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

TaDTGIP1-TaDTG6-BDel574-TaPIF1 module regulates drought stress response in wheat

Drought is a major environmental constraint to wheat production, yet the genetic and molecular mechanisms underlying drought tolerance remain poorly understood. A gain-of-function protein variant TaDTG6-BDel574 has been identified and positively regulates TaPIF1 transcription to enhance wheat drought tolerance. However, the precise molecular pathways driving this response are yet to be fully characterized. In this study, we demonstrate that TaPIF1 plays a crucial role in mediating wheat drought tolerance by regulating stomatal aperture to control transpiration. RNA sequencing combined with biochemical assays revealed that TaPIF1 directly binds to E-box elements to activate the expression of key stress-responsive genes, including TaABI5, TaRD17, and TaP5CS1. Notably, overexpression of TaABI5 enhances wheat drought tolerance by promoting stomatal closure, thereby reducing water loss. Furthermore, TaPIF1 interacts with TaABI5 and the bHLH transcription factor TaAKS1 to synergistically enhancing the transcriptional activation of TaABI5, TaRD17, and TaP5CS1. Additionally, our findings verified that TaDTGIP1 interacts with TaDTG6-BDel574 to attenuate its binding affinity and regulatory activity on the TaPIF1 promoter, thereby negatively regulating drought tolerance. Together, our findings unveil the molecular mechanisms underlying wheat drought stress response mediated by the TaDTGIP1-TaDTG6-BDel574-TaPIF1/TaABI5/TaAKS1-target regulatory module and identify potential candidate genes for breeding elite drought-tolerant wheat varieties.

Extranuclear function of Arabidopsis HOOKLESS1 regulates pleiotropic developmental processes in a non-cell-autonomous manner

N-acetyltransferase HOOKLESS1 (HLS1) in Arabidopsis, a recognized pivotal factor in hook formation, is also implicated in multifaceted developmental processes including plastochron and leaf senescence. However, its molecular function remains to be elucidated. We utilized LC-MS for plant hormone quantification and implemented a series of molecular genetic and physiological approaches to analyze HLS1 functions. Our findings indicate that HLS1 functions in tissues distant from its expression sites in the regulation of apical hook formation, plastochron, and leaf senescence, implying its non-cell-autonomous mode of action. We observed that extranuclear localization of HLS1 can complement the hls1 phenotype, suggesting a revision of our understanding that HLS1 is a nuclear-specific histone acetyltransferase. The double mutant of HLS1 and its paralog shows highly pleiotropic and enhanced phenotypes similar to those of the double mutant of the putative carboxypeptidase gene ALTERED MERISTEM PROGRAM1 (AMP1) and its paralog. Genetic analysis revealed that HLS1 and AMP1 act in the same genetic pathway to regulate the plastochron. No significant differences in the contents of known plant hormones were observed in the hls1 mutant. HLS1 is an N-acetyltransferase that functions in a non-cell-autonomous manner across pleiotropic developmental phenomena in conjunction with AMP1, potentially independent of known plant hormone signaling.

Identification and functional characterization of the MYB transcription factor GmMYBLJ in soybean leaf senescence

Leaf senescence is an important agronomic trait that significantly influences the quality and yield of soybeans. v-Myb avian myeloblastosis viral oncogene homolog (MYB) transcription factors are considered crucial regulators governing leaf senescence, which can be utilized to improve agronomic traits in crops. However, our knowledge regarding the functional roles of soybean MYBs in leaf senescence is extremely limited. In this study, GmMYBLJ, a CCA1-like MYB, was identified and functionally characterized with respect to leaf senescence. The GmMYBLJ protein is localized in the nucleus, and a high accumulation of its transcripts was observed in nodules and embryos. Notably, GmMYBLJ was highly expressed in soybean senescent leaves and was transcriptionally induced by dark or NaCl treatment, as confirmed by histochemical GUS staining analysis. Ectopic overexpression of GmMYBLJ in Arabidopsis not only led to earlier leaf senescence, reduced chlorophyll content, and increased MDA accumulation but also promoted the expression of several WRKY family transcription factors and senescence-associated genes, such as SAG12 and ORE1. Further investigation showed that overexpression of GmMYBLJ accelerated Arabidopsis leaf senescence under darkness and in response to Pst DC3000 infection. Moreover, transgenic soybean plants overexpressing GmMYBLJ grew faster and exhibited accelerated senescence under salt stress. DAB staining analysis showed that GmMYBLJ induced ROS accumulation in soybean hairy roots and Arabidopsis leaves. Collectively, our results provided useful information into the functional roles of GmMYBLJ in both age-dependent and stress-induced senescence.

