An abscisic acid-AtNAP transcription factor-SAG113 protein phosphatase 2C regulatory chain for controlling dehydration in senescing Arabidopsis leaves

ArNAC148 induces Acer rubrum leaf senescence by activating the transcription of the ABA receptor gene ArPYR13

Acer rubrum, an ornamental tree known for its stunning autumn colors, has an elusive molecular mechanism that governs its leaf senescence. We performed the genome-wide analysis of NAC transcription factor genes and PYRABACTIN RESISTANCE1-LIKE (PYLs) and found that ArNAC148 and ArPYL13 were significantly upregulated in senescing leaves as compared to mature leaves. Subcellular localization studies confirmed the nuclear localization of ArNAC148 and the cytoplasmic localization of ArPYL13. Electrophoretic mobility shift assay and yeast one-hybrid assay demonstrated that ArNAC148 directly binds to the promoter of ArPYL13. Luciferase reporter assays further showed that ArNAC148 activates the transcription of ArPYL13. The transient expression of ArNAC148 and ArPYL13 in tobacco leaves promoted chlorophyll degradation, increased H2O2 level, MDA contents, and electrolyte leakage in response to abscisic acid (ABA). Moreover, the virus-induced gene silencing of ArNAC148 and ArPYL13 in A. rubrum produced results that were opposite to those observed in transient expression experiments. Our findings suggest that ArNAC148 induces leaf senescence by directly activating the transcription of ArPYL13, providing insights into the ABA-mediated regulatory mechanisms governing leaf senescence in A. rubrum. This study offers new perspectives for researchers to explore the roles of NAC and PYL genes in regulating leaf senescence in woody ornamental plants.

The OsAGO2-OsNAC300-OsNAP module regulates leaf senescence in rice

Leaves play a crucial role in the growth and development of rice (Oryza sativa) as sites for the production of photosynthesis. Early leaf senescence leads to substantial drops in rice yields. Whether and how DNA methylation regulates gene expression and affects leaf senescence remains elusive. Here, we demonstrate that mutations in rice ARGONAUTE 2 (OsAGO2) lead to premature leaf senescence, with chloroplasts in Osago2 having lower chlorophyll content and an abnormal thylakoid structure compared with those from wild-type plants. We show that OsAGO2 associates with a 24-nt microRNA and binds to the promoter region of OsNAC300, which causes DNA methylation and suppressed expression of OsNAC300. Overexpressing OsNAC300 causes the similar premature leaf senescence as Osago2 mutants and knocking out OsNAC300 in the Osago2 mutant background suppresses the early senescence of Osago2 mutants. Based on yeast one-hybrid, dual-luciferase, and electrophoresis mobility shift assays, we propose that OsNAC300 directly regulates transcription of the key rice aging gene NAC-like, activated by APETALA3/PISTILLATA (OsNAP) to control leaf senescence. Our results unravel a previously unknown epigenetic regulatory mechanism underlying leaf senescence in which OsAGO2-OsNAC300-OsNAP acts as a key regulatory module of leaf senescence to maintain leaf function.

Arabidopsis WRKY1 promotes monocarpic senescence by integrative regulation of flowering, leaf senescence, and nitrogen remobilization

Monocarpic senescence, characterized by whole-plant senescence following a single flowering phase, is widespread in seed plants, particularly in crops, determining seed harvest time and quality. However, how external and internal signals are systemically integrated into monocarpic senescence remains largely unknown. Here, we report that the Arabidopsis thaliana transcription factor WRKY1 plays essential roles in multiple key steps of monocarpic senescence. WRKY1 expression is induced by age, salicylic acid (SA), and nitrogen (N) deficiency. Flowering and leaf senescence are accelerated in the WRKY1 overexpression lines but are delayed in the wrky1 mutants. The combined DNA affinity purification sequencing and RNA sequencing analyses uncover the direct target genes of WRKY1. Further studies show that WRKY1 coordinately regulates three processes in monocarpic senescence: (1) suppressing FLOWERING LOCUS C gene expression to initiate flowering, (2) inducing SA biosynthesis genes to promote leaf senescence, and (3) activating the N assimilation and transport genes to trigger N remobilization. In summary, our study reveals how one stress-responsive transcription factor, WRKY1, integrates flowering, leaf senescence, and N remobilization processes into monocarpic senescence, providing important insights into plant lifetime regulation.

Drought Protective Effects of Exogenous ABA and Kinetin on Lettuce: Sugar Content, Antioxidant Enzyme Activity, and Productivity

Drought is an environmental stressor that significantly impacts plant growth and development. Comprehending the complexity of drought stress and water utilization in the context of plant growth and development holds significant importance for sustainable agriculture. The aim of this study was to evaluate the effect of exogenously applied phytohormones on lettuce (Lactuca sativa L.) sugar content profiles and antioxidant enzyme activity and productivity. Lettuce plants were grown under normal and drought conditions in a growth chamber with a photoperiod of 14/10 h (day/night). Kinetin and abscisic acid were applied separately and in combinations when the second leaf was fully expanded. The results showed that sugar accumulation and productivity of the pretreated plants under drought were significantly higher than the controls. The perspective offered by this work showed that growth-related and stress-related phytohormones significantly influenced plant sugar metabolism, metabolic profiles, and productivity, thus enabling the control of yield and quality.

Expression and function identification of senescence-associated genes under continuous drought treatment in grapevine (Vitis vinifera L.) leaves

Natural leaf senescence is critical for plant fitness. Drought-induced premature leaf senescence affects grape yield and quality. However, reports on the regulatory mechanisms underlying premature leaf senescence under drought stress are limited. In this study, two-year-old potted 'Muscat Hamburg' grape plants were subjected to continuous natural drought treatment until mature leaves exhibited senescence symptoms. Physiological and biochemical indices related to drought stress and senescence were monitored. Transcriptome and transgenic Arabidopsis were used to perform expression analyses and functional identification of drought-induced senescence-associated genes. Twelve days of continuous drought stress was sufficient to cause various physiological disruptions and visible senescence symptoms in mature 'Muscat Hamburg' leaves. These disruptions included malondialdehyde and H2O2 accumulation, and decreased catalase activity and chlorophyll (Chl) levels. Transcriptome analysis revealed that most genes involved in photosynthesis and Chl synthesis were downregulated after 12 d of drought treatment. Three key Chl catabolic genes (SGR, NYC1, and PAO) were significantly upregulated. Overexpression of VvSGR in wild Arabidopsis further confirmed that SGR directly promoted early yellowing of cotyledons and leaves. In addition, drought treatment decreased expression of gibberellic acid signaling repressors (GAI and GAI1) and cytokinin signal components (AHK4, AHK2, RR22, RR9-1, RR9-2, RR6, and RR4) but significantly increased the expression of abscisic acid, jasmonic acid, and salicylic acid signaling components and responsive transcription factors (bZIP40/ABF2, WRKY54/75/70, ANAC019, and MYC2). Moreover, some NAC members (NAC0002, NAC019, and NAC048) may also be drought-induced senescence-associated genes. These results provide extensive information on candidate genes involved in drought-induced senescence in grape leaves.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01465-2.

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.

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.

