The Delayed Leaf Senescence Mutants of Arabidopsis, ore1, ore3, and ore9 are Tolerant to Oxidative Stress

Aging later but faster: how StCDF1 regulates senescence in Solanum tuberosum

In potato, maturity is assessed by leaf senescence, which, in turn, affects yield and tuber quality traits. Previously, we showed that the CYCLING DOF FACTOR1 (StCDF1) locus controls leaf maturity in addition to the timing of tuberization. Here, we provide evidence that StCDF1 controls senescence onset separately from senescence progression and the total life cycle duration. We used molecular-biological approaches (DNA-Affinity Purification Sequencing) to identify a direct downstream target of StCDF1, named ORESARA1 (StORE1S02), which is a NAC transcription factor acting as a positive senescence regulator. By overexpressing StORE1S02 in the long life cycle genotype, early onset of senescence was shown, but the total life cycle remained long. At the same time, StORE1S02 knockdown lines have a delayed senescence onset. Furthermore, we show that StORE1 proteins play an indirect role in sugar transport from source to sink by regulating expression of SWEET sugar efflux transporters during leaf senescence. This study clarifies the important link between tuber formation and senescence and provides insight into the molecular regulatory network of potato leaf senescence onset. We propose a complex role of StCDF1 in the regulation of potato plant senescence.

Transcription factors NtNAC028 and NtNAC080 form heterodimers to regulate jasmonic acid biosynthesis during leaf senescence in Nicotiana tabacum

Plant senescence, as a highly integrated developmental stage, involves functional degeneration and nutrient redistribution. NAM/ATAF1/CUC (NAC) transcription factors orchestrate various senescence-related signals and mediate the fine-tuning underlying plant senescence. Previous data revealed that knockout of either NtNAC028 or NtNAC080 leads to delayed leaf senescence in tobacco (Nicotiana tabacum), which implies that NtNAC028 and NtNAC080 play respective roles in the regulation of leaf senescence, although they share 91.87% identity with each other. However, the mechanism underlying NtNAC028- and NtNAC080-regulated leaf senescence remains obscure. Here, we determined that NtNAC028 and NtNAC080 activate a putative jasmonic acid (JA) biosynthetic gene, NtLOX3, and enhance the JA level in vivo. We found that NtNAC028 and NtNAC080 interact with each other and themselves through their NA-terminal region. Remarkably, only the dimerization between NtNAC028 and NtNAC080 stimulated the transcriptional activation activity, but not the DNA binding activity of this heterodimer on NtLOX3. Metabolome analysis indicated that overexpression of either NtNAC028 or NtNAC080 augments both biosynthesis and degradation of nicotine in the senescent stages. Thus, we conclude that NtNAC028 cooperates with NtNAC080 and forms a heterodimer to enhance NtLOX3 expression and JA biosynthesis to trigger the onset of leaf senescence and impact secondary metabolism in tobacco.

Overexpression of SLIM1 transcription factor accelerates vegetative development in Arabidopsis thaliana

The transcription factor Sulfur Limitation 1 (SLIM1) belongs to the plant-specific Ethylene Insenstive3-Like transcription factor family and is known to coordinate gene expression in response to sulfur deficiency. However, the roles of SLIM1 in nutrient-sufficient conditions have not been characterized. Employing constitutive SLIM1 overexpression (35S::SLIM1) and CRISPR/Cas9 mutant plants (slim1-cr), we identified several distinct phenotypes in nutrient-sufficient conditions in Arabidopsis thaliana. Overexpression of SLIM1 results in plants with approximately twofold greater rosette area throughout vegetative development. 35S::SLIM1 plants also bolt earlier and exhibit earlier downregulation of photosynthesis-associated genes and earlier upregulation of senescence-associated genes than Col-0 and slim1-cr plants. This suggests that overexpression of SLIM1 accelerates development in A. thaliana. Genome-wide differential gene expression analysis relative to Col-0 at three time points with slim1-cr and two 35S::SLIM1 lines allowed us to identify 1,731 genes regulated directly or indirectly by SLIM1 in vivo.

OsNAC103, a NAC Transcription Factor, Positively Regulates Leaf Senescence and Plant Architecture in Rice

Leaf senescence, the last stage of leaf development, is essential for crop yield by promoting nutrition relocation from senescence leaves to new leaves and seeds. NAC (NAM/ATAF1/ATAF2/CUC2) proteins, one of the plant-specific transcription factors, widely distribute in plants and play important roles in plant growth and development. Here, we identified a new NAC member OsNAC103 and found that it plays critical roles in leaf senescence and plant architecture in rice. OsNAC103 mRNA levels were dramatically induced by leaf senescence as well as different phytohormones such as ABA, MeJA and ACC and abiotic stresses including dark, drought and high salinity. OsNAC103 acts as a transcription factor with nuclear localization signals at the N terminal and a transcriptional activation signal at the C terminal. Overexpression of OsNAC103 promoted leaf senescence while osnac103 mutants delayed leaf senescence under natural condition and dark-induced condition, meanwhile, senescence-associated genes (SAGs) were up-regulated in OsNAC103 overexpression (OsNAC103-OE) lines, indicating that OsNAC103 positively regulates leaf senescence in rice. Moreover, OsNAC103-OE lines exhibited loose plant architecture with larger tiller angles while tiller angles of osnac103 mutants decreased during the vegetative and reproductive growth stages due to the response of shoot gravitropism, suggesting that OsNAC103 can regulate the plant architecture in rice. Taken together, our results reveal that OsNAC103 plays crucial roles in the regulation of leaf senescence and plant architecture in rice.

Reactive oxygen species may be involved in the distinctive biological effects of different doses of 12C6+ ion beams on Arabidopsis

Introduction

Heavy ion beam is a novel approach for crop mutagenesis with the advantage of high energy transfer line density and low repair effect after injury, however, little investigation on the biological effect on plant was performed. 50 Gy irradiation significantly stimulated the growth of Arabidopsis seedlings, as indicated by an increase in root and biomass, while 200 Gy irradiation significantly inhibited the growth of seedlings, causing a visible decrease in plant growth.

Methods

The Arabidopsis seeds were irradiated by 12C6+. Monte Carlo simulations were used to calculate the damage to seeds and particle trajectories by ion implantation. The seed epidermis received SEM detection and changes in its organic composition were detected using FTIR. Evidence of ROS and antioxidant systems were analyzed. RNA-seq and qPCR were used to detect changes in seedling transcript levels.

Results and discussion

Monte Carlo simulations revealed that high-dose irradiation causes various damage. Evidence of ROS and antioxidant systems implies that the emergence of phenotypes in plant cells may be associated with oxidative stress. Transcriptomic analysis of the seedlings demonstrated that 170 DEGs were present in the 50 Gy and 200 Gy groups and GO enrichment indicated that they were mainly associated with stress resistance and cell wall homeostasis. Further GO enrichment of DEGs unique to 50 Gy and 200 Gy revealed 58 50Gy-exclusive DEGs were enriched in response to oxidative stress and jasmonic acid entries, while 435 200 Gy-exclusive DEGs were enriched in relation to oxidative stress, organic cyclic compounds, and salicylic acid. This investigation advances our insight into the biological effects of heavy ion irradiation and the underlying mechanisms.