Identification and characterisation of 'No apical meristem; Arabidopsis transcription activation factor; Cup-shape cotyledon' (NAC) family transcription factors involved in sugar accumulation and abscisic acid signalling in grape (Vitis vinifera)

The 'No apical meristem; Arabidopsis transcription activation factor; Cup-shape cotyledon' (NAC) transcription factors are pivotal in plant development and stress response. Sucrose-non-fermenting-related protein kinase 1.2 (SnRK1) is a key enzyme in glucose metabolism and ABA signalling. In this study, we used grape (Vitis vinifera ) calli to explore NAC's roles in sugar and ABA pathways and its relationship with VvSnRK1.2 . We identified 19 VvNACs highly expressed at 90days after blooming, coinciding with grape maturity and high sugar accumulation, and 11 VvNACs randomly selected from 19 were demonstrated in response to sugar and ABA treatments. VvNAC26 showed significant response to sugar and ABA treatments, and its protein, as a nucleus protein, had transcriptional activation in yeast. We obtained the overexpression (OE-VvNAC26 ) and RNA-inhibition (RNAi-VvNAC26 ) of VvNAC26 in transgenic calli by Agrobacterium tumefaciens -mediated transformation. We found that VvNAC26 negatively influenced fructose content. Under sugar and ABA treatments, VvNAC26 negatively influenced the expression of most sugar-related genes, while positively influencing the expression of most ABA pathway-related genes. Dual-luciferase reporter experiments demonstrated that VvNAC26 significantly upregulates VvSnRK1.2 promoter expression in tobacco (Nicotiana benthamiana ) leaves, although this process in grape calli requires ABA. The levels of sugar content, sugar-related genes, and ABA-related genes fluctuated significantly in OE-VvNAC26 +RNAi-VvSnRK1.2 and OE-VvSnRK1.2 +RNAi-VvNAC26 transgenic calli. These findings indicated that VvNAC26 regulates sugar metabolism and ABA pathway, displaying synergistic interactions with VvSnRK1.2 .

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.

How abiotic stresses trigger sugar signaling to modulate leaf senescence?

Plants have evolved the adaptive capacity to mitigate the negative effect of external adversities at chemical, molecular, cellular, and physiological levels. This capacity is conferred by triggering the coordinated action of internal regulatory factors, in which sugars play an essential role in the regulating chloroplast degradation and leaf senescence under various stresses. In this review, we summarize the recent findings on the senescent-associated changes in carbohydrate metabolism and its relation to chlorophyl degradation, oxidative damage, photosynthesis inhibition, programmed cell death (PCD), and sink-source relation as affected by abiotic stresses. The action of sugar signaling in regulating the initiation and progression of leaf senescence under abiotic stresses involves interactions with various plant hormones, reactive oxygen species (ROS) burst, and protein kinases. This discussion aims to elucidate the complex regulatory network and molecular mechanisms that underline sugar-induced leaf senescence in response to various abiotic stresses. The imperative role of sugar signaling in regulating plant stress responses potentially enables the production of crop plants with modified sugar metabolism. This, in turn, may facilitate the engineering of plants with improved stress responses, optimal life span and higher yield achievement.

NACs, generalist in plant life

Plant-specific NAC proteins constitute a major transcription factor family that is well-known for its roles in plant growth, development, and responses to abiotic and biotic stresses. In recent years, there has been significant progress in understanding the functions of NAC proteins. NAC proteins have a highly conserved DNA-binding domain; however, their functions are diverse. Previous understanding of the structure of NAC transcription factors can be used as the basis for their functional diversity. NAC transcription factors consist of a target-binding domain at the N-terminus and a highly versatile C-terminal domain that interacts with other proteins. A growing body of research on NAC transcription factors helps us comprehend the intricate signalling network and transcriptional reprogramming facilitated by NAC-mediated complexes. However, most studies of NAC proteins have been limited to a single function. Here, we discuss the upstream regulators, regulatory components and targets of NAC in the context of their prospective roles in plant improvement strategies via biotechnology intervention, highlighting the importance of the NAC transcription factor family in plants and the need for further research.

