Living to Die and Dying to Live: The Survival Strategy behind Leaf Senescence

Nitrogen deposition favors later leaf senescence in woody species

China has experienced an unprecedented increase in nitrogen deposition over recent decades, threatening ecosystem structure, functioning, and resilience. However, the impact of elevated nitrogen deposition on the date of foliar senescence remains widely unexplored. Using 22,780 in situ observations and long-term satellite-based date of foliar senescence measures for woody species across China, we find that increased nitrogen deposition generally delays date of foliar senescence, with strong causal evidence observed at site-to-region scales. Changes in climate conditions and nitrogen deposition levels jointly controlled the direction of date of foliar senescence trends (advance or delay). The spatial variability of nitrogen deposition effects can be related to plant traits (e.g., nitrogen resorption and use efficiencies), climatic conditions, and soil properties. Moreover, elevated nitrogen deposition delays date of foliar senescence by promoting foliar expansion and enhancing plant productivity during the growing season, while its influence on evapotranspiration may either accelerate or delay date of foliar senescence depending on local water availability. This study highlights the critical role of nitrogen deposition in regulating date of foliar senescence trends, revealing a key uncertainty in modeling date of foliar senescence driven solely by climate change and its far-reaching implications for ecosystem-climate feedbacks.

Influence of Summer Drought on Post-Drought Resprouting and Leaf Senescence in Prunus spinosa L. Growing in a Common Garden

Understanding how woody plants cope with severe water shortages is critical, especially for regions where droughts are becoming more frequent and intense. We studied the effects of drought intensity, focusing on post-drought resprouting, autumn leaf senescence and the subsequent spring bud burst. Furthermore, we aimed to study population differentiation in the drought and post-drought responses. We performed a summer dry-out experiment in a common garden of potted Prunus spinosa L. (Rosaceae) saplings. We analysed responses across different visual stress symptom categories and examined differentiation between provenances from a local origin (Western Europe, Belgium), a lower latitude (Spain) and a higher latitude (Sweden). The chance of post-drought resprouting was greater for the more severely affected plants than for the less severely affected ones, and it occurred earlier. The plants that displayed wilting of the leaves during the drought had a leaf senescence 2.7 days earlier than the controls, whereas that of plants with 25 to 75% and more than 75% of desiccated leaves was 7 and 15 days later, respectively. During the drought, the local provenance was the first to develop visual symptoms compared to the other two provenances. However, among plants that exhibited no or only mild symptoms, this provenance also had a higher likelihood of post-drought resprouting. Among the control plants, the higher-latitude provenance displayed leaf senescence earlier, while the lower-latitude provenance senesced later compared to the local provenance. However, these differences in the timing of leaf senescence among the three provenances disappeared in treated plants with more than 25% of desiccated leaves due to the drought. Whereas leaf senescence could be earlier or later depending on the developed drought symptoms, the timing of bud burst was only delayed. Results indicate that resprouting and timing of leaf senescence are responsive to the severity of the experienced drought in a provenance-dependent way.

FaNAC047-FaNAC058 module coordinately promotes chlorophyll degradation and reactive oxygen species production during heat-induced leaf senescence in tall fescue

Leaf senescence can be triggered by various abiotic stresses. Among these, heat stress emerges as a pivotal environmental factor, particularly in light of the predicted rise in global temperatures. However, the molecular mechanism underlying heat-induced leaf senescence remains largely unexplored. As a cool-season grass species, tall fescue (Festuca arundinacea) is an ideal and imperative material for investigating heat-induced leaf senescence because heat stress easily triggers leaf senescence to influence its forage yield and turf quality. Here, we investigated the role of FaNAC047 in heat-induced leaf senescence. Overexpression of FaNAC047 promoted heat-induced leaf senescence in transgenic tall fescue that was evidenced by a more seriously destructive photosystem and higher accumulation of reactive oxygen species (ROS), whereas knockdown of FaNAC047 delayed leaf senescence. Further protein-DNA interaction assays indicated that FaNAC047 directly activated the transcriptions of NON-YELLOW COLORING 1 (FaNYC1), NYC1-like (FaNOL), and STAY-GREEN (FaSGR) but directly inhibited Catalases 2 (FaCAT2) expression, thereby promoting chlorophyll degradation and ROS accumulation. Subsequently, protein-protein interaction assays revealed that FaNAC047 physically interacted with FaNAC058 to enhance its regulatory effect on FaNYC1, FaNOL, FaSGR, and FaCAT2. Additionally, FaNAC047 could transcriptionally activate FaNAC058 expression to form a regulatory cascade, driving senescence progression. Consistently, the knockdown of FaNAC058 significantly delayed heat-induced leaf senescence. Collectively, our results reveal that FaNAC047-FaNAC058 module coordinately mediates chlorophyll degradation and ROS production to positively regulate heat-induced leaf senescence. The findings illustrate the molecular network of heat-induced leaf senescence for breeding heat-resistant plants.

CmHRE2L-CmACS6 transcriptional cascade negatively regulates waterlogging tolerance in Chrysanthemum

The role of ethylene as an initial signaling molecule in waterlogging stress is well-established. However, the complex molecular mechanisms underlying ethylene biosynthesis and its functional significance in chrysanthemums under waterlogging conditions have remained unclear. In this study, we observed an increase in the expression of 1-aminocyclopropane-1-carboxylate synthase 6 (CmACS6), which encodes a key enzyme responsible for ethylene biosynthesis, in response to waterlogging. This elevation increases ethylene production, induces leaf chlorosis, and enhances the chrysanthemum's sensitivity to waterlogging stress. Moreover, our analysis of upstream regulators revealed that the expression of CmACS6, in response to waterlogging, is directly upregulated by CmHRE2-like (Hypoxia Responsive ERF-like, CmHRE2L), an ethylene response factor. Notably, CmHRE2-L binds directly to the GCC-like motif in the promoter region of CmACS6. Genetic validation assays demonstrated that CmHRE2L was induced by waterlogging and contributed to ethylene production, consequently reducing waterlogging tolerance in a partially CmACS6-dependent manner. This study identified the regulatory module involving CmHRE2L and CmACS6, which governs ethylene biosynthesis in response to waterlogging stress.

Physiological and transcriptional reprogramming for salinity tolerance of endangered mangrove associate Hernandia nymphaeifolia

BACKGROUND

Hernandia nymphaeifolia is a typical mangrove associate with high ecological, ornamental, and medicinal values, but in China, it has become endangered in recent years, and an urgent protection is needed. Salinity is a key factor for growth and survival of mangrove seedlings, and thus a comprehensive understanding of salt tolerance in mangroves is important for their conservation and afforestation. However, little is known regarding salt-responsive mechanisms in H. nymphaeifolia.

RESULTS

In this study, we posed gradient salt treatments on H. nymphaeifolia seedlings and investigated their physiological and transcriptional reprogramming in response to salinity stress. The results revealed that hyper-salinity stress adversely impacted on leaf growth, cell integrity and photosynthetic performance of H. nymphaeifolia seedlings than those growing in fresh water or low salt conditions, mirroring its moderate salinity tolerance as a mangrove associate. Genes involved in osmotic sensing and regulation, reactive oxygen species (ROS) scavenging and ion homeostasis were differentially expressed to alleviate the destructive effects. Furthermore, our results identified some kinase-encoding genes as hub genes in co-expression networks, which may play a key role in regulating the synergistic expression of salt-responsive genes upon stress conditions.

CONCLUSION

This research enriches our knowledge of the molecular mechanisms underlying the salinity tolerance of mangrove associates, which can theoretically assist the conservation and restoration of H. nymphaeifolia. Our findings also provide valuable genetic resources for future potential bioengineering applications in the fields of agriculture and forestry.