CHLORIDE CHANNEL-b mediates vacuolar nitrate efflux to improve low nitrogen adaptation in Arabidopsis

The vacuole is an important organelle for nitrate storage, and the reuse of vacuolar nitrate under nitrate starvation helps plants adapt to low-nitrate environments. CHLORIDE CHANNEL-b (CLC-b) in the vacuolar membrane is a nitrate transporter; however, its regulation and effects on nitrate efflux have not been established. Here, we evaluated CLC-b expression and its effects on physiological parameters under low nitrate conditions. CLC-b expression increased significantly in the roots of wild-type Arabidopsis (Arabidopsis thaliana) Col-0 under nitrate starvation. Under low nitrate, clcb mutants showed reductions in chlorophyll content and xylem sap nitrate concentration, shoot/root nitrate ratios, shoot/root total N ratios, and biomass. CLC-b-overexpression yielded opposite phenotypes and increased nitrogen use efficiency. CLC-b mutants showed elevated chlorate tolerance and an increased proportion of vacuolar nitrate relative to the total protoplast nitrate content as compared to the wild type. Yeast 1-hybrid, EMSA, and chromatin immunoprecipitation (ChIP) experiments showed that HRS1 HOMOLOG2 (HHO2), the expression of which is downregulated under low nitrate, binds directly to the promoter of CLC-b. clcb/hho2 double mutants and HHO2-overexpressing clcb plants had similar phenotypes under low nitrate to those of clcb single mutants. Thus, CLC-b mediates vacuolar nitrate efflux and is negatively regulated by HHO2, providing a theoretical basis for improving plant adaptability to low nitrate.

Arabidopsis EIN2 represses ABA responses during germination and early seedling growth by inactivating HLS1 protein independently of the canonical ethylene pathway

The signaling pathways for the phytohormones ethylene and abscisic acid (ABA) have antagonistic effects on seed germination and early seedling establishment. However, the underlying molecular mechanisms remain unclear. In Arabidopsis thaliana, ETHYLENE INSENSITIVE 2 (EIN2) localizes to the endoplasmic reticulum (ER); although its biochemical function is unknown, it connects the ethylene signal with the key transcription factors EIN3 and EIN3-LIKE 1 (EIL1), leading to the transcriptional activation of ethylene-responsive genes. In this study, we uncovered an EIN3/EIL1-independent role for EIN2 in regulating the ABA response. Epistasis analysis demonstrated that this distinct role of EIN2 in the ABA response depends on HOOKLESS 1 (HLS1), the putative histone acetyltransferase acting as a positive regulator of ABA responses. Protein interaction assays supported a direct physical interaction between EIN2 and HLS1 in vitro and in vivo. Loss of EIN2 function resulted in an alteration of HLS1-mediated histone acetylation at the ABA-INSENSITIVE 3 (ABI3) and ABI5 loci, which promotes gene expression and the ABA response during seed germination and early seedling growth, indicating that the EIN2-HLS1 module contributes to ABA responses. Our study thus revealed that EIN2 modulates ABA responses by repressing HLS1 function, independently of the canonical ethylene pathway. These findings shed light on the intricate regulatory mechanisms underling the antagonistic interactions between ethylene and ABA signaling, with significant implications for our understanding of plant growth and development.

Role of EIN2-mediated ethylene signaling in regulating petal senescence, abscission, reproductive development, and hormonal crosstalk in tobacco

Ethylene plays a pivotal role in a wide range of developmental, physiological, and defense processes in plants. EIN2 (ETHYLENE INSENSITIVE2) is a key player in the ethylene signaling pathway. To characterize the role of EIN2 in processes, such as petal senescence, where it has been found to play important roles along with various other developmental and physiological processes, the tobacco (Nicotiana tabacum) ortholog of EIN2 (NtEIN2) was isolated and NtEIN2 silenced transgenic lines were generated using RNA interference (RNAi). Silencing of NtEIN2 compromised plant defense against pathogens. NtEIN2 silenced lines displayed significant delays in petal senescence, and pod maturation, and adversely affected pod and seed development. This study further dissected the petal senescence in ethylene insensitive lines, that displayed alteration in the pattern of petal senescence and floral organ abscission. Delay in petal senescence was possibly because of delayed aging processes within petal tissues. Possible crosstalk between EIN2 and AUXIN RESPONSE FACTOR 2 (ARF2) in regulating the petal senescence process was also investigated. Overall, these experiments indicated a crucial role for NtEIN2 in controlling diverse developmental and physiological processes, especially in petal senescence.

Transcription Factors-Regulated Leaf Senescence: Current Knowledge, Challenges and Approaches

Leaf senescence is a complex biological process regulated at multiple levels, including chromatin remodeling, transcription, post-transcription, translation, and post-translational modifications. Transcription factors (TFs) are crucial regulators of leaf senescence, with NAC and WRKY families being the most studied. This review summarizes the progress made in understanding the regulatory roles of these families in leaf senescence in Arabidopsis and various crops such as wheat, maize, sorghum, and rice. Additionally, we review the regulatory functions of other families, such as ERF, bHLH, bZIP, and MYB. Unraveling the mechanisms of leaf senescence regulated by TFs has the potential to improve crop yield and quality through molecular breeding. While significant progress has been made in leaf senescence research in recent years, our understanding of the molecular regulatory mechanisms underlying this process is still incomplete. This review also discusses the challenges and opportunities in leaf senescence research, with suggestions for possible strategies to address them.

Increasingly amplified stimulation mediated by TaNAC69-B is crucial for the leaf senescence in wheat

Leaf senescence involves massive multidimensional alterations, such as nutrient redistribution, and is closely related to crop yield and quality. No apical meristem, Arabidopsis transcription activation factor, and Cup-shaped cotyledon (NAC)-type transcription factors integrate various signals and modulate an enormous number of target genes to ensure the appropriate progression of leaf senescence. However, few leaf senescence-related NACs have been functionally characterized in wheat. Based on our previous RNA-sequencing (RNA-seq) data, we focused on a NAC family member, TaNAC69-B, which is increasingly expressed during leaf senescence in wheat. Overexpression of TaNAC69-B led to precocious leaf senescence in wheat and Arabidopsis, and affected several agricultural traits in transgenic wheat. Moreover, impaired expression of TaNAC69-B by virus-induced gene silencing retarded the leaf senescence in wheat. By RNA-seq and quantitative real-time polymerase chain reaction analysis, we confirmed that some abscisic acid (ABA) biosynthesis genes, including AAO3 and its ortholog in wheat, TraesCS2B02G270600 (TaAO3-B), were elevated by the overexpression of TaNAC69-B. Consistently, we observed more severe ABA-induced leaf senescence in TaNAC69-B-OE wheat and Arabidopsis plants. Furthermore, we determined that TaNAC69-B bound to the NAC binding site core (CGT) on the promoter regions of AAO3 and TaAO3-B. Moreover, we confirmed elevated ABA levels in TaNAC69-B-OE wheat lines. Although TaNAC69-B shares 39.83% identity (amino acid) with AtNAP, TaNAC69-B did not completely restore the delayed leaf senescence in the atnap mutant. Collectively, our results revealed a positive feedback loop, consisting of TaNAC69-B, ABA biosynthesis and leaf senescence, that is essential for the regulation of leaf senescence in wheat.

Involvement of microRNA164 in responses to heat stress in Arabidopsis

MicroRNAs (miRNAs) are considered to be integral parts of plant stress regulatory networks. Under long-term heat stress, miR164 is induced. Conversely, its targets are repressed. Transgenic overexpressors (164OE) and mutants of MIR164 (mir164) were used to study miR164's functions during heat responses. Target gene expression decreased in 164OE transgenic plants and increased in mir164a-4 and mir164b mutants. Under heat stress, the mir164 mutants presented heat-sensitive phenotypes, while 164OE transgenic plants showed better thermotolerance than wild-type (WT) plants. Overexpression of miR164 decreased heat-inhibition of hypocotyl lengths. Under heat stress, miR164 target genes modulated the expression of chlorophyll b reductase and chlorophyll catabolic genes, reducing the chlorophyll a/b ratio. More H₂O₂ accumulated in the mir164 mutants under heat stress, which may have caused oxidative damage. In addition, expression of HSPs was altered in the experimental plants compared to that of the WT. Overall, miR164 influenced target gene expression, altering development, chlorophyll a/b ratio, H₂O₂-caused damage, and HSPs expression under long-term heat stress. These phenomena, in turn, likely influence the thermotolerance of plants.

