients are disassembled and relocated to actively growing organs. Plant leaf senescence process can be triggered by developmental cues and environmental factors, proper regulation of this process is essential to improve crop yield. Protein kinases are enzymes that modify their substrates activities by changing the conformation, stability, and localization of those proteins, to play a crucial role in the leaf senescence process. Impressive progress has been made in understanding the role of different protein kinases in leaf senescence recently. This review focuses on the recent progresses in plant leaf senescence-related kinases. We summarize the current understanding of the function of kinases on senescence signal perception and transduction, to help us better understand how the orderly senescence degeneration process is regulated by kinases, and how the kinase functions in the intricate integration of environmental signals and leaf age information.

The Transcriptional Corepressor HOS15 Mediates Dark-Induced Leaf Senescence in Arabidopsis

Multiple endogenous and environmental signals regulate the intricate and highly complex processes driving leaf senescence in plants. A number of genes have been identified in a variety of plant species, including Arabidopsis, which influence leaf senescence. Previously, we have shown that HOS15 is a multifunctional protein that regulates several physiological processes, including plant growth and development under adverse environmental conditions. HOS15 has also been reported to form a chromatin remodeling complex with PWR and HDA9 and to regulate the chromatin structure of numerous genes. However, unlike PWR and HDA9, the involvement of HOS15 in leaf senescence is yet to be identified. Here, we report that HOS15, together with PWR and HDA9, promotes leaf senescence via transcriptional regulation of SAG12/29, senescence marker genes, and CAB1/RCBS1A, photosynthesis-related genes. The expression of ORE1, SAG12, and SAG29 was downregulated in hos15-2 plants, whereas the expression of photosynthesis-related genes, CAB1 and RCBS1A, was upregulated. HOS15 also promoted senescence through dark stress, as its mutation led to a much greener phenotype than that of the WT. Phenotypes of double and triple mutants of HOS15 with PWR and HDA9 produced phenotypes similar to those of a single hos15-2. In line with this observation, the expression levels of NPX1, APG9, and WRKY57 were significantly elevated in hos15-2 and hos15/pwr, hos15/hda9, and hos15/pwr/hda9 mutants compared to those in the WT. Surprisingly, the total H3 acetylation level decreased in age-dependent manner and under dark stress in WT; however, it remained the same in hos15-2 plants regardless of dark stress, suggesting that dark-induced deacetylation requires functional HOS15. More interestingly, the promoters of APG9, NPX1, and WRKY57 were hyperacetylated in hos15-2 plants compared to those in WT plants. Our data reveal that HOS15 acts as a positive regulator and works in the same repressor complex with PWR and HDA9 to promote leaf senescence through aging and dark stress by repressing NPX1, APG9, and WRKY57 acetylation.

What Worth the Garlic Peel

Plants have two types of reproduction: sexual, resulting in embryo production, and asexual, resulting in vegetative bodies commonly derived from stems and roots (e.g., bulb, tuber). Dead organs enclosing embryos (DOEEs, such as seed coat and pericarp) are emerging as central components of the dispersal unit acting to nurture the embryo and ensure its survival in the habitat. Here we wanted to investigate the properties of dead organs enclosing plant asexual reproductive bodies, focusing on the garlic (Allium sativum) bulb. We investigated the biochemical and biological properties of the outer peel enclosing the bulb and the inner peel enclosing the clove using various methodologies, including bioassays, proteomics, and metabolomics. The garlic peels differentially affected germination and post-germination growth, with the outer peel demonstrating a strong negative effect on seed germination of Sinapis alba and on post-germination growth of Brassica juncea. Proteome analysis showed that dead garlic peels possess 67 proteins, including chitinases and proteases, which retained their enzymatic activity. Among primary metabolites identified in garlic peels, the outer peel accumulated multiple sugars, including rhamnose, mannitol, sorbitol, and trehalose, as well as the modified amino acid 5-hydroxylysine, known as a major component of collagen, at a higher level compared to the clove and the inner peel. Growth of Escherichia coli and Staphylococcus aureus was promoted by garlic peel extracts but inhibited by clove extract. All extracts strongly inhibited spore germination of Fusarium oxysporum f.sp. melonis. Thus, the garlic peels not only provide physical protection to vegetative offspring but also appear to function as a refined arsenal of proteins and metabolites for enhancing growth and development, combating potential pathogens, and conferring tolerance to abiotic stresses.