Unraveling Morphological, Physiological, and Transcriptomic Alterations Underlying the Formation of Little Leaves in Phytoplasma-Infected Sweet Cherry Trees

Phytoplasmas are minute phytopathogenic bacteria that induce excessive vegetative growth, known as witches' broom (WB), in many infected plant species during the later stages of infection. The WB structure is characterized by densely clustered little (small) leaves, which are frequently accompanied by chlorosis (yellowing). The mechanisms behind the formation of little leaves within WB structures (LL-WB) are poorly understood. To address this gap, the LL-WB formation was extensively studied using sweet cherry virescence (SCV) phytoplasma-infected sweet cherry plants. Based on morphological examinations, signs of premature leaf senescence were observed in LL-WB samples, including reduced leaf size, chlorosis, and alterations in shape. Subsequent physiological analyses indicated decreased sucrose and glucose levels and changes in hormone concentrations in LL-WB samples. Additionally, the transcriptomic analysis revealed impaired ribosome biogenesis and DNA replication. As an essential process in protein production, the compromised ribosome biogenesis and the inhibited DNA replication led to cell cycle arrest, thus affecting leaf morphogenesis and further plant development. Moreover, the expression of marker genes involved in premature leaf senescence was significantly altered. These results indicate a complicated interplay between the development of leaves, premature leaf senescence, and pathogen-induced stress responses in SCV phytoplasma-infected sweet cherry trees. The results of this study provide insight into understanding the underlying molecular mechanisms driving the formation of little leaves and interactions between plants and pathogens. The findings might help control phytoplasma diseases in sweet cherry cultivation.

Metabolic strategies in hypoxic plants

Complex multicellular organisms have evolved in an oxygen-enriched atmosphere. Oxygen is therefore essential for all aerobic organisms, including plants, for energy production through cellular respiration. However, plants can experience hypoxia following extreme flooding events and also under aerated conditions in proliferative organs or tissues characterized by high oxygen consumption. When oxygen availability is compromised, plants adopt different strategies to cope with hypoxia and limited aeration. A common feature among different plant species is the activation of an anaerobic fermentative metabolism to provide ATP to maintain cellular homeostasis under hypoxia. Fermentation also requires many sugar substrates, which is not always feasible, and alternative metabolic strategies are thus needed. Recent findings have also shown that the hypoxic metabolism is also active in specific organs or tissues of the plant under aerated conditions. Here, we describe the regulatory mechanisms that control the metabolic strategies of plants and how they enable them to thrive despite challenging conditions. A comprehensive mechanistic understanding of the genetic and physiological components underlying hypoxic metabolism should help to provide opportunities to improve plant resilience under the current climate change scenario.

Integrating gene expression analysis and ecophysiological responses to water deficit in leaves of tomato plants

Soil water deficit (WD) significantly impacts plant survival and crop yields. Many gaps remain in our understanding of the synergistic coordination between molecular and ecophysiological responses delaying substantial drought-induced effects on plant growth. To investigate this synergism in tomato leaves, we combined molecular, ecophysiological, and anatomical methods to examine gene expression patterns and physio-anatomical characteristics during a progressing WD experiment. Four sampling points were selected for transcriptomic analysis based on the key ecophysiological responses of the tomato leaves: 4 and 5 days after WD (d-WD), corresponding to 10% and 90% decrease in leaf stomatal conductance; 8 d-WD, the leaf wilting point; and 10 d-WD, when air embolism blocks 12% of the leaf xylem water transport. At 4 d-WD, upregulated genes were mostly linked to ABA-independent responses, with larger-scale ABA-dependent responses occurring at 5 d-WD. At 8 d-WD, we observed an upregulation of heat shock transcription factors, and two days later (10 d-WD), we found a strong upregulation of oxidative stress transcription factors. Finally, we found that young leaves present a stronger dehydration tolerance than mature leaves at the same drought intensity level, presumably because young leaves upregulate genes related to increased callose deposition resulting in limiting water loss to the phloem, and related to increased cell rigidity by modifying cell wall structures. This combined dataset will serve as a framework for future studies that aim to obtain a more holistic WD plant response at the molecular, ecophysiological and anatomical level.

Influence of stem and leaf phenotypes, physiological responses and cellular ultrastructure on defoliated sugarcane cultivars

Defoliation is a primary agronomic traits, its variation depends on different plant species or cultivars. The present article assess the leaf morphological responses, oxidative metabolites and enzymatic activities at sheath base of sugarcane cultivars during defoliation stage of plant leaves. The mature leaf sheath of GT47 strongly wrapped to the stem, and no stem was exposed. The upper and lower edges of the immature fusing abscission zone were parallel, and slightly lower browning area (+ 3 to + 7 leaf position). The ROC22 cultivar was monitored highest leaf sheath-based cellulose and lignin content, followed by GT60 and GT47. Peroxidase activity was higher in leaf sheath base edge (ROC22) as compare to other cultivars. The malondialdehyde content was found highest in GT60, followed by ROC22, and GT47. The exo-β-1,4-glucanase/ cellobiohydrolase activity was found highest in the margin of GT47 than lateral and medial axis of ROC22 and GT60. The axis activity increased exponentially, and ROC22 gradually decreased from the periphery of the mid-axis and lower than GT47 and GT60 in the lateral and mid-axis of leaf. In conclusion, the mature leaves are easy to defoliate mainly loose leaf sheaths, large leaf sheath inclination angles, more deformation during the growth period of the abscission zone, early with large cracks, and slow browning process. Leaf sheaths with high fibre and lignin content showed significant hardness and thickness. The sugarcane cultivars showed positive correlation between peroxidase and malondialdehyde content with the browning process at the base of mature leaf sheaths.

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

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

Root Cortical Senescence Enhances Drought Tolerance in Cotton

The root cortical senescence (RCS) is closely associated with root absorptive function. However, characteristics and responses of RCS to drought stress in cotton have received little attention. This study subjected the drought-tolerant variety 'Guoxin 02' and the drought-sensitive variety 'Ji 228' to drought stress (8% PEG6000) and no-stress (0% PEG6000) treatments to determine the characteristics and responses of cotton RCS to drought stress. The results showed that the greater the distance from the root tip, the more severe the RCS occurrence under drought stress compared with non-stress treatment. The occurrence of RCS in 'Guoxin 02' increased by 14.03%-20.18% compared to 'Ji 228' under drought stress. The RCS was negatively correlated with root respiration but positively correlated with root length and leaf water potential. The silencing of RCS-related genes (GhSAG12 and GhbHLH121) can mitigate the drought-induced RCS phenomenon in cotton; however, reduced drought tolerance. Exogenous abscisic acid (ABA) treatment can promote RCS generation. Conversely, ABA synthesis exhibits contrasting effects. In summary, endogenous hormones regulated RCS, which reduced the root metabolic and seemingly achieved more resource redistribution to new roots, thereby fully utilize deep water resources. Thus, the study demonstrates the potential of RCS in improving the drought stress tolerance of cotton.