OsTZF1, a CCCH-tandem zinc finger protein gene, driven under own promoter produces no pleiotropic effects and confers salt and drought tolerance in rice

Different abiotic stresses induce OsTZF1, a tandem CCCH-type zinc finger domain gene, in rice. Here, we report that transgenic rice plants overexpressing OsTZF1 under own promoter (P_OsTZF1:OsTZF1-OX [for overexpression]) transferred to soil showed normal growth similar to vector control plants. The P_OsTZF1:OsTZF1-OX produced normal leaves without any lesion mimic phenotype and exhibited normal seed setting. The P_OsTZF1:OsTZF1-OX plants showed significantly increased tolerance to salt and drought stresses and enhanced post stress recovery. Microarray analysis revealed a total of 846 genes up-regulated and 360 genes down-regulated in P_OsTZF1:OsTZF1-OX salt-treated plants. Microarray analysis of P_OsTZF1:OsTZF1-OX plants showed the regulation of many abiotic stress tolerance genes. These results suggest that OsTZF1-OX under own promoter show abiotic stress tolerance and produces no pleiotropic effect on phenotype of transgenic rice plant.

Specific roles of strigolactones in plant physiology and remediation of heavy metals from contaminated soil

Strigolactones (SLs) have been implicated in various developmental processes of the plant, including the response against several abiotic stresses. It is well known as a class of endogenous phytohormones that regulates shoot branching, secondary growth and root morphology. This hormone facilitates plants in responding to nitrogen and phosphorus starvation by shaping the above and below ground structural design. SLs actively participate within regulatory networks of plant stress adaptation that are governed by phytohormones. Heavy metals (HMs) in soil are considered a serious environmental problem that causes various harmful effects on plants. SLs along with other plant hormones imply the role in plant architecture is far from being fully understood. Strategy to remove/remediation of HMs from the soil with the help of SLs has not been defined yet. Therefore, the present review aims to comprehensively provide an overview of SLs role in fine-tuning plant architectures, relation with other plant hormones under abiotic stress, and remediation of HMs contaminated soil using SLs.

Nutrient status regulates MED19a phase separation for ORESARA1-dependent senescence

The mediator complex is highly conserved in eukgaryotes and is integral for transcriptional responses. Mediator subunits associate with signal-responsive transcription factors (TF) to activate expression of specific signal-responsive genes. As the key TF of Arabidopsis thaliana senescence, ORESARA1 (ORE1) is required for nitrogen deficiency (-N) induced senescence; however, the mediator subunit that associates with ORE1 remains unknown. Here, we show that Arabidopsis MED19a associates with ORE1 to activate -N senescence-responsive genes. Disordered MED19a forms inducible nuclear condensates under -N that is regulated by decreasing MED19a lysine acetylation. MED19a carboxyl terminus (cMED19a) harbors a mixed-charged intrinsically disordered region (MC-IDR) required for ORE1 interaction and liquid-liquid phase separation (LLPS). Plant and human cMED19 are sufficient to form heterotypic condensates with ORE1. Human cMED19 MC-IDR, but not yeast cMED19 IDR, partially complements med19a suggesting functional conservation in evolutionarily distant eukaryotes. Phylogenetic analysis of eukaryotic cMED19 revealed that the MC-IDR could arise through convergent evolution. Our result of MED19 MC-IDR suggests that plant MED19 is regulated by phase separation during stress responses.

Transcriptome analysis reveals candidate genes involved in nitrogen deficiency stress in apples

Nitrogen is one of the macroelements required for plant growth and development and the identification of candidate genes involved in nitrogen deficiency stress is of great importance to the sustainable development of agriculture. Here, we found that the color of apple leaves changed from dark green to yellow-green, the malondialdehyde (MDA) content, soluble protein content, and proline content significantly increased, the chlorophyll content significantly decreased in response to nitrate deficiency stress. According to the physiological and biochemical changes of apple leaves during nitrate deficiency stress, nitrogen deficiency stress was divided into two stages: early nitrogen deficiency stage (ES) and late nitrogen deficiency stage (LS). Transcriptome sequencing was performed in these two stress stages. 5773 differential expression genes (DEGs) were identified in the early nitrogen deficiency stress stage and 6130 DEGs were identified in the late nitrogen deficiency stress stage. Functional analysis of these DEGs revealed that a large number of DEGs were enriched in 'porphyrin and chlorophyll metabolic' pathways, the 'photosynthesis' pathway, the 'photosynthesis-antenna protein' pathway, and the 'ABA', 'ETH', and 'JA' signal transduction pathways, and the metabolic networks of these pathways were constructed. In addition, overexpression of MdNAC4 weakened the tolerance of apple calli to nitrogen deficiency stress. Taken together, our results reveal possible pathways for apple adaptation to nitrogen deficiency stress and identify the function of MdNAC4, a key transcription factor regulating nitrogen deficiency stress, which enriches the molecular mechanism of apple adapting to a nitrogen deficiency environment.

The apple GARP family gene MdHHO3 regulates the nitrate response and leaf senescence

The regulation of plant gene expression by nitrate is a complex regulatory process. Here, we identified 90 GARP family genes in apples by genome-wide analysis. As a member of the GARP gene family, the expression of MdHHO3 (Malus domestica HYPERSENSITIVITY TO LOW PHOSPHATE-ELICITED PRIMARY ROOT SHORTENING1 HOMOLOG 3) is upregulated under N (nitrogen) supply. The results of DNA-binding site analysis and electrophoretic mobility shift assays (EMSA) showed that MdHHO3 binds to the motif-containing GAATC. Furthermore, MdHHO3 binds to its promoter sequence and inhibits its activity. In addition, the overexpression of MdHHO3 in apple calli resulted in less accumulation of nitrate in 35S:MdHHO3-GFP calli and downregulated the expression of the nitrate transport-related genes but upregulated the expression of the nitrate assimilation-related genes. Similarly, the expression of the nitrate transport-related genes was downregulated and the expression of the nitrate assimilation-related genes was upregulated in MdHHO3 overexpression Arabidopsis and tobacco plants. Interaction experiments showed that MdHHO3 could bind to the promoter MdNRT2.1 (NITRATE TRANSPORTER 2.1) and negatively regulate its expression. Moreover, the exposure of MdHHO3-overexpressing Arabidopsis and tobacco to nitrate deficiency resulted in an early senescence phenotype as compared to the WT plants. These results show that MdHHO3 can not only negatively regulate nitrate accumulation in response to nitrate but also promote early leaf senescence under nitrate deficiency. This information may be useful to further reveal the mechanism of the nitrate response and demonstrates that nitrate deficiency induces leaf senescence in apples.

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.

Stress, senescence, and specialized metabolites in bryophytes

Life on land exposes plants to varied abiotic and biotic environmental stresses. These environmental drivers contributed to a large expansion of metabolic capabilities during land plant evolution and species diversification. In this review we summarize knowledge on how the specialized metabolite pathways of bryophytes may contribute to stress tolerance capabilities. Bryophytes are the non-tracheophyte land plant group (comprising the hornworts, liverworts, and mosses) and rapidly diversified following the colonization of land. Mosses and liverworts have as wide a distribution as flowering plants with regard to available environments, able to grow in polar regions through to hot desert landscapes. Yet in contrast to flowering plants, for which the biosynthetic pathways, transcriptional regulation, and compound function of stress tolerance-related metabolite pathways have been extensively characterized, it is only recently that similar data have become available for bryophytes. The bryophyte data are compared with those available for angiosperms, including examining how the differing plant forms of bryophytes and angiosperms may influence specialized metabolite diversity and function. The involvement of stress-induced specialized metabolites in senescence and nutrient response pathways is also discussed.