Natural variation in STAYGREEN contributes to low-temperature tolerance in cucumber

Low-temperature (LT) stress threatens cucumber production globally; however, the molecular mechanisms underlying LT tolerance in cucumber remain largely unknown. Here, using a genome-wide association study (GWAS), we found a naturally occurring single nucleotide polymorphism (SNP) in the STAYGREEN (CsSGR) coding region at the gLTT5.1 locus associated with LT tolerance. Knockout mutants of CsSGR generated by clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated nuclease 9 exhibit enhanced LT tolerance, in particularly, increased chlorophyll (Chl) content and reduced reactive oxygen species (ROS) accumulation in response to LT. Moreover, the C-repeat Binding Factor 1 (CsCBF1) transcription factor can directly activate the expression of CsSGR. We demonstrate that the LT-sensitive haplotype CsSGRHapA , but not the LT-tolerant haplotype CsSGRHapG could interact with NON-YELLOW COLORING 1 (CsNYC1) to mediate Chl degradation. Geographic distribution of the CsSGR haplotypes indicated that the CsSGRHapG was selected in cucumber accessions from high latitudes, potentially contributing to LT tolerance during cucumber cold-adaptation in these regions. CsSGR mutants also showed enhanced tolerance to salinity, water deficit, and Pseudoperonospora cubensis, thus CsSGR is an elite target gene for breeding cucumber varieties with broad-spectrum stress tolerance. Collectively, our findings provide new insights into LT tolerance and will ultimately facilitate cucumber molecular breeding.

Transcriptome Analysis of Native Kentucky Bluegrass (Poa pratensis L.) in Response to Osmotic Stress

Kentucky bluegrass (Poa pratensis L.) is an important cool season turfgrass species with a high cold tolerance, but it is sensitive to drought. It is valuable for the applications of Kentucky bluegrass to improve its drought tolerance. However, little is known about the underlying drought mechanism. In the present study, transcriptomic profiling in the roots and leaves of the Kentucky bluegrass cultivar 'Qinghai', in response to osmotic stress in the form of treatment with 2 h and 50 h of 25% (v/v) PEG-6000, was analyzed. The results showed that a large number of genes were significantly up-regulated or down-regulated under osmotic stress. The majority of genes were up-regulated in leaves but down-regulated in roots after 2 h and 50 h of osmotic stress, among them were 350 up-regulated DEGs and 20 down-regulated DEGs shared in both leaves and roots. GO and KEGG analysis showed that carbohydrate metabolism, polyamine and amino acid metabolism and the plant hormone signaling pathway were enriched in the leaves and roots of 'Qinghai' after osmotic stress. The genes involving in carbohydrate metabolism were up-regulated, and sucrose, trehalose and raffinose levels were consistently increased. The genes involved in polyamine and amino acid metabolism were up-regulated in leaves in response to osmotic stress and several amino acids, such as Glu, Met and Val levels were increased, while the genes involved in photosynthesis, carbon fixation and citrate cycle in leaves were down-regulated. In addition, the genes involved in plant hormone biosynthesis and signal transduction were altered in leaves after osmotic stress. This study provided promising candidate genes for studying drought mechanisms in 'Qinghai' and improving the drought tolerance of Kentucky bluegrass and drought-sensitive crops.

A truncated ETHYLENE INSENSITIVE3-like protein, GhLYI, regulates senescence in cotton

Numerous endogenous and environmental signals regulate the intricate and highly orchestrated process of plant senescence. Ethylene (ET), which accumulates as senescence progresses, is a major promoter of leaf senescence. The master transcription activator ETHYLENE INSENSITIVE3 (EIN3) activates the expression of a wide range of downstream genes during leaf senescence. Here, we found that a unique EIN3-LIKE 1 (EIL1) gene, cotton LINT YIELD INCREASING (GhLYI), encodes a truncated EIN3 protein in upland cotton (Gossypium hirsutum L.) that functions as an ET signal response factor and a positive regulator of senescence. Ectopic expression or overexpression of GhLYI accelerated leaf senescence in both Arabidopsis (Arabidopsis thaliana) and cotton. Cleavage under targets and tagmentation (CUT&Tag) analyses revealed that SENESCENCE-ASSOCIATED GENE 20 (SAG20) was a target of GhLYI. Electrophoretic mobility shift assay (EMSA), yeast 1-hybrid (Y1H), and dual-luciferase transient expression assay confirmed that GhLYI directly bound the promoter of SAG20 to activate its expression. Transcriptome analysis revealed that transcript levels of a series of senescence-related genes, SAG12, NAC-LIKE, ACTIVATED by APETALA 3/PISTILLATA (NAP/ANAC029), and WRKY53, are substantially induced in GhLYI overexpression plants compared with wild-type (WT) plants. Virus-induced gene silencing (VIGS) preliminarily confirmed that knockdown of GhSAG20 delayed leaf senescence. Collectively, our findings provide a regulatory module involving GhLYI-GhSAG20 in controlling senescence in cotton.