The atypical Dof transcriptional factor OsDes1 contributes to stay-green, grain yield, and disease resistance in rice

The regulation of leaf senescence and disease resistance plays a crucial role in determining rice grain yield and quality, which are important to meet the ever-increasing food demands of the world. Here, we identified an atypical Dof transcriptional factor OsDes1 that contributes to the stay-green phenotype, grain yield, and disease resistance in rice. The expression level of OsDes1 is positively associated with stay-green in natural variations of japonica rice, suggesting that OsDes1 would be alternatively used in breeding programs. Mechanistically, OsDes1 targets the promoter of the Rieske FeS protein gene OsPetC to activate its expression and interacts with OsPetC to protect against its degradation, thus promoting stay-green and ultimately improving the grain yield. OsDes1 also binds to the promoter region of defense-related genes, such as OsPR1b, and activates their expression, leading to enhanced disease resistance. These findings offer a potential strategy for breeding rice to enhance grain yield and disease resistance.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Drought resistance strategies in minor millets: a review

MAIN CONCLUSION

The review discusses growth and drought-response mechanisms in minor millets under three themes: drought escape, drought avoidance and drought tolerance. Drought is one of the most prominent abiotic stresses impacting plant growth, performance, and productivity. In the context of climate change, the prevalence and severity of drought is expected to increase in many agricultural regions worldwide. Millets (coarse grains) are a group of small-seeded grasses cultivated in arid and semi-arid regions throughout the world and are an important source of food and feed for humans and livestock. Although minor millets, i.e., foxtail millet, finger millet, proso millet, barnyard millet, kodo millet and little millet are generally hardier and more drought-resistant than cereals and major millets (sorghum and pearl millet), understanding their responses, processes and strategies in response to drought is more limited. Here, we review drought resistance strategies in minor millets under three themes: drought escape (e.g., short crop cycle, short vegetative period, developmental plasticity and remobilization of assimilates), drought avoidance (e.g., root traits for better water absorption and leaf traits to control water loss), and drought tolerance (e.g., osmotic adjustment, maintenance of photosynthetic ability and antioxidant potential). Data from 'omics' studies are summarized to provide an overview of the molecular mechanisms important in drought tolerance. In addition, the final section highlights knowledge gaps and challenges to improving minor millets. This review is intended to enhance major cereals and millet per se in light of climate-related increases in aridity.

Seasonal switching of integrated leaf senescence controls in an evergreen perennial Arabidopsis

Evergreeness is a substantial strategy for temperate and boreal plants and is as common as deciduousness. However, whether evergreen plants switch foliage functions between seasons remains unknown. We conduct an in natura study of leaf senescence control in the evergreen perennial, Arabidopsis halleri. A four-year census of leaf longevity of 102 biweekly cohorts allows us to identify growth season (GS) and overwintering (OW) cohorts characterised by short and extended longevity, respectively, and to recognise three distinct periods in foliage functions, i.e., the growth, overwintering, and reproductive seasons. Photoperiods during leaf expansion separate the GS and OW cohorts, providing primal control of leaf senescence depending on the season, with leaf senescence being shut down during winter. Phenotypic and transcriptomic responses in field experiments indicate that shade-induced and reproductive-sink-triggered senescence are active during the growth and reproductive seasons, respectively. These secondary controls of leaf senescence cause desynchronised and synchronised leaf senescence during growth and reproduction, respectively. Conclusively, seasonal switching of leaf senescence optimises resource production, storage, and translocation for the season, making the evergreen strategy adaptively relevant.

Leaf gene expression trajectories during the growing season are consistent between sites and years in American beech

Transcriptomics provides a versatile tool for ecological monitoring. Here, through genome-guided profiling of transcripts mapping to 33 042 gene models, expression differences can be discerned among multi-year and seasonal leaf samples collected from American beech trees at two latitudinally separated sites. Despite a bottleneck due to post-Columbian deforestation, the single nucleotide polymorphism-based population genetic background analysis has yielded sufficient variation to account for differences between populations and among individuals. Our expression analyses during spring-summer and summer-autumn transitions for two consecutive years involved 4197 differentially expressed protein coding genes. Using Populus orthologues we reconstructed a protein-protein interactome representing leaf physiological states of trees during the seasonal transitions. Gene set enrichment analysis revealed gene ontology terms that highlight molecular functions and biological processes possibly influenced by abiotic forcings such as recovery from drought and response to excess precipitation. Further, based on 324 co-regulated transcripts, we focused on a subset of GO terms that could be putatively attributed to late spring phenological shifts. Our conservative results indicate that extended transcriptome-based monitoring of forests can capture diverse ranges of responses including air quality, chronic disease, as well as herbivore outbreaks that require activation and/or downregulation of genes collectively tuning reaction norms maintaining the survival of long living trees such as the American beech.

Phosphorylation of Thr-225 and Ser-262 on ERD7 Promotes Age-Dependent and Stress-Induced Leaf Senescence through the Regulation of Hydrogen Peroxide Accumulation in Arabidopsis thaliana

As the final stage of leaf development, leaf senescence is affected by a variety of internal and external signals including age and environmental stresses. Although significant progress has been made in elucidating the mechanisms of age-dependent leaf senescence, it is not clear how stress conditions induce a similar process. Here, we report the roles of a stress-responsive and senescence-induced gene, ERD7 (EARLY RESPONSIVE TO DEHYDRATION 7), in regulating both age-dependent and stress-induced leaf senescence in Arabidopsis. The results showed that the leaves of erd7 mutant exhibited a significant delay in both age-dependent and stress-induced senescence, while transgenic plants overexpressing the gene exhibited an obvious accelerated leaf senescence. Furthermore, based on the results of LC-MS/MS and PRM quantitative analyses, we selected two phosphorylation sites, Thr-225 and Ser-262, which have a higher abundance during senescence, and demonstrated that they play a key role in the function of ERD7 in regulating senescence. Transgenic plants overexpressing the phospho-mimetic mutant of the activation segment residues ERD7T225D and ERD7T262D exhibited a significantly early senescence, while the inactivation segment ERD7T225A and ERD7T262A displayed a delayed senescence. Moreover, we found that ERD7 regulates ROS accumulation by enhancing the expression of AtrbohD and AtrbohF, which is dependent on the critical residues, i.e., Thr-225 and Ser-262. Our findings suggest that ERD7 is a positive regulator of senescence, which might function as a crosstalk hub between age-dependent and stress-induced leaf senescence.

Non-targeted discovery of high-value bio-products in Nicotiana glauca L: a potential renewable plant feedstock

The evaluation of plant-based feedstocks is an important aspect of biorefining. Nicotiana glauca is a solanaceous, non-food crop that produces large amounts of biomass and is well adapted to grow in suboptimal conditions. In the present article, compatible sequential solvent extractions were applied to N. glauca leaves to enable the generation of enriched extracts containing higher metabolite content comparing to direct leaf extracts. Typically, between 60 to 100 metabolite components were identified within the fractions. The occurrence of plant fatty acids, fatty acid alcohols, alkanes, sterols and terpenoids was detected by gas liquid chromatography-mass spectrometry (GC-MS) and metabolite identification was confirmed by comparison of physico-chemical properties displayed by available authentic standards. Collectively, co-products such waxes, oils, fermentable sugars, and terpenoids were all identified and quantified. The enriched fractions of N. glauca revealed a high level of readily extractable hydrocarbons, oils and high value co-products. In addition, the saccharification yield and cell wall composition analyses in the stems revealed the potential of the residue material as a promising lignocellulosic substrate for the production of fermentable sugars. In conclusion a multifractional cascade for valuable compounds/commodities has been development, that uses N. glauca biomass. These data have enabled the evaluation of N. glauca material as a potential feedstock for biorefining.