E3 ubiquitin ligase MaRZF1 modulates high temperature-induced green ripening of banana by degrading MaSGR1

High temperatures (>24°C) prevent the development of a yellow peel on bananas called green ripening, owing to the inhibition of chlorophyll degradation. This phenomenon greatly reduces the marketability of banana fruit, but the mechanisms underlining high temperature-repressed chlorophyll catabolism need to be elucidated. Herein, we found that the protein accumulation of chlorophyll catabolic enzyme MaSGR1 (STAY-GREEN 1) was reduced when bananas ripened at high temperature. Transiently expressing MaSGR1 in banana peel showed its positive involvement in promoting chlorophyll degradation under high temperature, thereby weakening green ripening phenotype. Using yeast two-hybrid screening, we identified a RING-type E3 ubiquitin ligase, MaRZF1 (RING Zinc Finger 1), as a putative MaSGR1-interacting protein. MaRZF1 interacts with and targets MaSGR1 for ubiquitination and degradation via the proteasome pathway. Moreover, upregulating MaRZF1 inhibited chlorophyll degradation, and attenuated MaSGR1-promoted chlorophyll degradation in bananas during green ripening, indicating that MaRZF1 negatively regulates chlorophyll catabolism via the degradation of MaSGR1. Taken together, MaRZF1 and MaSGR1 form a regulatory module to mediate chlorophyll degradation associated with high temperature-induced green ripening in bananas. Therefore, our findings expand the understanding of posttranslational regulatory mechanisms of temperature stress-caused fruit quality deterioration.

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.

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.

Transcriptome Analysis and Metabolic Profiling Reveal the Key Regulatory Pathways in Drought Stress Responses and Recovery in Tomatoes

Drought stress is a major abiotic factor affecting tomato production and fruit quality. However, the genes and metabolites associated with tomato responses to water deficiency and rehydration are poorly characterized. To identify the functional genes and key metabolic pathways underlying tomato responses to drought stress and recovery, drought-susceptible and drought-tolerant inbred lines underwent transcriptomic and metabolomic analyses. A total of 332 drought-responsive and 491 rehydration-responsive core genes were robustly differentially expressed in both genotypes. The drought-responsive and rehydration-responsive genes were mainly related to photosynthesis-antenna proteins, nitrogen metabolism, plant-pathogen interactions, and the MAPK signaling pathway. Various transcription factors, including homeobox-leucine zipper protein ATHB-12, NAC transcription factor 29, and heat stress transcription factor A-6b-like, may be vital for tomato responses to water status. Moreover, 24,30-dihydroxy-12(13)-enolupinol, caffeoyl hawthorn acid, adenosine 5'-monophosphate, and guanosine were the key metabolites identified in both genotypes under drought and recovery conditions. The combined transcriptomic and metabolomic analysis highlighted the importance of 38 genes involved in metabolic pathways, the biosynthesis of secondary metabolites, the biosynthesis of amino acids, and ABC transporters for tomato responses to water stress. Our results provide valuable clues regarding the molecular basis of drought tolerance and rehydration. The data presented herein may be relevant for genetically improving tomatoes to enhance drought tolerance.

A pilot study on non-invasive in situ detection of phytochemicals and plant endogenous status using fiber optic infrared spectroscopy

Traditional methods for assessing plant health often lack the necessary attributes for continuous and non-destructive monitoring. In this pilot study, we present a novel technique utilizing a customized fiber optic probe based on attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) with a contact force control unit for non-invasive and continuous plant health monitoring. We also developed a normalized difference mid-infrared reflectance index through statistical analysis of spectral features, enabling differentiation of drought and age conditions in plants. Our research aims to characterize phytochemicals and plant endogenous status optically, addressing the need for improved analytical measurement methods for in situ plant health assessment. The probe configuration was optimized with a triple-loop tip and a 3 N contact force, allowing sensitive measurements while minimizing leaf damage. By combining polycrystalline and chalcogenide fiber probes, a comprehensive wavenumber range analysis (4000-900 cm-1) was achieved. Results revealed significant variations in phytochemical composition among plant species, for example, red spinach with the highest polyphenolic content and green kale with the highest lignin content. Petioles displayed higher lignin and cellulose absorbance values compared to veins. The technique effectively monitored drought stress on potted green bok choy plants in situ, facilitating the quantification of changes in water content, antioxidant activity, lignin, and cellulose levels. This research represents the first demonstration of the potential of fiber optic ATR-FTIR probes for non-invasive and rapid plant health measurements, providing insights into plant health and advancements in quantitative monitoring for indoor farming practices, bioanalytical chemistry, and environmental sciences.

MaNAC029 modulates ethylene biosynthesis and fruit quality and undergoes MaXB3-mediated proteasomal degradation during banana ripening

INTRODUCTIONS

Ethylene regulates ripening by activating various metabolic pathways that controlcolor, aroma, flavor, texture, and consequently, the quality of fruits. However, the modulation of ethylene biosynthesis and quality formation during banana fruit ripening remains unclear.

OBJECTIVES

The present study aimed to identify the regulatory module that regulates ethylene and fruit quality-related metabolisms during banana fruit ripening.

METHODS

We used RNA-seq to compare unripe and ripe banana fruits and identified a ripening-induced NAC transcription factor, MaNAC029. We further performed DNA affinity purification sequencing to identify the MaNAC029's target genes involved in ethylene biosynthesis and fruit quality formation, and electrophoretic mobility shift assay, chromatin immunoprecipitation with real-time polymerase chain reaction and dual luciferase assays to explore the underlying regulatory mechanisms. Immunoprecipitation combined with mass spectrometry, yeast two-hybrid assay, and bimolecular fluorescence complementation assay were used to screen and verify the proteins interacting with MaNAC029. Finally, the function of MaNAC029 and its interacting protein associated with ethylene biosynthesis and quality formation was verified through transient overexpression experiments in banana fruits.

RESULTS

The study identified a nucleus-localized, ripening-induced NAC transcription factor MaNAC029. It transcriptionally activated genes associated with ethylene biosynthesis and a variety of cellular metabolisms related to fruit quality formation (cell wall degradation, starch degradation, aroma compound synthesis, and chlorophyll catabolism) by directly modulating their promoter activity during ripening. Overexpression of MaNAC029 in banana fruits activated ethylene biosynthesis and accelerated fruit ripening and quality formation. Notably, the E3 ligase MaXB3 interacted with and ubiquitinated MaNAC029 protein, facilitating MaNAC029 proteasomal degradation. Consistent with this finding, MaXB3 overexpression attenuated MaNAC029-enhanced ethylene biosynthesis and quality formation.

CONCLUSION

Our findings demonstrate that a MaXB3-MaNAC029 module regulates ethylene biosynthesis and a series of cellular metabolisms related to fruit quality formation during banana ripening. These results expand the understanding of the transcriptional and post-translational mechanisms of fruit ripening and quality formation.

An insertion of transposon in DcNAP inverted its function in the ethylene pathway to delay petal senescence in carnation (Dianthus caryophyllus L.)