CLE42 delays leaf senescence by antagonizing ethylene pathway in Arabidopsis

Leaf senescence is the final stage of leaf development and is influenced by numerous internal and environmental factors. CLE family peptides are plant-specific peptide hormones that regulate various developmental processes. However, the role of CLE in regulating Arabidopsis leaf senescence remains unclear. Here, we found that CLE42 is a negative regulator of leaf senescence by using a CRISPR/Cas9-produced CLE mutant collection. The cle42 mutant displayed earlier senescence phenotypes, while overexpression of CLE42 delayed age-dependent and dark-induced leaf senescence. Moreover, application of the synthesized 12-amino-acid peptide (CLE42p) also delayed leaf senescence under natural and dark conditions. CLE42 and CLE41/44 displayed functional redundancy in leaf senescence, and the cle41 cle42 cle44 triple mutant displayed more pronounced earlier senescence phenotypes than any single mutant. Analysis of differentially expressed genes obtained by RNA-Seq methodology revealed that the ethylene pathway was suppressed by overexpressing CLE42. Moreover, CLE42 suppressed ethylene biosynthesis and thus promoted the protein accumulation of EBF, which in turn decreased the function of EIN3. Accordingly, mutation of EIN3/EIL1 or overexpression of EBF1 suppressed the earlier senescence phenotypes of the cle42 mutant. Together, our results reveal that the CLE peptide hormone regulates leaf senescence by communicating with the ethylene pathway.

Over-Expression of Larch DAL1 Accelerates Life-Cycle Progression in Arabidopsis

Homologs of Larix kaempferiDEFICIENS-AGAMOUS-LIKE 1 (LaDAL1) promote flowering in Arabidopsis. However, their functional role in the whole life-cycle is limited. Here, we analyzed the phenotypes and transcriptomes of Arabidopsis plants over-expressing LaDAL1. With respect to the defined life-cycle stage of Arabidopsis based on the meristem state, the results showed that LaDAL1 promoted seed germination, bolting, flower initiation, and global proliferative arrest, indicating that LaDAL1 accelerates the meristem reactivation, the transitions of vegetative meristem to inflorescence and flower meristem, and meristem arrest. As a marker gene of meristem, TERMINAL FLOWER 1 was down-regulated after LaDAL1 over-expression. These results reveal that LaDAL1 accelerates the life-cycle progression in Arabidopsis by promoting the transition of meristem fate, providing more and novel functional information about the conifer age-related gene DAL1.

Maize ZmBES1/BZR1-3 and -9 Transcription Factors Negatively Regulate Drought Tolerance in Transgenic Arabidopsis

The BRI1-EMS suppressor 1 (BES1)/brassinazole-resistant 1(BZR1) transcription factors play crucial roles in plant growth, development, and stress response. However, little is known about the function of maize’s BES1/BZR1s. In this study, the ZmBES1/BZR1-3 and ZmBES1/BZR1-9 genes were cloned from maize’s inbred line, B73, and they were functionally evaluated by analyzing their expression pattern, subcellular localization, transcriptional activation activity, as well as their heterologous expression in Arabidopsis, respectively. The results of the qRT-PCR showed that the ZmBES1/BZR1-3 and ZmBES1/BZR1-9 genes were predominantly expressed in the root, and their expression was significantly down-regulated by drought stress. The ZmBES1/BZR1-3 and ZmBES1/BZR1-9 proteins localized in the nucleus but showed no transcriptional activation activity as a monomer. Subsequently, it was found that the heterologous expression of the ZmBES1/BZR1-3 and ZmBES1/BZR1-9 genes in Arabidopsis decreased drought tolerance, respectively. The transgenic lines showed a more serious wilting phenotype, shorter root length, lower fresh weight, and higher relative electrolyte leakage (REL) and malondialdehyde (MDA) content compared to the control under drought stress. The RNA-sequencing data showed that the 70.67% and 93.27% differentially expressed genes (DEGs) were significantly down-regulated in ZmBES1/BZR1-3 and ZmBES1/BZR1-9 transgenic Arabidopsis, respectively. The DEGs of ZmBES1/BZR1-3 gene’s expressing lines were mainly associated with oxidative stress response and amino acid metabolic process and enriched in phenylpropanoid biosynthesis and protein processing in the endoplasmic reticulum. But the DEGs of the ZmBES1/BZR1-9 gene’s expressing lines were predominantly annotated with water deprivation, extracellular stimuli, and jasmonic acid and enriched in phenylpropanoid biosynthesis and plant hormone signal transduction. Moreover, ZmBES1/BZR1-9 increased stomatal aperture in transgenic Arabidopsis under drought stress. This study indicates that ZmBES1/BZR1-3 and ZmBES1/BZR1-9 negatively regulate drought tolerance via different pathways in transgenic Arabidopsis, and it provides insights into the underlying the function of BES1/BZR1s in crops.

Carbon Source Reduction Postpones Autumn Leaf Senescence in a Widespread Deciduous Tree

The growing-season length of temperate and boreal trees has a strong effect on the global carbon cycle. Yet, a poor understanding of the drivers of phenological processes, such as autumn leaf senescence in deciduous trees, limits our capacity to estimate growing-season lengths under climate change. While temperature has been shown to be an important driver of autumn leaf senescence, carbon source-sink dynamics have been proposed as a mechanism that could help explain variation of this important process. According to the carbon sink limitation hypothesis, senescence is regulated by the interplay between plant carbon source and sink dynamics, so that senescence occurs later upon low carbon inputs (source) and earlier upon low carbon demand (sink). Here, we manipulated carbon source-sink dynamics in birch saplings (Betula pendula) to test the relevance of carbon sink limitation for autumn leaf senescence and photosynthetic decline in a widespread deciduous tree. Specifically, we conducted a gradient of leaf and bud removal treatments and monitored the effects on autumnal declines in net photosynthesis and the timing of leaf senescence. In line with the carbon sink limitation hypothesis, we observed that leaf removal tended to increase total leaf-level autumn photosynthesis and delayed the timing of senescence. Conversely, we did not observe an effect of bud removal on either photosynthesis or senescence, which was likely caused by the fact that our bud removal treatment did not considerably affect the plant carbon sink. While we cannot fully rule out that the observed effect of leaf removal was influenced by possible treatment-level differences in leaf age or soil resource availability, our results provide support for the hypothesis of carbon sink limitation as a driver of growing-season length and move the scientific field closer to narrowing the uncertainty in climate change predictions.

A mutation in Arabidopsis SAL1 alters its in vitro activity against IP3 and delays developmental leaf senescence in association with lower ROS levels

Our manuscript is the first to find a link between activity of SAL1/OLD101 against IP₃ and plant leaf senescence regulation and ROS levels assigning a potential biological role for IP₃. Leaf senescence is a genetically programmed process that limits the longevity of a leaf. We identified and analyzed the recessive Arabidopsis stay-green mutation onset of leaf death 101 (old101). Developmental leaf longevity is extended in old101 plants, which coincided with higher peroxidase activity and decreased H₂O₂ levels in young 10-day-old, but not 25-day-old plants. The old101 phenotype is caused by a point mutation in SAL1, which encodes a bifunctional enzyme with inositol polyphosphate-1-phosphatase and 3' (2'), 5'-bisphosphate nucleotidase activity. SAL1 activity is highly specific for its substrates 3-polyadenosine 5-phosphate (PAP) and inositol 1, 4, 5-trisphosphate (IP₃), where it removes the 1-phosphate group from the IP₃ second messenger. The in vitro activity of recombinant old101 protein against its substrate IP₃ was 2.5-fold lower than that of wild type SAL1 protein. However, the in vitro activity of recombinant old101 mutant protein against PAP remained the same as that of the wild type SAL1 protein. The results open the possibility that the activity of SAL1 against IP₃ may affect the redox balance of young seedlings and that this delays the onset of leaf senescence.

CLE14 functions as a "brake signal" to suppress age-dependent and stress-induced leaf senescence by promoting JUB1-mediated ROS scavenging in Arabidopsis

Leaf senescence is an important developmental process in the plant life cycle and has a significant impact on agriculture. When facing harsh environmental conditions, monocarpic plants often initiate early leaf senescence as an adaptive mechanism to ensure a complete life cycle. Upon initiation, the senescence process is fine-tuned through the coordination of both positive and negative regulators. Here, we report that the small secreted peptide CLAVATA3/ESR-RELATED 14 (CLE14) functions in the suppression of leaf senescence by regulating ROS homeostasis in Arabidopsis. Expression of the CLE14-encoding gene in leaves was significantly induced by age, high salinity, abscisic acid (ABA), salicylic acid, and jasmonic acid. CLE14 knockout plants displayed accelerated progression of both natural and salinity-induced leaf senescence, whereas increased CLE14 expression or treatments with synthetic CLE14 peptides delayed senescence. CLE14 peptide treatments also delayed ABA-induced senescence in detached leaves. Further analysis showed that overexpression of CLE14 led to reduced ROS levels in leaves, where higher expression of ROS scavenging genes was detected. Moreover, CLE14 signaling resulted in transcriptional activation of JUB1, a NAC family transcription factor previously identified as a negative regulator of senescence. Notably, the delay of leaf senescence, reduction in H_{2O2 level, and activation of ROS scavenging genes by} CLE14 peptides were dependent on JUB1. Collectively, these results suggest that the small peptide CLE14 serves as a novel "brake signal" to regulate age-dependent and stress-induced leaf senescence through JUB1-mediated ROS scavenging.