Rice Basic Helix-Loop-Helix 079 (OsbHLH079) Delays Leaf Senescence by Attenuating ABA Signaling

Leaf senescence represents the final phase of leaf development and is characterized by a highly organized degenerative process involving the active translocation of nutrients from senescing leaves to growing tissues or storage organs. To date, a large number of senescence-associated transcription factors (sen-TFs) have been identified that regulate the initiation and progression of leaf senescence. Many of these TFs, including NAC (NAM/ATAF1/2/CUC2), WRKY, and MYB TFs, have been implicated in modulating the expression of downstream senescence-associated genes (SAGs) and chlorophyll degradation genes (CDGs) under the control of phytohormones. However, the involvement of basic helix-loop-helix (bHLH) TFs in leaf senescence has been less investigated. Here, we show that OsbHLH079 delays both natural senescence and dark-induced senescence: Overexpression of OsbHLH079 led to a stay-green phenotype, whereas osbhlh079 knockout mutation displayed accelerated leaf senescence. Similar to other sen-TFs, OsbHLH079 showed a gradual escalation in expression as leaves underwent senescence. During this process, the mRNA levels of SAGs and CDGs remained relatively low in OsbHLH079 overexpressors, but increased sharply in osbhlh079 mutants, suggesting that OsbHLH079 negatively regulates the transcription of SAGs and CDGs under senescence conditions. Additionally, we found that OsbHLH079 delays ABA-induced senescence. Subsequent RT-qPCR and dual-luciferase reporter assays revealed that OsbHLH079 downregulates the expression of ABA signaling genes, such as OsABF2, OsABF4, OsABI5, and OsNAP. Taken together, these results demonstrate that OsbHLH079 functions in delaying leaf yellowing by attenuating the ABA responses.

NtMYB1 and NtNCED1/2 control abscisic acid biosynthesis and tepal senescence in Chinese narcissus (Narcissus tazetta)

Chinese narcissus (Narcissus tazetta var. chinensis cv. 'Jinzhanyintai') is one of the 10 most famous traditional flowers of China, having a beautiful and highly ornamental flower with a rich fragrance. However, the flower longevity affects its commercial appeal. While petal senescence in Narcissus is ethylene-independent and abscisic acid-dependent, the regulatory mechanism has yet to be determined. In this study, we identified a R2R3-MYB gene (NtMYB1) from Narcissus tazetta and generated oeNtMYB1 and Ntmyb1 RNA interference mutants in Narcissus as well as an oeNtMYB1 construct in Arabidopsis. Overexpressing NtMYB1 in Narcissus or Arabidopsis led to premature leaf yellowing, an elevated level of total carotenoid, a reduced level of chlorophyll b, and a decrease in photosystem II fluorescence (Fv/Fm). A dual-luciferase assay and chromatin immunoprecipitation-quantitative PCR revealed that NtMYB1 directly binds to the promoter of NtNCED1 or NtNCED2 and activates NtNCED1/2 gene expression both in vitro and in vivo. Moreover, overexpressing NtMYB1 accelerated abscisic acid biosynthesis, up-regulated the content of zeatin and abscisic acid, and down-regulated the level of β-carotene and gibberellin A1, leading to petal senescence and leaf yellowing in Narcissus. This study revealed a regulatory process that is fundamentally different between non-photosynthetic organs and leaves.

Growth in fluctuating light buffers plants against photorespiratory perturbations

Photorespiration (PR) is the pathway that detoxifies the product of the oxygenation reaction of Rubisco. It has been hypothesized that in dynamic light environments, PR provides a photoprotective function. To test this hypothesis, we characterized plants with varying PR enzyme activities under fluctuating and non-fluctuating light conditions. Contrasting our expectations, growth of mutants with decreased PR enzyme levels was least affected in fluctuating light compared with wild type. Results for growth, photosynthesis and metabolites combined with thermodynamics-based flux analysis revealed two main causal factors for this unanticipated finding: reduced rates of photosynthesis in fluctuating light and complex re-routing of metabolic fluxes. Only in non-fluctuating light, mutants lacking the glutamate:glyoxylate aminotransferase 1 re-routed glycolate processing to the chloroplast, resulting in photooxidative damage through H2O2 production. Our results reveal that dynamic light environments buffer plant growth and metabolism against photorespiratory perturbations.

Multilayered regulation of developmentally programmed pre-anthesis tip degeneration of the barley inflorescence

Leaf and floral tissue degeneration is a common feature in plants. In cereal crops such as barley (Hordeum vulgare L.), pre-anthesis tip degeneration (PTD) starts with growth arrest of the inflorescence meristem dome, which is followed basipetally by the degeneration of floral primordia and the central axis. Due to its quantitative nature and environmental sensitivity, inflorescence PTD constitutes a complex, multilayered trait affecting final grain number. This trait appears to be highly predictable and heritable under standardized growth conditions, consistent with a developmentally programmed mechanism. To elucidate the molecular underpinnings of inflorescence PTD, we combined metabolomic, transcriptomic, and genetic approaches to show that barley inflorescence PTD is accompanied by sugar depletion, amino acid degradation, and abscisic acid responses involving transcriptional regulators of senescence, defense, and light signaling. Based on transcriptome analyses, we identified GRASSY TILLERS1 (HvGT1), encoding an HD-ZIP transcription factor, as an important modulator of inflorescence PTD. A gene-edited knockout mutant of HvGT1 delayed PTD and increased differentiated apical spikelets and final spikelet number, suggesting a possible strategy to increase grain number in cereals. We propose a molecular framework that leads to barley PTD, the manipulation of which may increase yield potential in barley and other related cereals.

Extensin and senescence: a cell wall connection

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Lu H, Niu X, Fan Y, Yuan Y, Huang L, Zhao B, Liu Y, Xiao F. 2023. The extensin protein SAE1 plays a role in leaf senescence and is targeted by the ubiquitin ligase SINA4 in tomato. Journal of Experimental Botany 74, 5635–5652.

Cadmium up Taking and Allocation in Wood Species Associated to Cacao Agroforestry Systems and Its Potential Role for Phytoextraction

Trees in cacao Agroforestry systems (AFS) may present a high potential for cadmium (Cd) phytoextraction, helping to reduce Cd in cacao (Theobroma cacao L.) plants grown in contaminated soils. To assess this potential, four forest fine-woody species commonly found in cacao high-productive sites in Colombia (Tabebuia rosea, Terminalia superba, Albizia guachapele, and Cariniana pyriformis) were exposed to contrasting CdCl2 contamination levels (0, 6, and 12 ppm) on a hydroponic medium. Growth dynamics, tolerance index (TI), and Cd concentration and allocation in leaves, stems, and roots were evaluated for up to 90 days after initial exposure. T. superba, A. guachapele, and C. pyriformis were classified as moderately tolerant (TI > 0.6), and T. rosea was considered a sensitive species (TI < 0.35) under 12 ppm Cd contamination. Despite showing a high stem Cd concentration, C. pyriformis also showed the lowest relative growth rate. Among the evaluated forest species, A. guachapele exhibited the highest Cd accumulation capacity per plant (2.02 mg plant-1) but also exhibited a higher Cd allocation to leaves (4%) and a strong decrease in leaf and stem dry mass after 90 days of exposure (~75% and 50% respectively, compared to control treatments). Taking together all the favorable features exhibited by T. superba as compared to other CAFS tree species and recognized phytoextractor tree species in the literature, such as Cd hyperaccumulation, high tolerance index, low Cd concentration in leaves, and high Cd allocation to the stem (harvestable as wood), this species is considered to have a high potential for cadmium phytoextraction in cocoa agroforestry systems.