Petal senescence is the final stage of flower development. Transcriptional regulation plays key roles in this process. However, whether and how post-transcriptional regulation involved is still largely unknown. Here, we identified an ethylene-induced NAC family transcription factor DcNAP in carnation (Dianthus caryophyllus L.). One allele, DcNAP-dTdic1, has an insertion of a dTdic1 transposon in its second exon. The dTdic1 transposon disrupts the structure of DcNAP and causes alternative splicing, which transcribes multiple domain-deleted variants (DcNAP2 and others). Conversely, the wild type allele DcNAP transcribes DcNAP1 encoding an intact NAC domain. Silencing DcNAP1 delays and overexpressing DcNAP1 accelerates petal senescence in carnation, while silencing and overexpressing DcNAP2 have the opposite effects, respectively. Further, DcNAP2 could interact with DcNAP1 and interfere the binding and activation activity of DcNAP1 to the promoters of its downstream target ethylene biosynthesis genes DcACS1 and DcACO1. Lastly, ethylene signalling core transcriptional factor DcEIL3-1 can activate the expression of DcNAP1 and DcNAP2 in the same way by binding their promoters. In summary, we discovered a novel mechanism by which DcNAP regulates carnation petal senescence at the post-transcriptional level. It may also provide a useful strategy to manipulate the NAC domains of NAC transcription factors for crop genetic improvement.

The ABA-AtNAP-SAG113 PP2C module regulates leaf senescence by dephoshorylating SAG114 SnRK3.25 in Arabidopsis

We previously reported that ABA inhibits stomatal closure through AtNAP-SAG113 PP2C regulatory module during leaf senescence. The mechanism by which this module exerts its function is unknown. Here we report the identification and functional analysis of SAG114, a direct target of the regulatory module. SAG114 encodes SnRK3.25. Both bimolecular fluorescence complementation (BiFC) and yeast two-hybrid assays show that SAG113 PP2C physically interacts with SAG114 SnRK3.25. Biochemically the SAG113 PP2C dephosphorylates SAG114 in vitro and in planta. RT-PCR and GUS reporter analyses show that SAG114 is specifically expressed in senescing leaves in Arabidopsis. Functionally, the SAG114 knockout mutant plants have a significantly bigger stomatal aperture and a much faster water loss rate in senescing leaves than those of wild type, and display a precocious senescence phenotype. The premature senescence phenotype of sag114 is epistatic to sag113 (that exhibits a remarkable delay in leaf senescence) because the sag113 sag114 double mutant plants show an early leaf senescence phenotype, similar to that of sag114. These results not only demonstrate that the ABA-AtNAP-SAG113 PP2C regulatory module controls leaf longevity by dephosphorylating SAG114 kinase, but also reveal the involvement of the SnRK3 family gene in stomatal movement and water loss during leaf senescence.

Wheat VQ Motif-Containing Protein VQ25-A Facilitates Leaf Senescence via the Abscisic Acid Pathway

Leaf senescence is an important factor affecting the functional transition from nutrient assimilation to nutrient remobilization in crops. The senescence of wheat leaves is of great significance for its yield and quality. In the leaf senescence process, transcriptional regulation is a committed step in integrating various senescence-related signals. Although the plant-specific transcriptional regulation factor valine-glutamine (VQ) gene family is known to participate in different physiological processes, its role in leaf senescence is poorly understood. We isolated TaVQ25-A and studied its function in leaf senescence regulation. TaVQ25-A was mainly expressed in the roots and leaves of wheat. The TaVQ25-A-GFP fusion protein was localized in the nuclei and cytoplasm of wheat protoplasts. A delayed senescence phenotype was observed after dark and abscisic acid (ABA) treatment in TaVQ25-A-silenced wheat plants. Conversely, overexpression of TaVQ25-A accelerated leaf senescence and led to hypersensitivity in ABA-induced leaf senescence in Arabidopsis. A WRKY type transcription factor, TaWRKY133, which is tightly related to the ABA pathway and affects the expression of some ABA-related genes, was found to interact with TaVQ25-A both in vitro and in vivo. Results of this study indicate that TaVQ25-A is a positive regulator of ABA-related leaf senescence and can be used as a candidate gene for wheat molecular breeding.

The NAC transcription factors SNAP1/2/3/4 are central regulators mediating high nitrogen responses in mature nodules of soybean

Legumes can utilize atmospheric nitrogen via symbiotic nitrogen fixation, but this process is inhibited by high soil inorganic nitrogen. So far, how high nitrogen inhibits N2 fixation in mature nodules is still poorly understood. Here we construct a co-expression network in soybean nodule and find that a dynamic and reversible transcriptional network underlies the high N inhibition of N2 fixation. Intriguingly, several NAC transcription factors (TFs), designated as Soybean Nitrogen Associated NAPs (SNAPs), are amongst the most connected hub TFs. The nodules of snap1/2/3/4 quadruple mutants show less sensitivity to the high nitrogen inhibition of nitrogenase activity and acceleration of senescence. Integrative analysis shows that these SNAP TFs largely influence the high nitrogen transcriptional response through direct regulation of a subnetwork of senescence-associated genes and transcriptional regulators. We propose that the SNAP-mediated transcriptional network may trigger nodule senescence in response to high nitrogen.

The NAC transcription factor ZmNAC132 regulates leaf senescence and male fertility in maize

Leaf senescence is an integral step in the final stages of plant development, as nutrient remobilization from leaves to sink organs is accomplished during this process. NACs compose a large superfamily of plant-specific TFs involved in multiple plant development processes. Here, we identified a maize NAC TF, ZmNAC132, involved in leaf senescence and male fertility. ZmNAC132 expression was tightly linked to leaf senescence in an age-dependent manner. Knockout of ZmNAC132 led to delays in chlorophyll degradation and leaf senescence, whereas overexpression of ZmNAC132 had the opposite effects. ZmNAC132 could bind to and transactivate the promoter of ZmNYE1, a major chlorophyll catabolic gene, to accelerate chlorophyll degradation during leaf senescence. Moreover, ZmNAC132 affected male fertility through the upregulation of ZmEXPB1, an expansin-encoding gene associated with sexual reproduction and other related genes. Together, the results show that ZmNAC132 participates in the regulation of leaf senescence and male fertility through the targeting of different downstream genes in maize.

Abscisic acid can augment, but is not essential for, autumnal leaf senescence

Senescence vividly marks the onset of the final stages of the life of a leaf, yet the triggers and drivers of this process are still not fully understood. The hormone abscisic acid (ABA) is an important regulator of leaf senescence in model herbs, but the function of this hormone has not been widely tested in deciduous trees. Here we investigate the importance of ABA as a driver of leaf senescence in winter deciduous trees. In four diverse species we tracked leaf gas exchange, water potential, chlorophyll content, and leaf ABA levels from the end of summer until leaves were abscised or died. We found that no change in ABA levels occurred at the onset of chlorophyll decline or throughout the duration of leaf senescence. To test whether ABA could enhance leaf senescence, we girdled branches to disrupt ABA export in the phloem. Girdling increased leaf ABA levels in two of the species, and this increase triggered an accelerated rate of chlorophyll decline in these species. We conclude that an increase in ABA level may augment leaf senescence in winter deciduous species but that it is not essential for this annual process.

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.