Arabidopsis metacaspase MC1 localizes in stress granules, clears protein aggregates, and delays senescence

Stress granules (SGs) are highly conserved cytoplasmic condensates that assemble in response to stress and contribute to maintaining protein homeostasis. These membraneless organelles are dynamic, disassembling once the stress is no longer present. Persistence of SGs due to mutations or chronic stress has been often related to age-dependent protein-misfolding diseases in animals. Here, we find that the metacaspase MC1 is dynamically recruited into SGs upon proteotoxic stress in Arabidopsis (Arabidopsis thaliana). Two predicted disordered regions, the prodomain and the 360 loop, mediate MC1 recruitment to and release from SGs. Importantly, we show that MC1 has the capacity to clear toxic protein aggregates in vivo and in vitro, acting as a disaggregase. Finally, we demonstrate that overexpressing MC1 delays senescence and this phenotype is dependent on the presence of the 360 loop and an intact catalytic domain. Together, our data indicate that MC1 regulates senescence through its recruitment into SGs and this function could potentially be linked to its remarkable protein aggregate-clearing activity.

Ethylene-mediated metabolic priming increases photosynthesis and metabolism to enhance plant growth and stress tolerance

Enhancing crop yields is a major challenge because of an increasing human population, climate change, and reduction in arable land. Here, we demonstrate that long-lasting growth enhancement and increased stress tolerance occur by pretreatment of dark grown Arabidopsis seedlings with ethylene before transitioning into light. Plants treated this way had longer primary roots, more and longer lateral roots, and larger aerial tissue and were more tolerant to high temperature, salt, and recovery from hypoxia stress. We attributed the increase in plant growth and stress tolerance to ethylene-induced photosynthetic-derived sugars because ethylene pretreatment caused a 23% increase in carbon assimilation and increased the levels of glucose (266%), sucrose/trehalose (446%), and starch (87%). Metabolomic and transcriptomic analyses several days posttreatment showed a significant increase in metabolic processes and gene transcripts implicated in cell division, photosynthesis, and carbohydrate metabolism. Because of this large effect on metabolism, we term this "ethylene-mediated metabolic priming." Reducing photosynthesis with inhibitors or mutants prevented the growth enhancement, but this was partially rescued by exogenous sucrose, implicating sugars in this growth phenomenon. Additionally, ethylene pretreatment increased the levels of CINV1 and CINV2 encoding invertases that hydrolyze sucrose, and cinv1;cinv2 mutants did not respond to ethylene pretreatment with increased growth indicating increased sucrose breakdown is critical for this trait. A model is proposed where ethylene-mediated metabolic priming causes long-term increases in photosynthesis and carbohydrate utilization to increase growth. These responses may be part of the natural development of seedlings as they navigate through the soil to emerge into light.

MORF2-mediated plastidial retrograde signaling is involved in stress response and skotomorphogenesis beyond RNA editing

Retrograde signaling modulates the expression of nuclear genome-encoded organelle proteins to adjust organelle function in response to environmental cues. MULTIPLE ORGANELLAR RNA EDITING FACTOR 2 (MORF2) was initially recognized as a plastidial RNA-editing factor but recently shown to interact with GUN1. Given the central role of GUN1 in chloroplast retrograde signaling and the unviable phenotype of morf2 mutants that is inconsistent with many viable mutants involved in RNA editing, we hypothesized that MORF2 has functions either dosage dependent or beyond RNA editing. Using an inducible Clustered Interspaced Short Palindromic Repeat interference (iCRISPRi) approach, we were able to reduce the MORF2 transcripts in a controlled manner. In addition to MORF2-dosage dependent RNA-editing errors, we discovered that reducing MORF2 by iCRISPRi stimulated the expression of stress responsive genes, triggered plastidial retrograde signaling, repressed ethylene signaling and skotomorphogenesis, and increased accumulation of hydrogen peroxide. These findings along with previous discoveries suggest that MORF2 is an effective regulator involved in plastidial metabolic pathways whose reduction can readily activate multiple retrograde signaling molecules possibly involving reactive oxygen species to adjust plant growth. In addition, our newly developed iCRISPRi approach provided a novel genetic tool for quantitative reverse genetics studies on hub genes in plants.