Changes in Polar Metabolites Content during Natural and Methyl-Jasmonate-Promoted Senescence of Ginkgo biloba Leaves

The present study clarified changes in the contents of polar metabolites (amino acids, organic acids, saccharides, cyclitols, and phosphoric acid) in leaf senescence in Ginkgo biloba with or without the application of methyl jasmonate (JA-Me) in comparison with those in naturally senescent leaf blades and petioles. The contents of most amino acids and citric and malic acids were significantly higher in abaxially, and that of myo-inositol was lower in abaxially JA-Me-treated leaves than in adaxially JA-Me-treated and naturally senescent leaves. The levels of succinic and fumaric acids in leaves treated adaxially substantially high, but not in naturally senescent leaves. In contrast, sucrose, glucose, and fructose contents were much lower in leaf blades and petioles treated abaxially with JA-Me than those treated adaxially. The levels of these saccharides were also lower compared with those in naturally senescent leaves. Shikimic acid and quinic acid were present at high levels in leaf blades and petioles of G. biloba. In leaves naturally senescent, their levels were higher compared to green leaves. The shikimic acid content was also higher in the organs of naturally yellow leaves than in those treated with JA-Me. These results strongly suggest that JA-Me applied abaxially significantly enhanced processes of primary metabolism during senescence of G. biloba compared with those applied adaxially. The changes in polar metabolites in relation to natural senescence were also discussed.

Impact of Plasma-Activated Water Treatment on Quality and Shelf-Life of Fresh Spinach Leaves Evaluated by Comprehensive Metabolomic Analysis

Fresh baby spinach leaves are popular in salads and are sold as chilled and plastic-packed products. They are of high nutritional value but very perishable due to microbial contamination and enzymatic browning resulting from leaf senescence. Therefore, innovative food processing methods such as plasma-activated water (PAW) treatment are being explored regarding their applicability for ensuring food safety. PAW’s impact on food quality and shelf-life extension has, however, not been investigated extensively in vegetables so far. In the present study, a comprehensive metabolomic analysis was performed to determine possible changes in the metabolite contents of spinach leaves stored in a refrigerated state for eight days. Liquid chromatography high-resolution mass spectrometry, followed by stringent biostatistics, was used to compare the metabolomes in control, tap-water-rinsed or PAW-rinsed samples. No significant differences were discernible between the treatment groups at the beginning or end of the storage period. The observed loss of nutrients and activation of catabolic pathways were characteristic of a transition into the senescent state. Nonetheless, the presence of several polyphenolic antioxidants and γ-linolenic acid in the PAW-treated leaves indicated a significant increase in stress resistance and health-promoting antioxidant capacity in the sample. Furthermore, the enhancement of carbohydrate-related metabolisms indicated a delay in the senescence development. These findings demonstrated the potential of PAW to benefit food quality and the shelf-life of fresh spinach leaves.

Ethephon-Induced Ethylene Enhances Protein Degradation in Source Leaves, but Its High Endogenous Level Inhibits the Development of Regenerative Organs in Brassica napus

To investigate the regulatory role of ethylene in the source-sink relationship for nitrogen remobilization, short-term effects of treatment with different concentrations (0, 25, 50, and 75 ppm) of ethephon (2-chloroethylphosphonic acid, an ethylene inducing agent) for 10 days (EXP 1) and long-term effects at 20 days (Day 30) after treatment with 100 ppm for 10 days (EXP 2) on protein degradation and amino acid transport in foliar sprayed mature leaves of Brassica napus (cv. Mosa) were determined. In EXP 1, endogenous ethylene concentration gradually increased in response to the treated ethephon concentration, leading to the upregulation of senescence-associated gene 12 (SAG12) expression and downregulation of chlorophyll a/b-binding protein (CAB) expression. Further, the increase in ethylene concentration caused a reduction in protein, Rubisco, and amino acid contents in the mature leaves. However, the activity of protease and expression of amino acid transporter (AAP6), an amino acid transport gene, were not significantly affected or slightly suppressed between the treatments with 50 and 75 ppm. In EXP 2, the enhanced ethylene level reduced photosynthetic pigments, leading to an inhibition of flower development without any pod development. A significant increase in protease activity, confirmed using in-gel staining of protease, was also observed in the ethephon-treated mature leaves. Ethephon application enhanced the expression of four amino acid transporter genes (AAP1, AAP2, AAP4, and AAP6) and the phloem loading of amino acids. Significant correlations between ethylene level, induced by ethephon application, and the descriptive parameters of protein degradation and amino acid transport were revealed. These results indicated that an increase in ethylene upregulated nitrogen remobilization in the mature leaves (source), which was accompanied by an increase in proteolytic activity and amino acid transport, but had no benefit to pod (sink) development.