ANAC087 transcription factor positively regulates age-dependent leaf senescence through modulating the expression of multiple target genes in Arabidopsis

Leaf senescence is the final stage of leaf development and appropriate onset and progression of leaf senescence are critical for reproductive success and fitness. Although great progress has been made in identifying key genes regulating leaf senescence and elucidating the underlining mechanisms in the model plant Arabidopsis, there is still a gap to understanding the complex regulatory network. In this study, we discovered that Arabidopsis ANAC087 transcription factor (TF) positively modulated leaf senescence. Expression of ANAC087 was induced in senescing leaves and the encoded protein acted as a transcriptional activator. Both constitutive and inducible overexpression lines of ANAC087 showed earlier senescence than control plants, whereas T-DNA insertion mutation and dominant repression of the ANAC087 delayed senescence rate. A quantitative reverse transcription-polymerase chain reaction (qRT-PCR) profiling showed that the expression of an array of senescence-associated genes was upregulated in inducible ANAC087 overexpression plants including BFN1, NYE1, CEP1, RbohD, SAG13, SAG15, and VPEs, which are involved in programmed cell death (PCD), chlorophyll degradation and reactive oxygen species (ROS) accumulation. In addition, electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation-quantitative polymerase chain reaction (ChIP-qPCR) assays demonstrated that ANAC087 directly bound to the canonical NAC recognition sequence (NACRS) motif in promoters of its target genes. Moreover, mutation of two representative target genes, BFN1 or NYE1 alleviated the senescence rate of ANAC087-overexpression plants, suggesting their genetic regulatory relationship. Taken together, this study indicates that ANAC087 serves as an important regulator linking PCD, ROS, and chlorophyll degradation to leaf senescence.

The NAC transcription factor MdNAC4 positively regulates nitrogen deficiency-induced leaf senescence by enhancing ABA biosynthesis in apple

Although it is well established that nitrogen (N) deficiency induces leaf senescence, the molecular mechanism of N deficiency-induced leaf senescence remains largely unknown. Here, we show that an abscisic acid (ABA)-responsive NAC transcription factor (TF) is involved in N deficiency-induced leaf senescence. The overexpression of MdNAC4 led to increased ABA levels in apple calli by directly activating the transcription of the ABA biosynthesis gene MdNCED2. In addition, MdNAC4 overexpression promoted N deficiency-induced leaf senescence. Further investigation showed that MdNAC4 directly bound the promoter of the senescence-associated gene (SAG) MdSAG39 and upregulated its expression. Interestingly, the function of MdNAC4 in promoting N deficiency-induced leaf senescence was enhanced in the presence of ABA. Furthermore, we identified an interaction between the ABA receptor protein MdPYL4 and the MdNAC4 protein. Moreover, MdPYL4 showed a function similar to that of MdNAC4 in ABA-mediated N deficiency-induced leaf senescence. These findings suggest that ABA plays a central role in N deficiency-induced leaf senescence and that MdPYL4 interacts with MdNAC4 to enhance the response of the latter to N deficiency, thus promoting N deficiency-induced leaf senescence. In conclusion, our results provide new insight into how MdNAC4 regulates N deficiency-induced leaf senescence.

Cytokinins regulate rice lamina joint development and leaf angle

Leaf angle is determined by lamina joint inclination and is an important agronomic trait that determines plant architecture, photosynthetic efficiency, and crop yield. Cytokinins (CKs) are phytohormones involved in shaping rice (Oryza sativa L.) architecture, but their role in leaf angle remains unknown. Here, we report that CK accumulation mediated by rice CK OXIDASE/DEHYDROGENASE3 (OsCKX3) controls lamina joint development and negatively regulates leaf angle. Phenotypic analysis showed that rice osckx3 mutants had smaller leaf angles, while the overexpression lines (OsCKX3-OE) had larger leaf angles. Histological sections indicated that the leaf inclination changes in the osckx3 and OsCKX3-OE lines resulted from asymmetric proliferation of the cells and vascular bundles in the lamina joint. Reverse transcription quantitative PCR, promoter-fused β-glucuronidase expression, and subcellular localization assays indicated that OsCKX3 was highly expressed in the lamina joint, and OsCKX3-GFP fusion protein localized to the endoplasmic reticulum. The enzyme assays using recombinant protein OsCKX3 revealed that OsCKX3 prefers trans-zeatin (tZ) and isopentenyladenine (iP). Consistently, tZ and iP levels increased in the osckx3 mutants but decreased in the OsCKX3 overexpression lines. Interestingly, agronomic trait analysis of the rice grown in the paddy field indicated that osckx3 displayed a smaller leaf angle and enhanced primary branch number, grain size, 1,000-grain weight, and flag leaf size. Collectively, our results revealed that enhancing CK levels in the lamina joint by disrupting OsCKX3 negatively regulates leaf angle, highlighting that the CK pathway can be engineered to reduce leaf angle in rice and possibly in other cereals.

Overexpression of MsSAG113 gene promotes leaf senescence in alfalfa via participating in the hormone regulatory network

Introduction

Alfalfa (Medicago sativa) is a kind of high quality leguminous forage species, which was widely cultivated in the world. Leaf senescence is an essential process in plant development and life cycle. Here, we reported the isolation and functional analysis of an alfalfa SENESCENCE-ASSOCIATED GENE113 (MsSAG113), which belongs to the PP2C family and mainly plays a role in promoting plant senescence.

Methods

In the study, Agrobacterium-mediated, gene expression analysis, next generation sequencing, DNA pull-down, yeast single hybridization and transient expression were used to identify the function of MsSAG113 gene.

Results

The MsSAG113 gene was isolated from alfalfa, and the transgenic plants were obtained by Agrobacterium-mediated method. Compared with the wildtype, transgenic plants showed premature senescence in leaves, especially when cultivated under dark conditions. Meanwhile, application of exogenous hormones ABA, SA, MeJA, obviously acclerated leaf senescence of transgenic plants. Furthermore, the detached leaves from transgenic plants turned yellow earlier with lower chlorophyll content. Transcriptome analysis identified a total of 1,392 differentially expressed genes (DEGs), involving 13 transcription factor families. Of which, 234 genes were related to phytohormone synthesis, metabolism and transduction. Pull-down assay and yeast one-hybrid assay confirmed that alfalfa zinc finger CCCH domain-containing protein 39 (MsC3H-39) could directly bind the upstream of MsSAG113 gene. In conclusion, the MsSAG113 gene plays a crucial role in promoting leaf senescence in alfalfa via participating in the hormone regulatory network.

Discussion

This provides an essential basis for further analysis on the regulatory network involving senescence-associated genes in alfalfa.

Low red/far-red ratio can induce cytokinin degradation resulting in the inhibition of tillering in wheat (Triticum aestivum L.)