Divergent Cross-Adaptation of Herbicide-Treated Wheat and Triticale Affected by Drought or Waterlogging

Widely used agrochemicals that do not exert negative effects on crops and selectively target weeds could influence plant resilience under unfavorable conditions. The cross-adaptation of wheat (Triticum aestivum L.) and triticale (×Triticosecale Wittm.) exposed to two environmental abiotic stressors (drought and waterlogging) was evaluated after treatment with a selective herbicide (Serrate®, Syngenta). The ambivalent effects of the herbicide on the two studied crops were particularly distinct in waterlogged plants, showing a significant reduction in wheat growth and better performance of triticale individuals exposed to the same combined treatment. Histochemical staining for the detection of reactive oxygen species (ROS) confirmed that the herbicide treatment increased the accumulation of superoxide anion in the flooded wheat plants, and this effect persisted in the younger leaves of the recovered individuals. Comparative transcript profiling of ROS scavenging enzymes (superoxide dismutase, peroxidase, glutathione reductase, and catalase) in stressed and recovered plants revealed crop-specific variations resulting from the unfavorable water regimes in combination with the herbicide treatment. Short-term dehydration was relatively well tolerated by the hybrid crop triticale and this aligned with the considerable upregulation of genes for L-Proline biosynthesis. Its drought resilience was diminished by herbicide application, as evidenced by increased ROS accumulation after prolonged water deprivation.

Miniaturized Vis-NIR handheld spectrometer for non-invasive pigment quantification in agritech applications

Advanced precision agriculture requires the objective measurement of the structural and functional properties of plants. Biochemical profiles in leaves can differ depending on plant growing conditions. By quantitatively detecting these changes, farm production processes can be optimized to achieve high-yield, high-quality, and nutrient dense agricultural products. To enable the rapid and non-destructive detection on site, this study demonstrates the development of a new custom-designed portable handheld Vis-NIR spectrometer that collects leaf reflectance spectra, wirelessly transfers the spectral data through Bluetooth, and provides both raw spectral data and processed information. The spectrometer has two preprogramed methods: anthocyanin and chlorophyll quantification. Anthocyanin content of red and green lettuce estimated with the new spectrometer showed an excellent correlation coefficient of 0.84 with those determined by a destructive gold standard biochemical method. The differences in chlorophyll content were measured using leaf senescence as a case study. Chlorophyll Index calculated with the handheld spectrometer gradually decreased with leaf age as chlorophyll degrades during the process of senescence. The estimated chlorophyll values were highly correlated with those obtained from a commercial fluorescence-based chlorophyll meter with a correlation coefficient of 0.77. The developed portable handheld Vis-NIR spectrometer could be a simple, cost-effective, and easy to operate tool that can be used to non-invasively monitor plant pigment and nutrient content efficiently.

The Arabidopsis thaliana onset of leaf death 12 mutation in the lectin receptor kinase P2K2 results in an autoimmune phenotype

BACKGROUND

Plant immunity relies on the perception of immunogenic signals by cell-surface and intracellular receptors and subsequent activation of defense responses like programmed cell death. Under certain circumstances, the fine-tuned innate immune system of plants results in the activation of autoimmune responses that cause constitutive defense responses and spontaneous cell death in the absence of pathogens.

RESULTS

Here, we characterized the onset of leaf death 12 (old12) mutant that was identified in the Arabidopsis accession Landsberg erecta. The old12 mutant is characterized by a growth defect, spontaneous cell death, plant-defense gene activation, and early senescence. In addition, the old12 phenotype is temperature reversible, thereby exhibiting all characteristics of an autoimmune mutant. Mapping the mutated locus revealed that the old12 phenotype is caused by a mutation in the Lectin Receptor Kinase P2-TYPE PURINERGIC RECEPTOR 2 (P2K2) gene. Interestingly, the P2K2 allele from Landsberg erecta is conserved among Brassicaceae. P2K2 has been implicated in pathogen tolerance and sensing extracellular ATP. The constitutive activation of defense responses in old12 results in improved resistance against Pseudomonas syringae pv. tomato DC3000.

CONCLUSION

We demonstrate that old12 is an auto-immune mutant and that allelic variation of P2K2 contributes to diversity in Arabidopsis immune responses.

Genetic portrait of polyamine transporters in barley: insights in the regulation of leaf senescence

Nitrogen (N) is one of the most expensive nutrients to supply, therefore, improving the efficiency of N use is essential to reduce the cost of commercial fertilization in plant production. Since cells cannot store reduced N as NH₃ or NH₄ ⁺, polyamines (PAs), the low molecular weight aliphatic nitrogenous bases, are important N storage compounds in plants. Manipulating polyamines may provide a method to increase nitrogen remobilization efficiency. Homeostasis of PAs is maintained by intricate multiple feedback mechanisms at the level of biosynthesis, catabolism, efflux, and uptake. The molecular characterization of the PA uptake transporter (PUT) in most crop plants remains largely unknown, and knowledge of polyamine exporters in plants is lacking. Bi-directional amino acid transporters (BATs) have been recently suggested as possible PAs exporters for Arabidopsis and rice, however, detailed characterization of these genes in crops is missing. This report describes the first systematic study to comprehensively analyze PA transporters in barley (Hordeum vulgare, Hv), specifically the PUT and BAT gene families. Here, seven PUTs (HvPUT1-7) and six BATs (HvBAT1-6) genes were identified as PA transporters in the barley genome and the detailed characterization of these HvPUT and HvBAT genes and proteins is provided. Homology modeling of all studied PA transporters provided 3D structures prediction of the proteins of interest with high accuracy. Moreover, molecular docking studies provided insights into the PA-binding pockets of HvPUTs and HvBATs facilitating improved understanding of the mechanisms and interactions involved in HvPUT/HvBAT-mediated transport of PAs. We also examined the physiochemical characteristics of PA transporters and discuss the function of PA transporters in barley development, and how they help barley respond to stress, with a particular emphasis on leaf senescence. Insights gained here could lead to improved barley production via modulation of polyamine homeostasis.

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.

Genomic architecture of leaf senescence in sorghum (Sorghum bicolor)

Leaf senescence in sorghum is primarily controlled by the progression, but not by the onset of senescence. The senescence-delaying haplotypes of 45 key genes accentuated from landraces to improved lines. Leaf senescence is a genetically programmed developmental process and plays a central role for plant survival and crop production by remobilising nutrients accumulated in senescent leaves. In theory, the ultimate outcome of leaf senescence is determined by the onset and progression of senescence, but how these two processes contribute to senescence is not fully illustrated in crops and the genetic basis for them is not well understood. Sorghum (Sorghum bicolor), which is known for the remarkable stay-green trait, is ideal for dissecting the genomic architecture underlying the regulation of senescence. In this study, a diverse panel of 333 sorghum lines was explored for the onset and progression of leaf senescence. Trait correlation analysis showed that the progression of leaf senescence, rather than the onset of leaf senescence, significantly correlated with variations of the final leaf greenness. This notion was further supported by GWAS, which identified 31 senescence-associated genomic regions containing 148 genes, of which 124 were related to the progression of leaf senescence. The senescence-delaying haplotypes of 45 key candidate genes were enriched in lines with extremely prolonged senescence duration, while senescence-promoting haplotypes in those with extremely accelerated senescence. Haplotype combinations of these genes could well explain the segregation of the senescence trait in a recombinant inbred population. We also demonstrated that senescence-delaying haplotypes of candidate genes were under strong selection during sorghum domestication and genetic improvement. Together, this research advanced our understanding of crop leaf senescence and provided a suite of candidate genes for functional genomics and molecular breeding.