Primary metabolic processes as drivers of leaf ageing

Ageing in plants is a highly coordinated and complex process that starts with the birth of the plant or plant organ and ends with its death. A vivid manifestation of the final stage of leaf ageing is exemplified by the autumn colours of deciduous trees. Over the past decades, technological advances have allowed plant ageing to be studied on a systems biology level, by means of multi-omics approaches. Here, we review some of these studies and argue that these provide strong support for basic metabolic processes as drivers for ageing. In particular, core cellular processes that control the metabolism of chlorophyll, amino acids, sugars, DNA and reactive oxygen species correlate with leaf ageing. However, while multi-omics studies excel at identifying correlative processes and pathways, molecular genetic approaches can provide proof that such processes and pathways control ageing, by means of knock-out and ectopic expression of predicted regulatory genes. Therefore, we also review historic and current molecular evidence to directly test the hypotheses unveiled by the systems biology approaches. We found that the molecular genetic approaches, by and large, confirm the multi-omics-derived hypotheses with notable exceptions, where there is scant evidence that chlorophyll and DNA metabolism are important drivers of leaf ageing. We present a model that summarises the core cellular processes that drive leaf ageing and propose that developmental processes are tightly linked to primary metabolism to inevitably lead to ageing and death.

Comparative proteomic analysis of tomato (Solanum lycopersicum L.) shoots reveals crosstalk between strigolactone and auxin

As one of the main vegetable crops cultivated in the world, the tomato has advantages of high yield and economic benefits, and plays an important role in promoting farmers' income and social and economic growth. However, lateral branches during the growth process of tomato consume considerable nutrients and reduce the yield of tomato. Phytohormones such as strigolactone and auxin can inhibit the formation of lateral branches. However, the mechanism of their interaction is not particularly clear. To better understand the effects of exogenous strigolactone and auxin on tomato, proteome analyses of tomato shoots treated with exogenous GR24 and indole acetic acid were performed using an integrated approach involving tandem mass tag (TMT) labeling and liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). We identified 6685 proteins, of which 5822 contained quantitative information. Many differentially expressed proteins (DEPs) were found in different comparisons, including 415, 148, and 130 DEPs in GR24 vs mock, IAA vs mock, and GR24 + IAA vs mock comparisons, respectively. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that 'photosynthesis - antenna proteins' were significantly enriched in three treatments. Our data can help reveal the interaction between strigolactone and auxin in tomato seedlings.

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.

Luxury zinc supply acts as antiaging agent and enhances reproductive fitness in Arabidopsis thaliana

Developmental senescence in plants is an age dependent process affected by phytohormones, nutrient status, and environmental factors, while the antiaging effects of zinc are recognized in humans. This study explores the possible influence of a high, non-toxic Zn-supply (12 μM) on senescence and reproductive fitness in A. thaliana. Auxin-resistance mutant, axr1-12, and auxin overexpressing YUCCA6 mutant, yuc6-1D, and their corresponding background genotypes were grown until complete rosette senescence to quantify the fruit biomass and seed number. Gene expression of different antioxidant, auxin and senescence-associated markers were analyzed after the onset of senescence. All mutants showed delayed developmental senescence. Luxury Zn delayed senescence in wild type, but not in the mutant genotypes. Excluding axr1-12 mutants, which showed very low expression of the auxin gene marker INDOLE-3-ACETIC ACID INDUCIBLE 2 (IAA2), enhanced expression of the senescence markers SENESCENCE-ASSOCIATED GENE 12 (SAG12) and AUXIN RESPONSE FACTOR 2 (ARF2) coincided with decreased expression of IAA2. Delayed senescence and total number of seeds per plant were related to higher expression of the peroxisomal antioxidant enzymes Cu/Zn superoxide dismutase (SOD3) and catalase (CAT2). These results evidence that high Zn-induced delayed senescence and improved reproductive fitness in Arabidopsis are related to an auxin-independent mechanism that retains antioxidant activity.

Transcriptome Changes Reveal the Molecular Mechanisms of Humic Acid-Induced Salt Stress Tolerance in Arabidopsis

Humic acid (HA) is a principal component of humic substances, which make up the complex organic matter that broadly exists in soil environments. HA promotes plant development as well as stress tolerance, however the precise molecular mechanism for these is little known. Here we conducted transcriptome analysis to elucidate the molecular mechanisms by which HA enhances salt stress tolerance. Gene Ontology Enrichment Analysis pointed to the involvement of diverse abiotic stress-related genes encoding HEAT-SHOCK PROTEINs and redox proteins, which were up-regulated by HA regardless of salt stress. Genes related to biotic stress and secondary metabolic process were mainly down-regulated by HA. In addition, HA up-regulated genes encoding transcription factors (TFs) involved in plant development as well as abiotic stress tolerance, and down-regulated TF genes involved in secondary metabolic processes. Our transcriptome information provided here provides molecular evidences and improves our understanding of how HA confers tolerance to salinity stress in plants.

Silicon-induced postponement of leaf senescence is accompanied by modulation of antioxidative defense and ion homeostasis in mustard (Brassica juncea) seedlings exposed to salinity and drought stress

Soil salinity and drought stress (DS) are the massive problem for worldwide agriculture. Both stresses together become more toxic to the plant growth and development. Silicon (Si) being the second most abundant element in the earth's crust, exerts beneficial effects on plants under both stress and non-stress conditions. However, limited information is available to substantiate the beneficial role of Si in delaying the premature leaf senescence and imparting tolerance of mustard (Brassica juncea L.) plants to salinity and DS. Therefore, the present study aimed to explore the role of Si (source K₂SiO₃) in chlorophyll (Chl) biosynthesis, nutrients uptake, relative water content (RWC), proline (Pro) metabolism, antioxidant system and delaying of premature leaf senescence in mustard plants under sodium chloride (NaCl) and DS conditions. Results of this study show that exogenous Si (1.7 mM) significantly delayed the salt plus DS-induced premature leaf senescence. This was further accompanied by the enhanced nutrients accumulation and activity of chlorophyll metabolizing enzymes [δ-aminolevulinic acid (δ-ALA) dehydratase and porphobilinogen deaminase] and levels of δ-ALA, and Chls a and b and also by decreased the Chl degradation and Chl degrading enzymes (Chlorophyllase, Chl-degrading peroxidase, pheophytinase) activity. Exogenous Si treatment induced redox homoeostasis in B. juncea L. plants, which is evident by a reduced generation of reactive oxygen species (ROS) resulting due to suppressed activity of their generating enzymes (glycolate oxidase and NADPH oxidase) and enhanced defence system. Furthermore, application of Si inhibited the activity of protease and triggered the activity of antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase) and plasma membrane H⁺-ATPase activity. In conclusion, all these results reveal that Si could help in the modulation of Chl metabolism, redox hemostasis, and the regulation of nutrients (nitrogen, phosphorus, Si and potassium) uptake in the mustard plants that lead to the postponement of premature leaf senescence under salinity plus DS.