Shoot branching is inhibited by a low red/far-red ratio (R/FR). Prior studies have shown that the R/FR suppressed Arabidopsis thaliana branching by promotes bud abscisic acid (ABA) accumulation directly. Given that wheat tiller buds are wrapped in leaf sheaths and may not respond rapidly to a R/FR, systemic cytokinin (CTK) may be more critical. Here, systemic hormonal signals including indole-3-acetic acid (IAA), gibberellins (GA) and CTK and bud ABA signals in wheat were tested under a low R/FR. The results showed that a low R/FR reduced the percentage of tiller occurrence of tiller IV and the tiller number per plant. The low R/FR did not rapidly induced ABA accumulation in the tiller IV because of the protection of the leaf sheath and had little effect on IAA content and signaling in the tiller nodes. The significant change in the CTK levels was observed earlier than those of other hormone (ABA, IAA and GA) and exogenous cytokinin restored the CTK levels and tiller number per plant under low R/FR conditions. Further analysis revealed that the decrease in cytokinin levels was mainly associated with upregulation of cytokinin degradation genes (TaCKX5, TaCKX11) in tiller nodes. In addition, exposure to a decreased R/FR upregulated the expression of GA biosynthesis genes (TaGA20ox1, TaGA3ox2), resulting in elevated GA levels, which might further promote CTK degradation in tiller nodes and inhibit tillering. Therefore, our results provide evidence that the enhancement of cytokinin degradation is a novel mechanism underlying the wheat tillering response to a low R/FR.

Overexpression of the AGL42 gene in cotton delayed leaf senescence through downregulation of NAC transcription factors

Premature leaf senescence negatively influences the physiology and yield of cotton plants. The conserved IDLNL sequence in the C-terminal region of AGL42 MADS-box determines its repressor potential for the down regulation of senescence-related genes. To determine the delay in premature leaf senescence, Arabidopsis AGL42 gene was overexpressed in cotton plants. The absolute quantification of transgenic cotton plants revealed higher mRNA expression of AGL42 compared to that of the non-transgenic control. The spatial expression of GUS fused with AGL42 and the mRNA level was highest in the petals, abscission zone (flower and bud), 8 days post anthesis (DPA) fiber, fresh mature leaves, and senescenced leaves. The mRNA levels of different NAC senescence-promoting genes were significantly downregulated in AGL42 transgenic cotton lines than those in the non-transgenic control. The photosynthetic rate and chlorophyll content were higher in AGL42 transgenic cotton lines than those in the non-transgenic control. Fluorescence in situ hybridization of the AG3 transgenic cotton line revealed a fluorescent signal on chromosome 1 in the hemizygous form. Moreover, the average number of bolls in the transgenic cotton lines was significantly higher than that in the non-transgenic control because of the higher retention of floral buds and squares, which has the potential to improve cotton fiber yield.

The S40 family members delay leaf senescence by promoting cytokinin synthesis

Leaf senescence is regulated by both endogenous hormones and environmental stimuli in a programmed and concerted way. The members of the S40 family have been reported to play roles in leaf senescence. Here we identified an S40 family member, CiS40-11, from Caragana intermedia. Phylogenetic analysis revealed that the CiS40-11 protein had the highest identity with AtS40-5 (AT1G11700) and AtS40-6 (AT1G61930) of Arabidopsis thaliana. CiS40-11 was highly expressed in leaves and was down-regulated after dark treatment. The subcellular localization analysis showed that CiS40-11 was a cytoplasm-nucleus dual-localized protein. Leaf senescence was delayed in both the CiS40-11 overexpressed A. thaliana and its transiently expressed C. intermedia. Transcriptomic analysis and endogenous hormones assay revealed that CiS40-11 inhibited leaf senescence via promoting the biosynthesis of cytokinins by blocking AtMYB2 expression in the CiS40-11 overexpression lines. Furthermore, overexpression of either AtS40-5 or AtS40-6 showed similar phenotype as the CiS40-11 overexpressing lines, while in the ats40-5a or ats40-6a mutants, the AtMYB2 expression was increased and their leaves exhibited a premature senescence phenotype. These results provide a new molecular mechanism of the S40 family in leaf senescence regulation of plants.

NAC transcription factor TgNAP promotes tulip petal senescence

Petal senescence is a crucial determinant for ornamental quality and economic value of floral crops. Salicylic acid (SA) and reactive oxygen species (ROS) are two prominent factors involved in plant senescence regulation. In this study, tulip TgNAP (NAC-like, activated by APETALA3/PISTILLATA) was characterized as positively regulating tulip petal senescence through dually regulating SA biosynthesis and ROS detoxification pathways. TgNAP was upregulated in senescing petals of tulip while exogenous SA and H2O2 treatments substantially promoted petal senescence in tulip. Silencing of TgNAP by VIGS assay delayed SA and H2O2-induced petal senescence in tulip, whereas overexpression of TgNAP promoted the senescence process in Arabidopsis (Arabidopsis thaliana) plants. Additionally, inhibition of SA biosynthesis prolonged the lifespan of TgNAP-silenced petal discs. Further evidence indicated that TgNAP activates the transcriptions of two key SA biosynthetic genes ISOCHORISMATE SYNTHASE 1 (TgICS1) and PHENYLALANINE AMMONIA-LYASE 1 (TgPAL1) through directly binding to their promoter regions. Meanwhile, TgNAP repressed ROS scavenging by directly inhibiting PEROXIDASE 12 (POD12) and POD17 expression. Taken together, these results indicate that TgNAP enhances SA biosynthesis and ROS accumulation to positively regulate petal senescence in tulip.

Characterization of NAC transcription factor NtNAC028 as a regulator of leaf senescence and stress responses

NAC proteins constitute one of the largest transcription factor families and are involved in regulation of plant development and stress responses. Our previous transcriptome analyses of tobacco revealed a significant increase in the expression of NtNAC028 during leaf yellowing. In this study, we found that NtNAC028 was rapidly upregulated in response to high salinity, dehydration, and abscisic acid (ABA) stresses, suggesting a vital role of this gene in abiotic stress response. NtNAC028 loss-of-function tobacco plants generated via CRISPR-Cas9 showed delayed leaf senescence and increased tolerance to drought and salt stresses. Meanwhile NtNAC028 overexpression led to precocious leaf senescence and hypersensitivity to abiotic stresses in Arabidopsis, indicating that NtNAC028 functions as a positive regulator of natural leaf senescence and a negative regulator of stress tolerance. Furthermore, NtNAC028-overexpressing Arabidopsis plants showed lower antioxidant enzyme activities, higher reactive oxygen species (ROS), and H2O2 accumulation under high salinity, resulted in more severe oxidative damage after salt stress treatments. On the other hand, NtNAC028 mutation in tobacco resulted in upregulated expression of ROS-scavenging and abiotic stress-related genes, higher antioxidant enzyme activities, and enhanced tolerance against abiotic stresses, suggesting that NtNAC028 might act as a vital regulator for plant stress response likely by mediating ROS scavenging ability. Collectively, our results indicated that the NtNAC028 plays a key regulatory role in leaf senescence and response to multiple abiotic stresses.