Esca grapevine disease involves leaf hydraulic failure and represents a unique premature senescence process

Xylem anatomy may change in response to environmental or biotic stresses. Vascular occlusion, an anatomical modification of mature xylem, contributes to plant resistance and susceptibility to different stresses. In woody organs, xylem occlusions have been examined as part of the senescence process, but their presence and function in leaves remain obscure. In grapevine, many stresses are associated with premature leaf senescence inducing discolorations and scorched tissue in leaves. However, we still do not know whether the leaf senescence process follows the same sequence of physiological events and whether leaf xylem anatomy is affected in similar ways. In this study, we quantified vascular occlusions in midribs from leaves with symptoms of the grapevine disease esca, magnesium deficiency and autumn senescence. We found higher amounts of vascular occlusions in leaves with esca symptoms (in 27% of xylem vessels on average), whereas the leaves with other symptoms (as well as the asymptomatic controls) had far fewer occlusions (in 3% of vessels). Therefore, we assessed the relationship between xylem occlusions and esca leaf symptoms in four different countries (California in the USA, France, Italy and Spain) and eight different cultivars. We monitored the plants over the course of the growing season, confirming that vascular occlusions do not evolve with symptom age. Finally, we investigated the hydraulic integrity of leaf xylem vessels by optical visualization of embolism propagation during dehydration. We found that the occlusions lead to hydraulic dysfunction mainly in the peripheral veins compared with the midribs in esca symptomatic leaves. These results open new perspectives on the role of vascular occlusions during the leaf senescence process, highlighting the uniqueness of esca leaf symptoms and its consequence on leaf physiology.

Postharvest Application of Acibenzolar-S-Methyl Activates Salicylic Acid Pathway Genes in Kiwifruit Vines

The plant defence inducer Actigard® (acibenzolar-S-methyl [ASM]) is applied before flowering and after fruit harvest to control bacterial canker in kiwifruit caused by Pseudomonas syringae pv. actinidiae. Pre-flowering application of ASM is known to upregulate defence gene expression; however, the effect of postharvest ASM on defence gene expression in the vine is unknown. In this study, the expression of eight "defence marker" genes was measured in the leaves of Actinidia chinensis var. chinensis, "Zesy002," and Actinidia chinensis var. deliciosa, "Hayward," vines after postharvest treatment with ASM and/or copper. There were two orchards per cultivar with harvest dates approximately three weeks apart for investigating potential changes in responsiveness to ASM during the harvest period. In all trials, postharvest ASM induced the expression of salicylic-acid-pathway defence genes PR1, PR2, PR5, BAD, DMR6, NIMIN2, and WRKY70. Gene upregulation was the greatest at 1 day and 7 days after treatment and declined to the control level after 3 weeks. In "Zesy002", the ASM-induced response was greater at the early harvest site than at the late harvest site. This decline was concomitant with leaf yellowing and a reduction in RNA yield. Effects of postharvest ASM on gene expression did not persist into the following spring, nor were vines conditioned to respond more strongly to pre-flowering ASM application.

Lipid transporter LSR1 positively regulates leaf senescence in Arabidopsis

Senescence is the final stage in the life history of a leaf, whereby plants relocate nutrients from leaves to other developing organs. Recent efforts have begun to focus on understanding the network-based molecular mechanism that incorporates various environmental signals and leaf age information and involves a complex process with the coordinated actions of multiple pathways. Here, we identified a novel participant, named LSR1 (Leaf Senescence Related 1), that involved in the regulation of leaf senescence. Loss-of-function lsr1-1 mutant showed delayed leaf senescence whereas the overexpression of LSR1 accelerated senescence. LSR1 encodes a lipid transfer protein, and the results show that the protein is located in chloroplast and intercellular space. The LSR1 may be involved in the regulation of leaf senescence by transporting lipids in plants.

Strigolactones in Plants and Their Interaction with the Ecological Microbiome in Response to Abiotic Stress

Phytohormones play an essential role in enhancing plant tolerance by responding to abiotic stresses, such as nutrient deficiency, drought, high temperature, and light stress. Strigolactones (SLs) are carotenoid derivatives that occur naturally in plants and are defined as novel phytohormones that regulate plant metabolism, growth, and development. Strigolactone assists plants in the acquisition of defensive characteristics against drought stress by initiating physiological responses and mediating the interaction with soil microorganisms. Nutrient deficiency is an important abiotic stress factor, hence, plants perform many strategies to survive against nutrient deficiency, such as enhancing the efficiency of nutrient uptake and forming beneficial relationships with microorganisms. Strigolactone attracts various microorganisms and provides the roots with essential elements, including nitrogen and phosphorus. Among these advantageous microorganisms are arbuscular mycorrhiza fungi (AMF), which regulate plant metabolic activities through phosphorus providing in roots. Bacterial nodulations are also nitrogen-fixing microorganisms found in plant roots. This symbiotic relationship is maintained as the plant provides organic molecules, produced in the leaves, that the bacteria could otherwise not independently generate. Related stresses, such as light stress and high-temperature stress, could be affected directly or indirectly by strigolactone. However, the messengers of these processes are unknown. The most prominent connector messengers have been identified upon the discovery of SLs and the understanding of their hormonal effect. In addition to attracting microorganisms, these groups of phytohormones affect photosynthesis, bridge other phytohormones, induce metabolic compounds. In this article, we highlighted the brief information available on SLs as a phytohormone group regarding their common related effects. In addition, we reviewed the status and described the application of SLs and plant response to abiotic stresses. This allowed us to comprehend plants' communication with the ecological microbiome as well as the strategies plants use to survive under various stresses. Furthermore, we identify and classify the SLs that play a role in stress resistance since many ecological microbiomes are unexplained.

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

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

Autophagy: a game changer for plant development and crop improvement

Manipulation of autophagic pathway represents a tremendous opportunity for designing climate-smart crops with improved yield and better adaptability to changing environment. For exploiting autophagy to its full potential, identification and comprehensive characterization of adapters/receptor complex and elucidation of its regulatory network in crop plants is highly warranted.  Autophagy is a major intracellular trafficking pathway in eukaryotes involved in vacuolar degradation of cytoplasmic constituents, mis-folded proteins, and defective organelles. Under optimum conditions, autophagy operates at a basal level to maintain cellular homeostasis, but under stressed conditions, it is induced further to provide temporal stress relief. Our understanding of this highly dynamic process has evolved exponentially in the past few years with special reference to several plant-specific roles of autophagy. Here, we review the most recent advances in the field of autophagy in plants and discuss its potential implications in designing crops with improved stress and disease-tolerance, enhanced yield potential, and improved capabilities for producing metabolites of high economic value. We also assess the current knowledge gaps and the possible strategies to develop a robust module for biotechnological application of autophagy to enhance bioeconomy and sustainability of agriculture.

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.

Genome-Wide Identification and Expression Analysis of Senescence-Associated Genes in Grapevine (Vitis vinifera L.) Reveal Their Potential Functions in Leaf Senescence Order

Natural leaf senescence is an acclimation strategy that enables plants to reallocate nutrients. In the present study, interestingly, we found that the basal mature leaves of grapevine primary shoots (P) exhibited the earliest senescence, followed by the apical young leaves of secondary shoots (ST), and then the basal mature leaves of secondary shoots (S). The Chl level decreased with the extent of leaf senescence. According to the genome-wide identification and expression analysis, sixteen senescence-associated genes (SAGs) involved in Chl breakdown were identified in the grapevine genome. Their expression patterns showed that the transcript changes in VvSGR, VvPPH2, and VvFtsH6-2 corresponded to the changes in Chl content among P, S, and ST. The changes in the transcription of VvNYC1, VvSGR, VvPAO1, VvPAO2, VvPAO4, VvPPH1, VvPPH3, and VvFtsH6-1 only contributed to low Chl levels in P. The cis-element analysis indicated that these SAGs possessed several light- and hormone-responsive elements in their promoters. Among them, ABA-responsive elements were found in twelve of the sixteen promoters of SAGs. Correspondingly, ABA-signaling components presented various changes in transcription among P, S, and ST. The transcription changes in VvbZIP45 and VvSnRK2.1 were similar to those in VvSGR, VvPPH2, and VvFtsH6-2. The other nine ABA-signaling components, which included VvRCAR2, VvRCAR4, VvRCAR6, VvRCAR7, VvRCAR2, VvPP2C4, VvPP2C9, VvbZIP25, and VvSnRK2.3, were highly expressed in P but there was no difference between S and ST, with similar expression patterns for VvNYC1, VvSGR, VvPAO1, VvPAO2, VvPAO4, VvPPH1, VvPPH3, and VvFtsH6-1. These results suggested that the senescence of P and ST could be regulated by different members of Chl breakdown-related SAGs and ABA-signaling components. These findings provide us with important candidate genes to further study the regulation mechanism of leaf senescence order in grapevine.