The Eruca sativa Genome and Transcriptome: A Targeted Analysis of Sulfur Metabolism and Glucosinolate Biosynthesis Pre and Postharvest

Rocket (Eruca sativa) is a source of health-related metabolites called glucosinolates (GSLs) and isothiocyanates (ITCs) but little is known of the genetic and transcriptomic mechanisms responsible for regulating pre and postharvest accumulations. We present the first de novo reference genome assembly and annotation, with ontogenic and postharvest transcriptome data relating to sulfur assimilation, transport, and utilization. Diverse gene expression patterns related to sulfur metabolism, GSL biosynthesis, and glutathione biosynthesis are present between inbred lines of rocket. A clear pattern of differential expression determines GSL abundance and the formation of hydrolysis products. One breeding line sustained GSL accumulation and hydrolysis product formation throughout storage. Multiple copies of MYB28, SLIM1, SDI1, and ESM1 have increased and differential expression postharvest, and are associated with GSLs and hydrolysis product formation. Two glucosinolate transporter gene (GTR2) copies were found to be associated with increased GSL accumulations in leaves. Monosaccharides (which are essential for primary metabolism and GSL biosynthesis, and contribute to the taste of rocket) were also quantified in leaves, with glucose concentrations significantly correlated with the expression of numerous GSL-related genes. Significant negative correlations were observed between the expression of glutathione synthetase (GSH) genes and those involved in GSL metabolism. Breeding line "B" showed increased GSH gene expression and low GSL content compared to two other lines where the opposite was observed. Co-expression analysis revealed senescence (SEN1) and oxidative stress-related (OXS3) genes have higher expression in line B, suggesting that postharvest deterioration is associated with low GSL concentrations.

Developmentally controlled changes during Arabidopsis leaf development indicate causes for loss of stress tolerance with age

Leaf senescence is the final stage of leaf development and is induced by the gradual occurrence of age-related changes (ARCs). The process of leaf senescence has been well described, but the cellular events leading to this process are still poorly understood. By analysis of progressively ageing, but not yet senescing, Arabidopsis thaliana rosette leaves, we aimed to better understand processes occurring prior to the onset of senescence. Using gene expression analysis, we found that as leaves mature, genes responding to oxidative stress and genes involved in stress hormone biosynthesis and signalling were up-regulated. A decrease in primary metabolites that provide protection against oxidative stress was a possible explanation for the increased stress signature. The gene expression and metabolomics changes occurred concomitantly to a decrease in drought, salinity, and dark stress tolerance of individual leaves. Importantly, stress-related genes showed elevated expression in the early ageing mutant old5 and decreased expression in the delayed ageing mutant ore9. We propose that the decreased stress tolerance with age results from the occurrence of senescence-inducing ARCs that is integrated into the leaf developmental programme, and that this ensures a timely and certain death.

Allies or Enemies: The Role of Reactive Oxygen Species in Developmental Processes of Black Cottonwood (Populus trichocarpa)

In contrast to aboveground organs (stems and leaves), developmental events and their regulation in underground organs, such as pioneer and fine roots, are quite poorly understood. The objective of the current study was to achieve a better understanding of the physiological and molecular role of reactive oxygen species (ROS) and ROS-related enzymes in the process of stem and pioneer root development in black cottonwood (Populus trichocarpa), as well as in the senescence of leaves and fine roots. Results of a transcriptomic analysis revealed that primary/secondary growth and senescence are accompanied by substantial changes in the expression of genes related to oxidative stress metabolism. We observed that some mechanisms common for above- and under-ground organs, e.g., the expression of superoxide dismutase (SOD) genes and SOD activity, declined during stems' and pioneer roots' development. Moreover, the localization of hydrogen peroxide (H₂O₂) and superoxide (O₂•^-) in the primary and secondary xylem of stems and pioneer roots confirms their involvement in xylem cell wall lignification and the induction of programmed cell death (PCD). H₂O₂ and O₂•^- in senescing fine roots were present in the same locations as demonstrated previously for ATG8 (AuTophaGy-related) proteins, implying their participation in cell degradation during senescence, while O₂•^- in older leaves was also localized similarly to ATG8 in chloroplasts, suggesting their role in chlorophagy. ROS and ROS-related enzymes play an integral role in the lignification of xylem cell walls in Populus trichocarpa, as well as the induction of PCD during xylogenesis and senescence.

Integrative roles of nitric oxide and hydrogen sulfide in melatonin-induced tolerance of pepper (Capsicum annuum L.) plants to iron deficiency and salt stress alone or in combination

There seems to be no report in the literature on the effect of melatonin (MT) in relieving the detrimental effects of combined application of salt stress (SS) and iron deficiency (ID). Therefore, the effect of MT on the accumulation/synthesis of endogenous nitric oxide (NO) and hydrogen sulphide (H₂ S) and how far these molecules are involved in MT-improved tolerance to the combined application of ID and SS in pepper (Capsicum annuum L) were tested. Hence, two individual trials were set up. The treatments in the first experiment comprised: Control, ID (0.1 mM FeSO₄ ), SS (100 mM NaCl) and ID + SS. The detrimental effects of combined stresses were more prominent than those by either of the single stress, with respect to growth, oxidative stress and antioxidant defense attributes. Single stress or both in combination improved the endogenous H₂ S and NO, and foliar-applied MT (100 µM) led to a further increase in NO and H₂ S levels. In the second experiment, 0.1 mM scavenger of NO, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO) and that of H₂ S, hypotuarine (HT) were applied along with MT to get further evidence whether NO and H₂ S are involved in MT-induced tolerance to ID and SS. MT combined with cPTIO and HT under a single or combined stress showed that NO effect was reversed by the NO scavenger, cPTIO, alone but the H₂ S effect was inhibited by both scavengers. These findings suggested that tolerance to ID and SS induced by MT may be involved in downstream signal crosstalk between NO and H₂ S.

Genetic Interaction Among Phytochrome, Ethylene and Abscisic Acid Signaling During Dark-Induced Senescence in Arabidopsis thaliana

Leaf senescence is induced by various internal and external stimuli. Dark-induced senescence has been extensively investigated, but the detailed mechanism underlying it is not well understood. The red light/far-red light receptor phytochrome B and its downstream transcription factors, PYHTOCHROME INTERACTING FACTORs (PIFs) 4 and 5, are known to play an important role in dark-induced senescence. Furthermore, the senescence-inducing phytohormones, ethylene and abscisic acid (ABA) are reported to be involved in dark-induced senescence. In this study, we analyzed the relationship between ethylene, ABA and PIFs in dark-induced leaf senescence. A triple mutant of the core ABA signaling components; SNF1-related protein kinases 2D (SRK2D), SRK2E, and SRK2I, displayed an ABA insensitive phenotype in ABA-induced senescence, whilst the ethylene insensitive mutant ein2 demonstrated low sensitivity to ABA, suggesting that ethylene signaling is involved in ABA-induced senescence. However, the pif4 pif5 mutant did not display low sensitivity to ABA, suggesting that PIF4 and PIF5 act upstream of ABA signaling. Although PIF4 and PIF5 reportedly regulate ethylene production, the triple mutant ein2 pif4 pif5 showed a stronger delayed senescence phenotype than ein2 or pif4 pif5, suggesting that EIN2 and PIF4/PIF5 partially regulate leaf senescence independently of each other. While direct target genes for PIF4 and PIF5, such as LONG HYPOCOTYL IN FAR-RED1 (HFR1) and PHYTOCHROME INTERACTING FACTOR 3-LIKE 1 (PIL1), showed transient upregulation under dark conditions (as is seen in the shade avoidance response), expression of STAY GREEN1 (SGR1), ORESARA1 (ORE1) and other direct target genes of PIF5, continued to increase during dark incubation. It is possible that transcription factors other than PIF4 and PIF5 are involved in the upregulation of SGR1 and ORE1 at a later stage of dark-induced senescence. Possible candidates are senescence-induced senescence regulators (SIRs), which include the NAC transcription factors ORE1 and AtNAP. In fact, ORE1 is known to bind to the SGR1 promoter and promotes its expression. It is therefore inferred that the phytochrome-PIF pathway regulates initial activation of senescence and subsequently, induced SIRs reinforce leaf senescence during dark-induced senescence.