Transmembrane potential, an indicator in situ reporting cellular senescence and stress response in plant tissues

BACKGROUND

Plant cells usually sustain a stable membrane potential due to influx and/or efflux of charged ions across plasma membrane. With the growth and development of plants, different tissues and cells undergo systemic or local programmed decline. Whether the membrane potential of plasma membrane could report senescence signal of plant tissues and cells is unclear.

RESULTS

We applied a maneuverable transmembrane potential (TMP) detection method with patch-clamp setup to examine the senescence signal of leaf tissue cells in situ over the whole life cycle in Arabidopsis thaliana. The data showed that the TMPs of plant tissues and cells were varied at different growth stages, and the change of TMP was higher at the vegetative growth stage than at the reproductive stage of plant growth. The distinct change of TMP was detectable between the normal and the senescent tissues and cells in several plant species. Moreover, diverse abiotic stimuli, such as heat stress, hyperpolarized the TMP in a short time, followed by depolarized membrane potential with the senescence occurring. We further examined the TMP of plant chloroplasts, which also indicates the senescence signal in organelles.

CONCLUSIONS

This convenient TMP detection method can report the senescence signal of plant tissues and cells, and can also indicate the potential of plant tolerance to environmental stress.

Phytochrome interacting factor regulates stomatal aperture by coordinating red light and abscisic acid

Stomata are crucial valves coordinating the fixation of carbon dioxide by photosynthesis and water loss through leaf transpiration. Phytochrome interacting factors (PIFs) are negative regulators of red light responses that belong to the basic helix-loop-helix family of transcription factors. Here, we show that the rice (Oryza sativa) PIF family gene OsPIL15 acts as a negative regulator of stomatal aperture to control transpiration in rice. OsPIL15 reduces stomatal aperture by activating rice ABSCISIC ACID INSENSITIVE 5 (OsABI5), which encodes a critical positive regulator of ABSCISIC ACID (ABA) signaling in rice. Moreover, OsPIL15 interacts with the NIGT1/HRS1/HHO family transcription factor rice HRS1 HOMOLOG 3 (OsHHO3) to possibly enhance the regulation of stomatal aperture. Notably, we discovered that the maize (Zea mays) PIF family genes ZmPIF1 and ZmPIF3, which are homologous to OsPIL15, are also involved in the regulation of stomatal aperture in maize, indicating that PIF-mediated regulation of stomatal aperture may be conserved in the plant lineage. Our findings explain the molecular mechanism by which PIFs play a role in red-light-mediated stomatal opening, and demonstrate that PIFs regulate stomatal aperture by coordinating the red light and ABA signaling pathways.

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.

HFR1, a bHLH Transcriptional Regulator from Arabidopsis thaliana, Improves Grain Yield, Shade and Osmotic Stress Tolerances in Common Wheat

Common wheat, Triticum aestivum, is the most widely grown staple crop worldwide. To catch up with the increasing global population and cope with the changing climate, it is valuable to breed wheat cultivars that are tolerant to abiotic or shade stresses for density farming. Arabidopsis LONG HYPOCOTYL IN FAR-RED 1 (AtHFR1), a photomorphogenesis-promoting factor, is involved in multiple light-related signaling pathways and inhibits seedling etiolation and shade avoidance. We report that overexpression of AtHFR1 in wheat inhibits etiolation phenotypes under various light and shade conditions, leading to shortened plant height and increased spike number relative to non-transgenic plants in the field. Ectopic expression of AtHFR1 in wheat increases the transcript levels of TaCAB and TaCHS as observed previously in Arabidopsis, indicating that the AtHFR1 transgene can activate the light signal transduction pathway in wheat. AtHFR1 transgenic seedlings significantly exhibit tolerance to osmotic stress during seed germination compared to non-transgenic wheat. The AtHFR1 transgene represses transcription of TaFT1, TaCO1, and TaCO2, delaying development of the shoot apex and heading in wheat. Furthermore, the AtHFR1 transgene in wheat inhibits transcript levels of PHYTOCHROME-INTERACTING FACTOR 3-LIKEs (TaPIL13, TaPIL15-1B, and TaPIL15-1D), downregulating the target gene STAYGREEN (TaSGR), and thus delaying dark-induced leaf senescence. In the field, grain yields of three AtHFR1 transgenic lines were 18.2–48.1% higher than those of non-transgenic wheat. In summary, genetic modification of light signaling pathways using a photomorphogenesis-promoting factor has positive effects on grain yield due to changes in plant architecture and resource allocation and enhances tolerances to osmotic stress and shade avoidance response.