Long-term adaptation of Arabidopsis thaliana to far-red light

Vascular plants use carotenoids and chlorophylls a and b to harvest solar energy in the visible region (400-700 nm), but they make little use of the far-red (FR) light. Instead, some cyanobacteria have developed the ability to use FR light by redesigning their photosynthetic apparatus and synthesizing red-shifted chlorophylls. Implementing this strategy in plants is considered promising to increase crop yield. To prepare for this, a characterization of the FR light-induced changes in plants is necessary. Here, we explore the behaviour of Arabidopsis thaliana upon exposure to FR light by following the changes in morphology, physiology and composition of the photosynthetic complexes. We found that after FR-light treatment, the ratio between the photosystems and their antenna size drastically readjust in an attempt to rebalance the energy input to support electron transfer. Despite a large increase in PSBS accumulation, these adjustments result in strong photoinhibition when FR-adapted plants are exposed to light again. Crucially, FR light-induced changes in the photosynthetic membrane are not the result of senescence, but are a response to the excitation imbalance between the photosystems. This indicates that an increase in the FR absorption by the photosystems should be sufficient for boosting photosynthetic activity in FR light.

Chloroplast dismantling in leaf senescence

In photosynthetic plant cells, chloroplasts act as factories of metabolic intermediates that support plant growth. Chloroplast performance is highly influenced by environmental cues. Thus, these organelles have the additional function of sensing ever changing environmental conditions, thereby playing a key role in harmonizing the growth and development of different organs and in plant acclimation to the environment. Moreover, chloroplasts constitute an excellent source of metabolic intermediates that are remobilized to sink tissues during senescence so that chloroplast dismantling is a tightly regulated process that plays a key role in plant development. Stressful environmental conditions enhance the generation of reactive oxygen species (ROS) by chloroplasts, which may lead to oxidative stress causing damage to the organelle. These environmental conditions trigger mechanisms that allow the rapid dismantling of damaged chloroplasts, which is crucial to avoid deleterious effects of toxic by-products of the degradative process. In this review, we discuss the effect of redox homeostasis and ROS generation in the process of chloroplast dismantling. Furthermore, we summarize the structural and biochemical events, both intra- and extraplastid, that characterize the process of chloroplast dismantling in senescence and in response to environmental stresses.

Senescence-Associated Glycine max (Gm)NAC Genes: Integration of Natural and Stress-Induced Leaf Senescence

Leaf senescence is a genetically regulated developmental process that can be triggered by a variety of internal and external signals, including hormones and environmental stimuli. Among the senescence-associated genes controlling leaf senescence, the transcriptional factors (TFs) comprise a functional class that is highly active at the onset and during the progression of leaf senescence. The plant-specific NAC (NAM, ATAF, and CUC) TFs are essential for controlling leaf senescence. Several members of Arabidopsis AtNAC-SAGs are well characterized as players in elucidated regulatory networks. However, only a few soybean members of this class display well-known functions; knowledge about their regulatory circuits is still rudimentary. Here, we describe the expression profile of soybean GmNAC-SAGs upregulated by natural senescence and their functional correlation with putative AtNAC-SAGs orthologs. The mechanisms and the regulatory gene networks underlying GmNAC081- and GmNAC030-positive regulation in leaf senescence are discussed. Furthermore, new insights into the role of GmNAC065 as a negative senescence regulator are presented, demonstrating extraordinary functional conservation with the Arabidopsis counterpart. Finally, we describe a regulatory circuit which integrates a stress-induced cell death program with developmental leaf senescence via the NRP-NAC-VPE signaling module.