Transcription Factors Associated with Leaf Senescence in Crops

Leaf senescence is a complex mechanism controlled by multiple genetic and environmental variables. Different crops present a delay in leaf senescence with an important impact on grain yield trough the maintenance of the photosynthetic leaf area during the reproductive stage. Additionally, because of the temporal gap between the onset and phenotypic detection of the senescence process, candidate genes are key tools to enable the early detection of this process. In this sense and given the importance of some transcription factors as hub genes in senescence pathways, we present a comprehensive review on senescence-associated transcription factors, in model plant species and in agronomic relevant crops. This review will contribute to the knowledge of leaf senescence process in crops, thus providing a valuable tool to assist molecular crop breeding.

Effects of nitrate deficiency on nitrate assimilation and chlorophyll synthesis of detached apple leaves

Nitrogen is one of the most important nutrients for plant growth and development. Nitrate nitrogen (NO₃^--N) is the main form of nitrogen taken up by plants. Understanding the effects of exogenous NO₃^--N on nitrogen metabolism at the gene expression and enzyme activity levels during nitrogen assimilation and chlorophyll synthesis is important for increasing nitrogen utilization efficiency. In this study, cell morphology, NO₃^--N uptake rates, the expression of key genes related to nitrogen assimilation and chlorophyll synthesis and enzyme activity in apple leaves under NO₃^--N deficiency were investigated. The results showed that the cell morphology of apple leaves was irreversibly deformed due to NO₃^--N deficiency. NO₃^--N was absorbed slightly one day after NO₃^--N deficiency treatment and effluxed after 3 days. The relative expression of genes encoding nitrogen assimilation enzymes and the activity of such enzymes decreased significantly after 1 day of NO₃^--N deficiency treatment. After treatment for 14 days, gene expression was upregulated, enzyme activity was increased, and NO₃^--N content was increased. NO₃^--N deficiency hindered the transformation of 5-aminobilinic acid (ALA) to porphobilinogen (PBG), suggesting a possible route by which NO₃^--N levels affect chlorophyll synthesis. Collectively, the results indicate that NO₃^--N deficiency affects enzyme activity by altering the expression of key genes in the nitrogen assimilation pathway, thereby suppressing NO₃^--N absorption and assimilation. NO₃^--N deficiency inhibits the synthesis of the chlorophyll precursor PBG, thereby hindering chlorophyll synthesis.

AtWDS1 negatively regulates age-dependent and dark-induced leaf senescence in Arabidopsis

Although the involvement of ROS (reactive oxygen species) in leaf senescence is well known, the factors governing this accumulation of ROS are not fully characterized. In this study, analysis of transgenic overexpressing and knock out lines of AtWDS1 (encoding a WD repeat protein), indicates that AtWDS1 negatively regulates age-dependent and dark-induced leaf senescence. Furthermore, we observed ROS accumulation and altered tolerance of oxidative stress in atwds1 plants, as well as upregulated expression of oxidative stress-responsive genes. The location of an EGFP-AtWDS1 fusion protein in the nucleus of transformed cells and plants indicates that AtWDS1 is a nuclear protein, and, using a Dual-Luciferase assay, we showed that AtWDS1 can act as a transcription activator. However, the lack of a nuclear localization sequence in AtWDS1 suggests that its presence in the nucleus must depend on interactions with other proteins. Indeed, we found that AtWDS1 interacts directly with AtRanBPM, and that mutation of the AtRanBPM gene results in partial mislocalization of AtWDS1 in the cytoplasm. Together, these results suggest a role for AtWDS1 as a novel modulator of redox homeostasis, which responds to developmental and stress signals to regulate leaf senescence.

Methods for the Simple and Reliable Assessment of Barley Sensitivity to Abiotic Stresses During Early Development

Physiological assays that facilitate screening for various types of responses to abiotic stresses are well established for model plants such as Arabidopsis; however, there is a need to optimize similar tests for cereal crops, including barley. We have developed a set of stress assays to characterize the response of different barley lines during two stages of development-seed germination and seedling growth. The assays presented, including the response to osmotic, salt, oxidative stresses, and exogenously applied abscisic acid, can be used for forward screening of populations after mutagenesis as well as for phenotyping of already isolated mutants, cultivars, or breeding lines. As well as protocols for stress treatments, we also provide methods for plant stress response evaluation, such as chlorophyll a fluorescence (ChlF) and image analysis.

Exogenous Melatonin Enhances Cold, Salt and Drought Stress Tolerance by Improving Antioxidant Defense in Tea Plant (Camellia sinensis (L.) O. Kuntze)

Melatonin is a biological hormone that plays crucial roles in stress tolerance. In this study, we investigated the effect of exogenous melatonin on abiotic stress in the tea plant. Under cold, salt and drought stress, increasing malondialdehyde levels and decreasing maximum photochemical efficiency of PSII were observed in tea leaves. Meanwhile, the levels of reactive oxygen species (ROS) increased significantly under abiotic stress. Interestingly, pretreatment with melatonin on leaves alleviated ROS burst, decreased malondialdehyde levels and maintain high photosynthetic efficiency. Moreover, 100 μM melatonin-pretreated tea plants showed high levels of glutathione and ascorbic acid and increased the activities of superoxide dismutase, peroxidase, catalase and ascorbate peroxidase under abiotic stress. Notably, melatonin treatments can positively up-regulate the genes (CsSOD, CsPOD, CsCAT and CsAPX) expression of antioxidant enzyme biosynthesis. Taken together, our results confirmed that melatonin protects tea plants against abiotic stress-induced damages through detoxifying ROS and regulating antioxidant systems.

Natural allelic variation of GVS1 confers diversity in the regulation of leaf senescence in Arabidopsis

Leaf senescence affects plant fitness. Plants that evolve in different environments are expected to acquire distinct regulations of leaf senescence. However, the adaptive and evolutionary roles of leaf senescence are largely unknown. We investigated leaf senescence in 259 natural accessions of Arabidopsis by quantitatively assaying dark-induced senescence responses using a high-throughput chlorophyll fluorescence imaging system. A meta-analysis of our data with phenotypic and climatic information demonstrated biological and environmental links with leaf senescence. We further performed genome-wide association mapping to identify the genetic loci underlying the diversity of leaf senescence responses. We uncovered a new locus, Genetic Variants in leaf Senescence (GVS1), with high similarity to reductase, where a single nonsynonymous nucleotide substitution at GVS1 mediates the diversity of the senescence trait. Loss-of-function mutations of GVS1 in Columbia-0 delayed leaf senescence and increased sensitivity to oxidative stress, suggesting that this GVS1 variant promotes optimal responses to developmental and environmental signals. Intriguingly, gvs1 loss-of-function mutants display allele- and accession-dependent phenotypes, revealing the functional diversity of GVS1 alleles not only in leaf senescence, but also oxidative stress. Our discovery of GVS1 as the genetic basis of natural variation in senescence programs reinforces its adaptive potential in modulating life histories across diverse environments.

Cold-priming of chloroplast ROS signalling is developmentally regulated and is locally controlled at the thylakoid membrane

24 h exposure to 4 °C primes Arabidopsis thaliana in the pre-bolting rosette stage for several days against full cold activation of the ROS responsive genes ZAT10 and BAP1 and causes stronger cold-induction of pleiotropically stress-regulated genes. Transient over-expression of thylakoid ascorbate peroxidase (tAPX) at 20 °C mimicked and tAPX transcript silencing antagonized cold-priming of ZAT10 expression. The tAPX effect could not be replaced by over-expression of stromal ascorbate peroxidase (sAPX) demonstrating that priming is specific to regulation of tAPX availability and, consequently, regulated locally at the thylakoid membrane. Arabidopsis acquired cold primability in the early rosette stage between 2 and 4 weeks. During further rosette development, primability was widely maintained in the oldest leaves. Later formed and later maturing leaves were not primable demonstrating that priming is stronger regulated with plant age than with leaf age. In 4-week-old plants, which were strongest primable, the memory was fully erasable and lost seven days after priming. In summary, we conclude that cold-priming of chloroplast-to-nucleus ROS signalling by transient post-stress induction of tAPX transcription is a strategy to modify cell signalling for some time without affecting the alertness for activation of cold acclimation responses.