Identification and Functional Analysis of Key Autophosphorylation Residues of Arabidopsis Senescence Associated Receptor-like Kinase

Reversible protein phosphorylation mediated by protein kinases and phosphatases plays important roles in the regulation of leaf senescence. We previously reported that the senescence-associated leucine-rich repeat receptor-like kinase AtSARK autophosphorylates on both serine/threonine and tyrosine residues and functions as a positive regulator of Arabidopsis leaf senescence; the senescence-suppressed protein phosphatase SSPP interacts with and dephosphorylates the cytoplasmic domain of AtSARK, thereby negatively regulating leaf senescence. Here, 27 autophosphorylation residues of AtSARK were revealed by mass spectrometry analysis, and six of them, including two Ser, two Thr, and two Tyr residues, were further found to be important for the biological functions of AtSARK. All site-directed mutations of these six residues that resulted in decreased autophosphorylation level of AtSARK could significantly inhibit AtSARK-induced leaf senescence. In addition, mutations mimicking the dephosphorylation form of Ser384 (S384A) or the phosphorylation form of Tyr413 (Y413E) substantially reduced the interaction between AtSARK and SSPP. All results suggest that autophosphorylation of AtSARK is essential for its functions in promoting leaf senescence. The possible roles of S384 and Y413 residues in fine-tuning the interaction between AtSARK and SSPP are discussed herein.

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.

Short daily ultraviolet exposure enhances intrinsic water-use efficiency and delays senescence in Micro-Tom tomato plants

Ultraviolet (UV) radiation, unless present at high doses, is recognised as a regulator of plant growth and some specific processes. The present study investigated the influence of short daily UV irradiation (15min/day, 11days) on leaf gas exchange and some biochemical and molecular markers of leaf senescence (such as stomata movements, chlorophyll breakdown, anthocyanin production, senescence-associated genes) in Micro-Tom tomato plants. The UV-induced reduction of g s (stomatal conductance) during the treatment was associated with the modified expression of some genes involved in the control of stomatal movements. We hypothesise a two-step regulation of stomatal closure involving salicylic and abscisic acid hormones. The temporal changes of g s and A net (net photosynthetic CO2 assimilation rate) along with the pigment behaviour, suggest a possible delay of leaf senescence in treated plants, confirmed by the expression levels of genes related to senescence such as SAG113 and DFR . The UV potential to induce a persistent partial inhibition of g s without severely affecting A net led to an increased iWUE (intrinsic water-use efficiency) during the 11-day treatment, suggesting a priming effect of short daily UV radiation towards drought conditions potentially useful in reducing the excess water use in agriculture.

New Advances in the Regulation of Leaf Senescence by Classical and Peptide Hormones

Leaf senescence is the last stage of leaf development, manifested by leaf yellowing due to the loss of chlorophyll, along with the degradation of macromolecules and facilitates nutrient translocation from the sink to the source tissues, which is essential for the plants' fitness. Leaf senescence is controlled by a sophisticated genetic network that has been revealed through the study of the molecular mechanisms of hundreds of senescence-associated genes (SAGs), which are involved in multiple layers of regulation. Leaf senescence is primarily regulated by plant age, but also influenced by a variety of factors, including phytohormones and environmental stimuli. Phytohormones, as important signaling molecules in plant, contribute to the onset and progression of leaf senescence. Recently, peptide hormones have been reported to be involved in the regulation of leaf senescence, enriching the significance of signaling molecules in controlling leaf senescence. This review summarizes recent advances in the regulation of leaf senescence by classical and peptide hormones, aiming to better understand the coordinated network of different pathways during leaf senescence.

A positive feedback regulatory loop, SA-AtNAP-SAG202/SARD1-ICS1-SA, in SA biosynthesis involved in leaf senescence but not defense response

Salicylic acid (SA) is an important plant hormone that regulates defense responses and leaf senescence. It is imperative to understand upstream factors that regulate genes of SA biosynthesis. SAG202/SARD1 is a key regulator for isochorismate synthase 1 (ICS1) induction and SA biosynthesis in defense responses. The regulatory mechanism of SA biosynthesis during leaf senescence is not well understood. Here we show that AtNAP, a senescence-specific NAC family transcription factor, directly regulates a senescence-associated gene named SAG202 as revealed in yeast one-hybrid and in planta assays. Inducible overexpreesion of AtNAP and SAG202 lead to high levels of SA and precocious senescence in leaves. Individual knockout mutants of sag202 and ics1 have markedly reduced SA levels and display a significantly delayed leaf senescence phenotype. Furthermore, SA positively feedback regulates AtNAP and SAG202. Our research has uncovered a unique positive feedback regulatory loop, SA-AtNAP-SAG202-ICS1-SA, that operates to control SA biosynthesis associated with leaf senescence but not defense response.

PlMYB308 Regulates Flower Senescence by Modulating Ethylene Biosynthesis in Herbaceous Peony

Herbaceous peony is an important cut-flower plant cultivated worldwide, but its short vase life substantially restricts its economic value. It is well established that endogenous hormones regulate the senescence process, but their molecular mechanism in flower senescence remains unclear. Here, we isolated a MYB transcription factor gene, PlMYB308, from herbaceous peony flowers, based on transcriptome data. Quantitative real-time PCR analysis showed that PlMYB308 is strongly up-regulated in senescing petals, and its expression was induced by abscisic acid or ethylene and reduced by gibberellin in petals. Treatment with abscisic acid or ethylene accelerated herbaceous peony petal senescence, and gibberellin delayed the process. PlMYB308 silencing delayed peony flower senescence and dramatically increased gibberellin, but reduced ethylene and abscisic acid levels in petals. PlMYB308 ectopic overexpression in tobacco accelerated flower senescence and reduced gibberellin, but increased ethylene and abscisic acid accumulation. Correspondingly, five endogenous hormone biosynthetic genes showed variable expression levels in petals after PlMYB308 silencing or overexpression. A dual-luciferase assay and yeast one-hybrid analysis showed that PlMYB308 specifically binds the PlACO1 promoter. Moreover, treatment with ethylene and 1-MCP can accelerate PlMYB308 silencing-reduced senescence and delay PlMYB308- overexpression-induced senescence. We also found that PlACO1 silencing delayed senescence in herbaceous peony petals. Taken together, our results suggest that the PlMYB308-PlACO1 regulatory checkpoints positively mediate the production of ethylene, and thus contribute to senescence in herbaceous peony flowers.

Dissecting the Molecular Regulation of Natural Variation in Growth and Senescence of Two Eutrema salsugineum Ecotypes

Salt cress (Eutrema salsugineum, aka Thellungiella salsuginea) is an extremophile and a close relative of Arabidopsis thaliana. To understand the mechanism of selection of complex traits under natural variation, we analyzed the physiological and proteomic differences between Shandong (SD) and Xinjiang (XJ) ecotypes. The SD ecotype has dark green leaves, short and flat leaves, and more conspicuous taproots, and the XJ ecotype had greater biomass and showed clear signs of senescence or leaf shedding with age. After 2-DE separation and ESI-MS/MS identification, between 25 and 28 differentially expressed protein spots were identified in shoots and roots, respectively. The proteins identified in shoots are mainly involved in cellular metabolic processes, stress responses, responses to abiotic stimuli, and aging responses, while those identified in roots are mainly involved in small-molecule metabolic processes, oxidation-reduction processes, and responses to abiotic stimuli. Our data revealed the evolutionary differences at the protein level between these two ecotypes. Namely, in the evolution of salt tolerance, the SD ecotype highly expressed some stress-related proteins to structurally adapt to the high salt environment in the Yellow River Delta, whereas the XJ ecotype utilizes the specialized energy metabolism to support this evolution of the short-lived xerophytes in the Xinjiang region.