Age-Dependent Abiotic Stress Resilience in Plants

Developmental age is a strong determinant of stress responses in plants. Differential susceptibility to various environmental stresses is widely observed at both the organ and whole-plant level. While it is clear that age determines stress susceptibility, the causes, regulatory mechanisms, and functions are only now beginning to emerge. Compared with concepts on age-related biotic stress resilience, advancements in the abiotic stress field are relatively limited. In this review, we focus on current knowledge of ontogenic resistance to abiotic stresses, highlighting examples at the organ (leaf) and plant level, preceded by an overview of the relevant concepts in plant aging. We also discuss age-related abiotic stress resilience mechanisms, speculate on their functional relevance, and outline outstanding questions.

Comparative Transcriptome-Based Mining of Senescence-Related MADS, NAC, and WRKY Transcription Factors in the Rapid-Senescence Line DLS-91 of Brassica rapa

Leaf senescence is a developmental process induced by various molecular and environmental stimuli that may affect crop yield. The dark-induced leaf senescence-91 (DLS-91) plants displayed rapid leaf senescence, dramatically decreased chlorophyll contents, low photochemical efficiencies, and upregulation of the senescence-associated marker gene BrSAG12-1. To understand DLS molecular mechanism, we examined transcriptomic changes in DLS-91 and control line DLS-42 following 0, 1, and 4 days of dark treatment (DDT) stages. We identified 501, 446, and 456 DEGs, of which 16.7%, 17.2%, and 14.4% encoded TFs, in samples from the three stages. qRT-PCR validation of 16 genes, namely, 7 MADS, 6 NAC, and 3 WRKY, suggested that BrAGL8-1, BrAGL15-1, and BrWRKY70-1 contribute to the rapid leaf senescence of DLS-91 before (0 DDT) and after (1 and 4 DDT) dark treatment, whereas BrNAC046-2, BrNAC029-2/BrNAP, and BrNAC092-1/ORE1 TFs may regulate this process at a later stage (4 DDT). In-silico analysis of cis-acting regulatory elements of BrAGL8-1, BrAGL42-1, BrNAC029-2, BrNAC092-1, and BrWRKY70-3 of B. rapa provides insight into the regulation of these genes. Our study has uncovered several AGL-MADS, WRKY, and NAC TFs potentially worthy of further study to understand the underlying mechanism of rapid DLS in DLS-91.

Root system architecture, physiological and transcriptional traits of soybean (Glycine max L.) in response to water deficit: A review

Drought stress is the main limiting factor for global soybean growth and production. Genetic improvement for water and nutrient uptake efficiency is critical to advance tolerance and enable more sustainable and resilient production, underpinning yield growth. The identification of quantitative traits and genes related to water and nutrient uptake will enhance our understanding of the mechanisms of drought tolerance in soybean. This review summarizes drought stress in the context of the physiological traits that enable effective acclimation, with a particular focus on roots. Genes controlling root system architecture play an important role in water and nutrient availability, and therefore important targets for breeding strategies to improve drought tolerance. This review highlights the candidate genes that have been identified as regulators of important root traits and responses to water stress. Progress in our understanding of the function of particular genes, including GmACX1, GmMS and GmPEPCK are discussed in the context of developing a system-based platform for genetic improvement of drought tolerance in soybean.

Contrasting roles of GmNAC065 and GmNAC085 in natural senescence, plant development, multiple stresses and cell death responses