ARF2-PIF5 interaction controls transcriptional reprogramming in the ABS3-mediated plant senescence pathway

One of the hallmarks of plant senescence is the global transcriptional reprogramming coordinated by a plethora of transcription factors (TFs). However, mechanisms underlying the interactions between different TFs in modulating senescence remain obscure. Previously, we discovered that plant ABS3 subfamily MATE transporter genes regulate senescence and senescence-associated transcriptional changes. In a genetic screen for mutants suppressing the accelerated senescence phenotype of the gain-of-function mutant abs3-1D, AUXIN RESPONSE FACTOR 2 (ARF2) and PHYTOCHROME-INTERACTING FACTOR 5 (PIF5) were identified as key TFs responsible for transcriptional regulation in the ABS3-mediated senescence pathway. ARF2 and PIF5 (as well as PIF4) interact directly and function interdependently to promote senescence, and they share common target genes such as key senescence promoting genes ORESARA 1 (ORE1) and STAY-GREEN 1 (SGR1) in the ABS3-mediated senescence pathway. In addition, we discovered reciprocal regulation between ABS3-subfamily MATEs and the ARF2 and PIF5/4 TFs. Taken together, our findings reveal a regulatory paradigm in which the ARF2-PIF5/4 functional module facilitates the transcriptional reprogramming in the ABS3-mediated senescence pathway.

Genome-wide H3K9 acetylation level increases with age-dependent senescence of flag leaf in rice

Flag leaf senescence is an important biological process that drives the remobilization of nutrients to the growing organs of rice. Leaf senescence is controlled by genetic information via gene expression and histone modification, but the precise mechanism is as yet unclear. Here, we analysed genome-wide acetylated lysine residue 9 of histone H3 (H3K9ac) enrichment by chromatin immunoprecipitation-sequencing (ChIP-seq), and examined its association with transcriptomes by RNA-seq during flag leaf aging in rice (Oryza sativa). We found that genome-wide H3K9 acetylation levels increased with age-dependent senescence in rice flag leaf, and there was a positive correlation between the density and breadth of H3K9ac with gene expression and transcript elongation. During flag leaf aging, we observed 1249 up-regulated differentially expressed genes (DEGs) and 996 down-regulated DEGs, showing a strong relationship between temporal changes in gene expression and gain/loss of H3K9ac. We produced a landscape of H3K9 acetylation-modified gene expression targets that include known senescence-associated genes, metabolism-related genes, as well as miRNA biosynthesis-related genes. Our findings reveal a complex regulatory network of metabolism- and senescence-related pathways mediated by H3K9ac, and elucidate patterns of H3K9ac-mediated regulation of gene expression during flag leaf aging in rice.

Insights into the Genomic Regions and Candidate Genes of Senescence-Related Traits in Upland Cotton via GWAS

Senescence is the last stage of plant development and is controlled by both internal and external factors. Premature senescence significantly affects the yield and quality of cotton. However, the genetic architecture underlying cotton senescence remains unclear. In this study, genome-wide association studies (GWAS) were performed based on 3,015,002 high-quality SNP markers from the resequencing data of 355 upland cotton accessions to detect genomic regions for cotton senescence. A total of 977 candidate genes within 55 senescence-related genomic regions (SGRs), SGR1-SGR55, were predicted. Gene ontology (GO) analysis of candidate genes revealed that a set of biological processes was enriched, such as salt stress, ethylene processes, and leaf senescence. Furthermore, in the leaf senescence GO term, one candidate gene was focused on: Gohir.A12G270900 (GhMKK9), located in SGR36, which encodes a protein of the MAP kinase kinase family. Quantitative real-time PCR (qRT-PCR) analysis showed that GhMKK9 was up-regulated in old cotton leaves. Overexpression of GhMKK9 in Arabidopsis accelerated natural leaf senescence. Virus-induced gene silencing (VIGS) of GhMKK9 in cotton increased drought tolerance. These results suggest that GhMKK9 is a positive regulator and might be involved in drought-induced senescence in cotton. The results provide new insights into the genetic basis of cotton senescence and will be useful for improving cotton breeding in the future.

Metabolic control of arginine and ornithine levels paces the progression of leaf senescence

Leaf senescence can be induced by stress or aging, sometimes in a synergistic manner. It is generally acknowledged that the ability to withstand senescence-inducing conditions can provide plants with stress resilience. Although the signaling and transcriptional networks responsible for a delayed senescence phenotype, often referred to as a functional stay-green trait, have been actively investigated, very little is known about the subsequent metabolic adjustments conferring this aptitude to survival. First, using the individually darkened leaf (IDL) experimental setup, we compared IDLs of wild-type (WT) Arabidopsis (Arabidopsis thaliana) to several stay-green contexts, that is IDLs of two functional stay-green mutant lines, oresara1-2 (ore1-2) and an allele of phytochrome-interacting factor 5 (pif5), as well as to leaves from a WT plant entirely darkened (DP). We provide compelling evidence that arginine and ornithine, which accumulate in all stay-green contexts-likely due to the lack of induction of amino acids (AAs) transport-can delay the progression of senescence by fueling the Krebs cycle or the production of polyamines (PAs). Secondly, we show that the conversion of putrescine to spermidine (SPD) is controlled in an age-dependent manner. Thirdly, we demonstrate that SPD represses senescence via interference with ethylene signaling by stabilizing the ETHYLENE BINDING FACTOR1 and 2 (EBF1/2) complex. Taken together, our results identify arginine and ornithine as central metabolites influencing the stress- and age-dependent progression of leaf senescence. We propose that the regulatory loop between the pace of the AA export and the progression of leaf senescence provides the plant with a mechanism to fine-tune the induction of cell death in leaves, which, if triggered unnecessarily, can impede nutrient remobilization and thus plant growth and survival.

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.

Apple SINA E3 ligase MdSINA3 negatively mediates JA-triggered leaf senescence by ubiquitinating and degrading the MdBBX37 protein

Jasmonic acid (JA) induces chlorophyll degradation and leaf senescence. B-box (BBX) proteins play important roles in the modulation of leaf senescence, but the molecular mechanism of BBX protein-mediated leaf senescence remains to be further studied. Here, we identified the BBX protein MdBBX37 as a positive regulator of JA-induced leaf senescence in Malus domestica (apple). Further studies showed that MdBBX37 interacted with the senescence regulatory protein MdbHLH93 to enhance its transcriptional activation on the senescence-associated gene MdSAG18, thereby promoting leaf senescence. Moreover, the JA signaling repressor MdJAZ2 interacted with MdBBX37 and interfered with the interaction between MdBBX37 and MdbHLH93, thereby negatively mediating MdBBX37-promoted leaf senescence. In addition, the E3 ubiquitin ligase MdSINA3 delayed MdBBX37-promoted leaf senescence through targeting MdBBX37 for degradation. The MdJAZ2-MdBBX37-MdbHLH93-MdSAG18 and MdSINA3-MdBBX37 modules realized the precise modulation of JA on leaf senescence. In parallel, our data demonstrate that MdBBX37 was involved in abscisic acid (ABA)- and ethylene-mediated leaf senescence through interacting with the ABA signaling regulatory protein MdABI5 and ethylene signaling regulatory protein MdEIL1, respectively. Taken together, our results not only reveal the role of MdBBX37 as an integration node in JA-, ABA- and ethylene-mediated leaf senescence, but also provide new insights into the post-translational modification of BBX proteins.