One AP2/ERF Transcription Factor Positively Regulates Pi Uptake and Drought Tolerance in Poplar

Drought decreases the inorganic phosphate (Pi) supply of soil, resulting in Pi starvation of plants, but the molecular mechanism of how plants, especially the perennial trees, are tolerant to drought stress and Pi starvation, is still elusive. In this study, we identified an AP2/ERF transcription factor gene, PalERF2, from Populus alba var. pyramidalis, and it was induced by both mannitol treatment and Pi starvation. Overexpressing and knocking-down of PalERF2 both enhanced and attenuated tolerance to drought stress and Pi deficiency compared to WT, respectively. Moreover, the overexpression of PalERF2 up-regulated the expression levels of Pi starvation-induced (PSI) genes and increased Pi uptake under drought conditions; however, its RNAi poplar showed the opposite phenotypes. Subsequent analysis indicated that PalERF2 directly modulated expressions of drought-responsive genes PalRD20 and PalSAG113, as well as PSI genes PalPHL2 and PalPHT1;4, through binding to the DRE motifs on their promoters. These results clearly indicate that poplars can recruit PalERF2 to increase the tolerance to drought and also elevate Pi uptake under drought stress.

CitWRKY28 and CitNAC029 promote the synthesis of cuticular wax by activating CitKCS gene expression in citrus fruit

CitWRKY28 and CitNAC029 are involved in cuticular wax synthesis as indicated by the comparative analysis of fruit aliphatic wax content between Citrus reticulata and Citrus trifoliata and gene co-expression analysis. Cuticular wax covers the fruit surface, playing important roles in reduction of fruit water loss and resistance to pathogen invasion. However, there is limited research on the synthesis and transcriptional regulation of cuticular wax in citrus fruit. In this study, we characterized the variations of aliphatic wax in HJ (Citrus reticulata) and ZK (Citrus trifoliata) from young fruit to mature fruit, as well as performed transcriptome sequencing on 27 samples at different fruit developmental stages. The results revealed that the ZK fruit always had a higher aliphatic wax content than the HJ fruit during development. qRT-PCR analysis demonstrated that two KCS genes, CitKCS1 and CitKCS12, had the most significant difference in expression between HJ and ZK. Furthermore, a heterologous expression assay in Arabidopsis indicated that CitKCS1 and CitKCS12 are involved in cuticular wax synthesis. Subsequently, gene co-expression network analysis screened CitWRKY28 and CitNAC029. Dual luciferase and EMSA assays indicated that CitWRKY28 might bind to the promoter of CitKCS1 and CitKCS12 and CitNAC029 might bind to that of CitKCS1 to activate their expression. Moreover, CitWRKY28 and CitNAC029 could promote the accumulation of cuticular wax in Arabidopsis leaves. Our findings provide new insights into the synthesis and regulation of cuticular wax and valuable information for further mining of wax-related genes in citrus fruit.

Transcriptome profiling of Arabidopsis thaliana roots in response to allelopathic effects of Conyza canadensis

The molecular mechanisms underlying allelopathy and their role in the interactions between invasive weeds and native species remain unclear. In this study, we aimed to explore the physiological and molecular response of plant roots of a native species to allelopathy from an invasive weed. We examined the growth and development of roots of native Arabidopsis thaliana for a 2-week period after being treated with aqueous extracts at different concentrations from invasive Conyza canadensis. Extracts with higher concentration in the Murashige and Skoog (MS) media (i.e., 4 mg of extract/mL of MS) significantly affected the root growth of A. thaliana. Roots of A. thaliana displayed weakened root tip activity and an accumulation of reactive oxygen species (ROS) in response to extracts from C. canadensis. The transcriptome analysis of A. thaliana roots exposed to phytotoxicity revealed differentially expressed genes (DEGs) involved in cell wall formation, abiotic stress, transporter genes and signal transduction. We found that genes associated with nutrient transport, such as major facilitator superfamily (MFS) and amino acid permease (AAP3) transporters as well as genes involved in stress response, including leucine-rich repeat receptor-like protein kinases (LRR-RLKs) were down-regulated. In addition, we found that many transcription factors associated with plant stress (such as APETALA2/ethylene response factors) were up-regulated while others (e.g., zinc-finger proteins) were down-regulated. Allelochemicals from C. canadensis also induced the up-regulation of detoxification (DTX) genes, ROS related genes, calcineurin B-like interacting protein kinases (CIPKs) and calmodulin. Overall, our findings provided insights into allelopathy in C. canadensis at the molecular level, and contributes to our understanding of invasion mechanisms of alien plant species. CLINICAL TRIALS REGISTRATION: This study does not contain any studies with clinical trials performed by any of the authors.

Involvement of NAC transcription factor NaNAC29 in Alternaria alternata resistance and leaf senescence in Nicotiana attenuata

NAC-LIKE, ACTIVATED BY AP3/PI (NAP) is a NAC transcription factor regulating leaf senescence in Arabidopsis thaliana. In wild tobacco Nicotiana attenuata, a nuclear localized NAC transcription factor NaNAC29 was identified to be highly elicited after inoculation of Alternaria alternata, a notorious necrotic fungus on tobacco species. The NaNAC29 possesses similar tertiary structure to NAP with 60% amino acid identity. However, it remains unknown the role of NaNAC29 in plant defense responses to A. alternata and leaf senescence in N. attenuata. In this paper, Defensin-like protein 1 (NaDLP1) was highly induced in N. attenuata after A. alternata inoculation and bigger lesions were developed in NaDLP1-silenced plants. Interestingly, A. alternata-induced NaDLP1 was reduced by 76% in VIGS NaNAC29 plants and by 61% in JA deficient irAOC plants at 3 days post inoculation. The regulation of NaDLP1 expression by NaNAC29 was clearly independent on JA pathway, since exogenous methyl jasmonate treatment could not complement the induction levels of NaDLP1 in NaNAC29-silenced plants to the levels in WT plants. Otherwise, the expression of NaNAC29 was low expressed in young leaves but highly in senescent leaves and dark-treated leaves. NaNAC29-silenced plants, which were generated by virus-induced gene silencing (VIGS NaNAC29), showed delayed senescence phenotype. In addition, constitutive over-expression of NaNAC29 in A. thaliana could rescue the delayed-senescence phenotype of nap and caused precocious leaf senescence of wild-type Col-0 plants. All the data above demonstrate that NaNAC29 is a NAP homolog in N. attenuata participating in the defense responses to A. alternata by regulation of a defensin protein NaDLP1 and promoting leaf senescence.