NACs are plant-specific transcription factors involved in controlling plant development, stress responses, and senescence. As senescence-associated genes (SAGs), NACs integrate age- and stress-dependent pathways that converge to programmed cell death (PCD). In Arabidopsis, NAC-SAGs belong to well-characterized regulatory networks, poorly understood in soybean. Here, we interrogated the soybean genome and provided a comprehensive analysis of senescence-associated Glycine max (Gm) NACs. To functionally examine GmNAC-SAGs, we selected GmNAC065, a putative ortholog of Arabidopsis ANAC083/VNI2 SAG, and the cell death-promoting GmNAC085, an ANAC072 SAG putative ortholog, for analyses. Expression analysis of GmNAC065 and GmNAC085 in soybean demonstrated (i) these cell death-promoting GmNACs display contrasting expression changes during age- and stress-induced senescence; (ii) they are co-expressed with functionally different gene sets involved in stress and PCD, and (iii) are differentially induced by PCD inducers. Furthermore, we demonstrated GmNAC065 expression delays senescence in Arabidopsis, a phenotype associated with enhanced oxidative performance under multiple stresses, higher chlorophyll, carotenoid and sugar contents, and lower stress-induced PCD compared to wild-type. In contrast, GmNAC085 accelerated stress-induced senescence, causing enhanced chlorophyll loss, ROS accumulation and cell death, decreased antioxidative system expression and activity. Accordingly, GmNAC065 and GmNAC085 targeted functionally contrasting sets of downstream AtSAGs, further indicating that GmNAC85 and GmNAC065 regulators function inversely in developmental and environmental PCD.

Transcriptome comparison of different ploidy reveals the mechanism of photosynthetic efficiency superiority of triploid poplar

Triploid poplars have obvious growth advantages, especially in leaf development and photosynthetic characteristics, but the molecular mechanism has not been revealed yet. In order to better understand the regulation mechanisms of leaf and chlorophyll development in the triploid poplars, we combined the leaf phenotypic data with the transcriptomic data of the 5th, 10th, and 25th leaves from triploid and diploid poplars, using weighted gene co-expression network analysis (WGCNA), and revealed that PpnGRF5-1 had a strong correlation with leaf development and net photosynthetic rate (Pn). PpnGRF5-1 overexpression transgenic plants showed that the leaf area, Pn, and chlorophyll concentration were significantly increased. Transcriptomic data analysis of the third leaf from PpnGRF5-1 overexpression transgenic plants showed that PpnGRF5-1 could up-regulate the expression levels of chlorophyll synthesis genes and down-regulate the transcription of chlorophyll degradation enzymes. Overall, our studies have greatly expanded our understanding of the molecular mechanisms regulating triploid growth dominance.

PRAT Proteins Operate in Organellar Protein Import and Export in Arabidopsis thaliana

Chloroplasts need to import preproteins and amino acids from the cytosol during their light-induced differentiation. Similarly, chloroplasts have to export organic matter including proteins and amino acids during leaf senescence. Members of the PRAT (preprotein and amino acid transporter) family are candidate transporters for both processes. Here, we defined the role of two small PRAT gene families, At4g26670 and At5g55510 (HP20 subfamily) versus At3g49560 and At5g24650 (HP30 subfamily) during greening of etiolated plants and during leaf senescence. Using a combination of reverse genetics, protein biochemistry and physiological tools, evidence was obtained for a role of chloroplast HP20, HP30 and HP30-2 in protein, but not amino acid, import into chloroplasts. HP20, HP30 and HP30-2 form larger complexes involved in the uptake of transit sequence-less cytosolic precursors. In addition, we identified a fraction of HP30-2 in mitochondria where it served a similar function as found for chloroplasts and operated in the uptake of transit sequence-less cytosolic precursor proteins. By contrast, HP22 was found to act in the export of proteins from chloroplasts during leaf senescence, and thus its role is entirely different from that of its orthologue, HP20. HP22 is part of a unique protein complex in the envelope of senescing chloroplasts that comprises at least 11 proteins and contains with HP65b (At5g55220) a protein that is related to the bacterial trigger factor chaperone. An ortholog of HP65b exists in the cyanobacterium Synechocystis and has previously been implicated in protein secretion. Whereas plants depleted of either HP22 or HP65b or even both were increasingly delayed in leaf senescence and retained much longer stromal chloroplast constituents than wild-type plants, HP22 overexpressors showed premature leaf senescence that was associated with accelerated losses of stromal chloroplast proteins. Together, our results identify the PRAT protein family as a unique system for importing and exporting proteins from chloroplasts.