Arabidopsis NITROGEN LIMITATION ADAPTATION regulates ORE1 homeostasis during senescence induced by nitrogen deficiency

Nitrogen is an important macronutrient in plants and its deficiency induces rapid leaf senescence. Two genes, ORE1 and NITROGEN LIMITATION ADAPTATION (NLA), have been implicated in regulating the senescence process but their relationship is unclear^1,2. Here, we show that nla and pho2 (also known as ubc24) plants develop rapid leaf senescence under nitrogen-starvation condition, whereas ore1 and nla/ore1 and pho2 (ubc24)/ore1 plants stay green. These results suggest that ORE1 acts downstream of NLA and PHO2 (UBC24). NLA interacts with ORE1 in the nucleus and regulates its stability through polyubiquitination using PHO2 (UBC24) as the E2 conjugase. Our findings identified ORE1 as a downstream target of NLA/PHO2 (UBC24) and showed that post-translational regulation of ORE1 levels determines leaf senescence during nitrogen deficiency.

ORESARA15, a PLATZ transcription factor, mediates leaf growth and senescence in Arabidopsis

Plant leaves undergo a series of developmental changes from leaf primordium initiation through growth and maturation to senescence throughout their life span. Although the mechanisms underlying leaf senescence have been intensively elucidated, our knowledge of the interrelationship between early leaf development and senescence is still fragmentary. We isolated the oresara15-1Dominant (ore15-1D) mutant, which had an extended leaf longevity and an enlarged leaf size, from activation-tagged lines of Arabidopsis. Plasmid rescue identified that ORE15 encodes a PLANT A/T-RICH SEQUENCE- AND ZINC-BINDING PROTEIN family transcription factor. Phenotypes of ore15-1D and ore15-2, a loss-of-function mutant, were evaluated through physiological and anatomical analyses. Microarray, quantitative reverse transcription polymerase chain reaction, and chromatin immunoprecipitation as well as genetic analysis were employed to reveal the molecular mechanism of ORE15 in the regulation of leaf growth and senescence. ORE15 enhanced leaf growth by promoting the rate and duration of cell proliferation in the earlier stage and suppressed leaf senescence in the later stage by modulating the GROWTH-REGULATING FACTOR (GRF)/GRF-INTERACTING FACTOR regulatory pathway. Our study highlighted a molecular conjunction through ORE15 between growth and senescence, which are two temporally separate developmental processes during leaf life span.

Irreversible strigolactone recognition: a non-canonical mechanism for hormone perception

Unveiling of hormone perception is central to comprehending hormone action. It is generally recognized that an active hormone molecule binds its receptor to initiate hormone signaling, subsequently dissociates from its receptor without being changed, and then initiates the next round of hormone perception. However, recent studies discovered that the α/β hydrolase DWARF14 serves as a non-canonical receptor for the plant hormone strigolactone (SL) to generate the active form of SL which remains covalently bound in an irreversible manner, triggering SL signal transduction. In this short review, we will discuss the recent advances in uncovering this unprecedented non-canonical mechanism for hormone perception.

Integrated mRNA, sRNA, and degradome sequencing reveal oilseed rape complex responses to Sclerotinia sclerotiorum (Lib.) infection

Sclerotinia stem rot (SSR), caused by the fungal pathogen Sclerotinia sclerotiorum, is a devastating disease resulting in yield losses and decreases in seed quality in oilseed rape (Brassica napus) worldwide. However, the molecular mechanisms underlying the response of oilseed rape to S. sclerotiorum infection at the transcriptional and post-transcriptional levels are poorly understood. Here, we used an integrated omics approach (transcriptome, sRNAome, and degradome sequencing) on the Illumina platform to compare the RNA expression and post-transcriptional profiles of oilseed rape plants inoculated or not with S. sclerotiorum. In total, 7,065 differentially expressed genes (DEGs) compared with the mock-inoculated control at 48 hours post inoculation were identified. These DEGs were associated with protein kinases, signal transduction, transcription factors, hormones, pathogenesis-related proteins, secondary metabolism, and transport. In the sRNA-Seq analysis, 77 known and 176 novel miRNAs were identified; however, only 10 known and 41 novel miRNAs were differentially expressed between the samples inoculated or not with S. sclerotiorum. Degradome sequencing predicted 80 cleavage sites with 64 miRNAs. Integrated mRNA, sRNA and degradome sequencing analysis reveal oilseed rape complex responses to S. sclerotiorum infection. This study provides a global view of miRNA and mRNA expression profiles in oilseed rape following S. sclerotiorum infection.

Stress-induced senescence and plant tolerance to abiotic stress

Senescence is an age-dependent process, ultimately leading to plant death, that in annual crop plants overlaps with the reproductive stage of development. Research on the molecular and biochemical mechanisms of leaf senescence has revealed a multi-layered regulatory network operating to control age-dependent processes. Abiotic stress-induced senescence challenges source-sink relationships and results in significant reduction in crop yields. Although processes associated with plant senescence are well studied, the mechanisms regulating stress-induced senescence are not well known. Here, we discuss the effects of abiotic stress on crop productivity, mechanisms associated with stress-induced senescence, and the possible use of these mechanisms for the generation of plant stress tolerance. We emphasize the involvement of source strength and stability of the photosynthetic apparatus in this process, and suggest a possible role of a perennial plant life strategy for the amelioration of stress-induced senescence.

Study of Cotton Leaf Senescence Induced by Alternaria alternata Infection

Premature leaf senescence in cotton, which often happens during the mid to late growth period, has been occurring with an increasing frequency in many cotton-growing areas and causing serious reduction in yield and quality. One of the key factors causing cotton leaf senescence is the infection of Alternaria leaf spot pathogens (Alternaria species), which often happens when cotton plants encounter adverse environmental conditions, such as chilling stress and physiological impairment. Stressed cotton leaves are apt to be infected by Alternaria leaf spot pathogens (Alternaria alternata) because of the reduction in disease resistance, leading to the initiation of leaf senescence. Here we describe the induction of cotton leaf senescence by Alternaria alternata infection, including the evaluation of the disease index and measure of physiological impairment associated with cotton leaf senescence and analysis of possible molecular mechanism using microarray.

Recent advances in molecular basis for strigolactone action

Strigolactones (SLs) are a very special class of plant hormones, which act as endogenous signals to regulate shoot branching in plants, and also serve as rhizosphere signals to regulate interactions of host plants with heterologous organisms such as symbiotic arbuscular mycorrhizal fungi and parasitic weeds. In this short review, we give a brief description of novel discoveries in SL biosynthesis pathway, and mainly summarize the recent advances in SL perception and signal transduction.

Exogenous GR24 Alleviates Cadmium Toxicity by Reducing Cadmium Uptake in Switchgrass (Panicum virgatum) Seedlings

Strigolactones (SLs) are classified into plant hormones, playing a key role as a mediator of plant growth in response to several abiotic stresses, including drought and salinity. However, the role of SLs in cadmium (Cd)-induced stress to plants is still unknown. The physiological responses of switchgrass (Panicum virgatum) stressed in 10 μmol L⁻¹ Cd to exogenous synthetic SLs analog, GR24 were studied in hydroponics. The Cd stress significantly caused the adverse effects on plant growth and root morphology, inhibited photosynthesis, but boosted lipid peroxidation of Switchgrass seedlings. After treatment of 1 μmol L⁻¹ GR24, the above adverse effects caused by Cd stress were significantly alleviated, mainly reflects in improvement of shoot biomass, relative water content, root development, chlorophyll contents, activities of typical antioxidant enzymes, nutrient uptake. The reason for exogenous GR24 alleviating cadmium toxicity might be owing to that exogenous GR24 promoted the content of endogenous SLs, increased some essential element Fe (iron), Zn (zinc), Mn (manganese) and Cu (copper) uptake and reduced cadmium uptake, accumulation and partition in shoot of switchgrass seedlings.