Characteristic and mechanism analysis of sludge polymer by adding carboxymethyl cellulose-g-acrylic acid for sludge utilization

Herein, a nutrient water retention agent is prepared by fully mixing sludge with carboxymethyl cellulose-g-acrylic acid (CMC-g-AA) gel and nanoscale zero-valent iron (nZVI) using polymer modifying curing technology. Experimental results show that when CMC:AA = 1:12 and CMC-g-AA gel content is 50%, sludge polymer has better water absorption and retention performance and the water retention time is extended for ∼14 days. At the same time, sludge polymer can preserve the characteristics of nutrient-rich elements and organic matter and promote plant growth. The addition of nZVI has a significant impact on reducing the risk of heavy metal toxic leaching in sludge. Moreover, analysis of variance and multiple comparisons shows that sludge polymer's particle size and water absorption times have significant effects on the water absorption and retention properties of sludge polymer. Scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy and ¹³C-nuclear magnetic resonance analyses show that the addition of an appropriate amount of gel could increase the number of hydrophilic groups and hydrophilic mineral components in sludge polymer, increase its overall porosity and improve its water absorption and retention properties.

Analysis of Physiological and Transcriptomic Differences between a Premature Senescence Mutant (GSm) and Its Wild-Type in Common Wheat (Triticum aestivum L.)

Simple Summary

Early leaf senescence is an important agronomic trait that affects crop yield and quality. To understand the molecular mechanism of early leaf senescence, a wheat (Triticum aestivum L.) premature leaf senescence mutant (GSm) and its wild type were employed in this study. We compared the physiological characteristics and transcriptome of wheat leaves between the wild type (WT) and the mutant at two-time points. Physiological characteristics and differentially expressed gene (DEG) analysis revealed many genes and metabolic pathways that were closely related to senescence. These results will not only support further gene cloning and functional analysis of GSm, but also facilitate the study of leaf senescence in wheat.

Abstract

Premature leaf senescence has a profound influence on crop yield and quality. Here, a stable premature senescence mutant (GSm) was obtained from the common wheat (Triticum aestivum L.) cultivar Chang 6878 by mutagenesis with ethyl methanesulfonate. The differences between the GSm mutant and its wild-type (WT) were analyzed in terms of yield characteristics, photosynthetic fluorescence indices, and senescence-related physiological parameters. RNA sequencing was used to reveal gene expression differences between GSm and WT. The results showed that the yield of GSm was considerably lower than that of WT. The net photosynthetic rate, transpiration rate, maximum quantum yield, non-photochemical quenching coefficient, photosynthetic electron transport rate, soluble protein, peroxidase activity, and catalase activity all remarkably decreased in flag leaves of GSm, whereas malondialdehyde content distinctively increased compared with those of WT. The analysis of differentially expressed genes indicated blockade of chlorophyll and carotenoid biosynthesis, accelerated degradation of chlorophyll, and diminished photosynthetic capacity in mutant leaves; brassinolide might facilitate chlorophyll breakdown and consequently accelerate leaf senescence. NAC genes positively regulated the senescence process. Compared with NAC genes, expression of WRKY and MYB genes was induced earlier in the mutant possibly due to increased levels of reactive oxygen species and plant hormones (e.g., brassinolide, salicylic acid, and jasmonic acid), thereby accelerating leaf senescence. Furthermore, the antioxidant system played a role in minimizing oxidative damage in the mutant. These results provides novel insight into the molecular mechanisms of premature leaf senescence in crops.

Mathematical Modeling Reveals That Sucrose Regulates Leaf Senescence via Dynamic Sugar Signaling Pathways

Sucrose (Suc) accumulation is one of the key indicators of leaf senescence onset, but little is known about its regulatory role. Here, we found that application of high (120-150 mM) and low levels (60 mM) of Suc to young leaf (YL) and fully expanded leaf (FEL) discs, respectively, decreased chlorophyll content and maximum photosynthetic efficiency. Electrolyte leakage and malondialdehyde levels increased at high Suc concentrations (90-120 mM in YL and 60 and 150 mM in FEL discs). In FEL discs, the senescence-associated gene NtSAG12 showed a gradual increase in expression with increased Suc application; in contrast, in YL discs, NtSAG12 was upregulated with low Suc treatment (60 mM) but downregulated at higher levels of Suc. In YL discs, trehalose-6-phosphate (T6P) accumulated at a low half-maximal effective concentration (EC50) of Suc (1.765 mM). However, T6P levels declined as trehalose 6 phosphate synthase (TPS) content decreased, resulting in the maximum velocity of sucrose non-fermenting-1-related protein kinase (SnRK) and hexokinase (HXK) occurring at higher level of Suc. We therefore speculated that senescence was induced by hexose accumulation. In FEL discs, the EC50 of T6P occurred at a low concentration of Suc (0.9488 mM); T6P levels progressively increased with higher TPS content, which inhibited SnRK activity with a dissociation constant (Kd) of 0.001475 U/g. This confirmed that the T6P-SnRK complex induced senescence in detached FEL discs.

Single-molecule long-read sequencing analysis improves genome annotation and sheds new light on the transcripts and splice isoforms of Zoysia japonica

BACKGROUND

Zoysia japonica is an important warm-season turfgrass used worldwide. Although the draft genome sequence and a vast amount of next-generation sequencing data have been published, the current genome annotation and complete mRNA structure remain incomplete. Therefore, to analyze the full-length transcriptome of Z. japonica, we used the PacBio single-molecule long-read sequencing method in this study.

RESULTS

First, we generated 37,056 high-confidence non-redundant transcripts from 16,005 gene loci. Next, 32,948 novel transcripts, 913 novel gene loci, 8035 transcription factors, 89 long non-coding RNAs, and 254 fusion transcripts were identified. Furthermore, 15,675 alternative splicing events and 5325 alternative polyadenylation sites were detected. In addition, using bioinformatics analysis, the underlying transcriptional mechanism of senescence was explored based on the revised reference transcriptome.

CONCLUSION

This study provides a full-length reference transcriptome of Z. japonica using PacBio single-molecule long-read sequencing for the first time. These results contribute to our knowledge of the transcriptome and improve the knowledge of the reference genome of Z. japonica. This will also facilitate genetic engineering projects using Z. japonica.

Overexpression of AHL9 accelerates leaf senescence in Arabidopsis thaliana

BACKGROUND

Leaf senescence, the final stage of leaf growth and development, is regulated by numerous internal factors and environmental cues. Ethylene is one of the key senescence related hormones, but the underlying molecular mechanism of ethylene-induced leaf senescence remains poorly understood.

RESULTS

In this study, we identified one AT-hook like (AHL) protein, AHL9, as a positive regulator of leaf senescence in Arabidopsis thaliana. Overexpression of AHL9 significantly accelerates age-related leaf senescence and promotes dark-induced leaf chlorosis. The early senescence phenotype observed in AHL9 overexpressing lines is inhibited by the ethylene biosynthesis inhibitor aminooxyacetic acid suggesting the involvement of ethylene in the AHL9-associated senescence. RNA-seq and quantitative reverse transcription PCR (qRT-PCR) data identified numerous senescence-associated genes differentially expressed in leaves of AHL9 overexpressing transgenic plants.

CONCLUSIONS

Our investigation demonstrates that AHL9 functions in accelerating the leaf senescence process via ethylene synthesis or signalling.