AtWAKL10, a Cell Wall Associated Receptor-Like Kinase, Negatively Regulates Leaf Senescence in Arabidopsis thaliana

Receptor-like kinases (RLKs) constitute a large group of cell surface receptors that play crucial roles in multiple biological processes. However, the function of most RLKs in plants has not been extensively explored, and much less for the class of cell wall associated kinases (WAKs) and WAK-like kinases (WAKLs). In this study, analyses of developmental expression patterns uncovered a putative role of AtWAKL10 in modulating leaf senescence, which was further investigated at physiological and molecular levels. The expression level of AtWAKL10 increased with the developmental progression and was rapidly upregulated in senescing leaf tissues. The promoter of AtWAKL10 contains various defense and hormone responsive elements, and its expression could be significantly induced by exogenous ABA, JA and SA. Moreover, the loss-of-function atwakl10 mutant showed earlier senescence along the course of natural development and accelerated leaf senescence under darkness and hormonal stresses, while plants overexpressing AtWAKL10 showed an opposite trend. Additionally, some defense and senescence related WRKY transcription factors could bind to the promoter of AtWAKL10. In addition, deletion and overexpression of AtWAKL10 caused several specific transcriptional alterations, including genes involved in cell extension, cell wall modification, defense response and senescence related WRKYs, which may be implicated in regulatory mechanisms adopted by AtWAKL10 in controlling leaf senescence. Taken together, these results revealed that AtWAKL10 negatively regulated leaf senescence.

Ethylene and salicylic acid synergistically accelerate leaf senescence in Arabidopsis

The phytohormones ethylene and salicylic acid (SA) have long been known to promote senescence, but their interplay during this process remains elusive. Here we report the synergistic effects of ethylene and SA on promoting leaf senescence in Arabidopsis. EIN3, a key transcription factor of ethylene signaling, physically interacted with the core SA signaling regulator NPR1 in senescing leaves. EIN3 and NPR1 synergistically promoted the expression of the senescence-associated genes ORE1 and SAG29. The senescence phenotype was more delayed for the ein3eil1npr1 triple mutant than ein3eil1 or npr1 with ethylene or/and SA treatment. NPR1-promoted leaf senescence may depend on functional EIN3/EIL1.

Stem girdling affects the onset of autumn senescence in aspen in interaction with metabolic signals

Autumn senescence in aspen (Populus tremula) is precisely timed every year to relocate nutrients from leaves to storage organs before winter. Here we demonstrate how stem girdling, which leads to the accumulation of photosynthates in the crown, influences senescence. Girdling resulted in an early onset of senescence, but the chlorophyll degradation was slower and nitrogen more efficiently resorbed than during normal autumn senescence. Girdled stems accumulated or retained anthocyanins potentially providing photoprotection in senescing leaves. Girdling of one stem in a clonal stand sharing the same root stock did not affect senescence in the others, showing that the stems were autonomous in this respect. One girdled stem with unusually high chlorophyll and nitrogen contents maintained low carbon-to-nitrogen (C/N) ratio and did not show early senescence or depleted chlorophyll level unlike the other girdled stems suggesting that the responses depended on the genotype or its carbon and nitrogen status. Metabolite analysis highlighted that the tricarboxylic acid (TCA) cycle, salicylic acid pathway, and redox homeostasis are involved in the regulation of girdling-induced senescence. We propose that disrupted sink-source relation and C/N status can provide cues through the TCA cycle and phytohormone signaling to override the phenological control of autumn senescence in the girdled stems.

Dark-Induced Barley Leaf Senescence - A Crop System for Studying Senescence and Autophagy Mechanisms

This review synthesizes knowledge on dark-induced barley, attached, leaf senescence (DILS) as a model and discusses the possibility of using this crop system for studying senescence and autophagy mechanisms. It addresses the recent progress made in our understanding of DILS. The following aspects are discussed: the importance of chloroplasts as early targets of DILS, the role of Rubisco as the largest repository of recoverable nitrogen in leaves senescing in darkness, morphological changes of these leaves other than those described for chloroplasts and metabolic modifications associated with them, DILS versus developmental leaf senescence transcriptomic differences, and finally the observation that in DILS autophagy participates in the circulation of cell components and acts as a quality control mechanism during senescence. Despite the progression of macroautophagy, the symptoms of degradation can be reversed. In the review, the question also arises how plant cells regulate stress-induced senescence via autophagy and how the function of autophagy switches between cell survival and cell death.