The leaf senescence-promoting transcription factor AtNAP activates its direct target gene CYTOKININ OXIDASE 3 to facilitate senescence processes by degrading cytokinins

Cytokinins (CKs) are a class of adenine-derived plant hormones that plays pervasive roles in plant growth and development including cell division, morphogenesis, lateral bud outgrowth, leaf expansion and senescence. CKs as a "fountain of youth" prolongs leaf longevity by inhibiting leaf senescence, and therefore must be catabolized for senescence to occur. AtNAP, a senescence-specific transcription factor has a key role in promoting leaf senescence. The role of AtNAP in regulating CK catabolism is unknown. Here we report the identification and characterization of AtNAP-AtCKX3 (cytokinin oxidase 3) module by which CKs are catabolized during leaf senescence in Arabidopsis. Like AtNAP, AtCKX3 is highly upregulated during leaf senescence. When AtNAP is chemically induced AtCKX3 is co-induced; and when AtNAP is knocked out, the expression of AtCKX3 is abolished. AtNAP physically binds to the cis element of the AtCKX3 promoter to direct its expression as revealed by yeast one-hybrid assays and in planta experiments. Leaves of the atckx3 knockout lines have higher CK concentrations and a delayed senescence phenotype compared with those of WT. In contrast, leaves with inducible expression of AtCKX3 have lower CK concentrations and exhibit a precocious senescence phenotype compared with WT. This research reveals that AtNAP transcription factor-AtCKX3 module regulates leaf senescence by connecting two antagonist plant hormones abscisic acid and CKs.

Regulation of rose petal dehydration tolerance and senescence by RhNAP transcription factor via the modulation of cytokinin catabolism

Petals and leaves share common evolutionary origins but have different phenotypic characteristics, such as the absence of stomata in the petals of most angiosperm species. Plant NAC transcription factor, NAP, is involved in ABA responses and regulates senescence-associated genes, and especially those that affect stomatal movement. However, the regulatory mechanisms and significance of NAP action in senescing astomatous petals is unclear. A major limiting factor is failure of flower opening and accelerated senescence. Our goal is to understand the finely regulatory mechanism of dehydration tolerance and aging in rose flowers. We functionally characterized RhNAP, an AtNAP-like transcription factor gene that is induced by dehydration and aging in astomatous rose petals. Cytokinins (CKs) are known to delay petal senescence and we found that a cytokinin oxidase/dehydrogenase gene 6 (RhCKX6) shares similar expression patterns with RhNAP. Silencing of RhNAP or RhCKX6 expression in rose petals by virus induced gene silencing markedly reduced petal dehydration tolerance and delayed petal senescence. Endogenous CK levels in RhNAP- or RhCKX6-silenced petals were significantly higher than those of the control. Moreover, RhCKX6 expression was reduced in RhNAP-silenced petals. This suggests that the expression of RhCKX6 is regulated by RhNAP. Yeast one-hybrid experiments and electrophoresis mobility shift assays showed that RhNAP binds to the RhCKX6 promoter in heterologous in vivo system and in vitro, respectively. Furthermore, the expression of putative signal transduction and downstream genes of ABA-signaling pathways were also reduced due to the repression of PP2C homolog genes by RhNAP in rose petals. Taken together, our study indicates that the RhNAP/RhCKX6 interaction represents a regulatory step enhancing dehydration tolerance in young rose petals and accelerating senescence in mature petals in a stomata-independent manner.

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.

Leaf senescence: progression, regulation, and application

Leaf senescence, the last stage of leaf development, is a type of postmitotic senescence and is characterized by the functional transition from nutrient assimilation to nutrient remobilization which is essential for plants' fitness. The initiation and progression of leaf senescence are regulated by a variety of internal and external factors such as age, phytohormones, and environmental stresses. Significant breakthroughs in dissecting the molecular mechanisms underpinning leaf senescence have benefited from the identification of senescence-altered mutants through forward genetic screening and functional assessment of hundreds of senescence-associated genes (SAGs) via reverse genetic research in model plant Arabidopsis thaliana as well as in crop plants. Leaf senescence involves highly complex genetic programs that are tightly tuned by multiple layers of regulation, including chromatin and transcription regulation, post-transcriptional, translational and post-translational regulation. Due to the significant impact of leaf senescence on photosynthesis, nutrient remobilization, stress responses, and productivity, much effort has been made in devising strategies based on known senescence regulatory mechanisms to manipulate the initiation and progression of leaf senescence, aiming for higher yield, better quality, or improved horticultural performance in crop plants. This review aims to provide an overview of leaf senescence and discuss recent advances in multi-dimensional regulation of leaf senescence from genetic and molecular network perspectives. We also put forward the key issues that need to be addressed, including the nature of leaf age, functional stay-green trait, coordination between different regulatory pathways, source-sink relationship and nutrient remobilization, as well as translational researches on leaf senescence.

PHYTOCHROME INTERACTING FACTORS PIF4 and PIF5 promote heat stress induced leaf senescence in Arabidopsis

Leaf senescence can be triggered by multiple abiotic stresses including darkness, nutrient limitation, salinity, and drought. Recently, heatwaves have been occurring more frequently, and they dramatically affect plant growth and development. However, the underlying molecular networks of heat stress-induced leaf senescence remain largely uncharacterized. Here we showed that PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 proteins could efficiently promote heat stress-induced leaf senescence in Arabidopsis. Transcriptomic profiling analysis revealed that PIF4 and PIF5 are likely to function through multiple biological processes including hormone signaling pathways. Further, we characterized NAC019, SAG113, and IAA29 as direct transcriptional targets of PIF4 and PIF5. The transcription of NAC019, SAG113, and IAA29 changes significantly in daytime after heat treatment. In addition, we demonstrated that PIF4 and PIF5 proteins were accumulated during the recovery after heat treatment. Moreover, we showed that heat stress-induced leaf senescence is gated by the circadian clock, and plants might be more actively responsive to heat stress-induced senescence during the day. Taken together, our findings proposed important roles for PIF4 and PIF5 in mediating heat stress-induced leaf senescence, which may help to fully illustrate the molecular network of heat stress-induced leaf senescence in higher plants and facilitate the generation of heat stress-tolerant crops.

CmNAC73 Mediates the Formation of Green Color in Chrysanthemum Flowers by Directly Activating the Expression of Chlorophyll Biosynthesis Genes HEMA1 and CRD1

Chrysanthemum is one of the most beautiful and popular flowers in the world, and the flower color is an important ornamental trait of chrysanthemum. Compared with other flower colors, green flowers are relatively rare. The formation of green flower color is attributed to the accumulation of chlorophyll; however, the regulatory mechanism of chlorophyll metabolism in chrysanthemum with green flowers remains largely unknown. In this study, we performed Illumina RNA sequencing on three chrysanthemum materials, Chrysanthemum vestitum and Chrysanthemum morifolium cultivars ‘Chunxiao’ and ‘Green anna’, which produce white, light green and dark green flowers, respectively. Based on the results of comparative transcriptome analysis, a gene encoding a novel NAC family transcription factor, CmNAC73, was found to be highly correlated to chlorophyll accumulation in the outer whorl of ray florets in chrysanthemum. The results of transient overexpression in chrysanthemum leaves showed that CmNAC73 acts as a positive regulator of chlorophyll biosynthesis. Furthermore, transactivation and yeast one-hybrid assays indicated that CmNAC73 directly binds to the promoters of chlorophyll synthesis-related genes HEMA1 and CRD1. Thus, this study uncovers the transcriptional regulation of chlorophyll synthesis-related genes HEMA1 and CRD1 by CmNAC73 and provides new insights into the development of green flower color in chrysanthemum and chlorophyll metabolism in plants.