The RPN12a proteasome subunit is essential for the multiple hormonal homeostasis controlling the progression of leaf senescence

The 26S proteasome is a conserved multi-subunit machinery in eukaryotes. It selectively degrades ubiquitinated proteins, which in turn provides an efficient molecular mechanism to regulate numerous cellular functions and developmental processes. Here, we studied a new loss-of-function allele of RPN12a, a plant ortholog of the yeast and human structural component of the 19S proteasome RPN12. Combining a set of biochemical and molecular approaches, we confirmed that a rpn12a knock-out had exacerbated 20S and impaired 26S activities. The altered proteasomal activity led to a pleiotropic phenotype affecting both the vegetative growth and reproductive phase of the plant, including a striking repression of leaf senescence associate cell-death. Further investigation demonstrated that RPN12a is involved in the regulation of several conjugates associated with the auxin, cytokinin, ethylene and jasmonic acid homeostasis. Such enhanced aptitude of plant cells for survival in rpn12a contrasts with reports on animals, where 26S proteasome mutants generally show an accelerated cell death phenotype.

Reevaluation of ethylene role in Arabidopsis cauline leaf abscission induced by water stress and rewatering

It was previously reported that cauline leaf abscission in Arabidopsis is induced by a cycle of water stress and rewatering, which is regulated by the complex of INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), HAESA (HAE), and HAESA-LIKE2 (HSL2) kinases. However, the involvement of ethylene in this process was ruled out. Because this conclusion contradicts the well-established role of ethylene in organ abscission induced by a cycle of water stress and rewatering, our present study was aimed to reevaluate the possible involvement of ethylene in this process. For this purpose, we examined the endogenous ethylene production during water stress and following rewatering, as well as the effects of exogenous ethylene and 1-methylcyclopropene (1-MCP), on cauline leaf abscission of Arabidopsis wild type. Additionally, we examined whether this stress induces cauline leaf abscission in ethylene-insensitive Arabidopsis mutants. The results of the present study demonstrated that ethylene production rates increased significantly in cauline leaves at 4 h after rewatering of stressed plants and remained high for at least 24 h in plants water-stressed to 40 and 30% of system weight. Ethylene treatment applied to well-watered plants induced cauline leaf abscission, which was inhibited by 1-MCP. Cauline leaf abscission was also inhibited by 1-MCP applied during a cycle of water stress and rewatering. Finally, no abscission occurred in two ethylene-insensitive mutants, ein2-1 and ein2-5, following a cycle of water stress and rewatering. Taken together, these results clearly indicate that ethylene is involved in Arabidopsis cauline leaf abscission induced by water stress and rewatering. Our results show that ethylene is involved in Arabidopsis cauline leaf abscission induced by water stress and rewatering, similar to leaf abscission in other plants.

Editing of the starch branching enzyme gene SBE2 generates high-amylose storage roots in cassava

The production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme gene SBE2 was firstly achieved. High-amylose cassava (Manihot esculenta Crantz) is desirable for starch industrial applications and production of healthier processed food for human consumption. In this study, we report the production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme 2 (SBE2). Mutations in two targeted exons of SBE2 were identified in all regenerated plants; these mutations, which included nucleotide insertions, and short or long deletions in the SBE2 gene, were classified into eight mutant lines. Three mutants, M6, M7 and M8, with long fragment deletions in the second exon of SBE2 showed no accumulation of SBE2 protein. After harvest from the field, significantly higher amylose (up to 56% in apparent amylose content) and resistant starch (up to 35%) was observed in these mutants compared with the wild type, leading to darker blue coloration of starch granules after quick iodine staining and altered starch viscosity with a higher pasting temperature and peak time. Further ¹H-NMR analysis revealed a significant reduction in the degree of starch branching, together with fewer short chains (degree of polymerization [DP] 15-25) and more long chains (DP>25 and especially DP>40) of amylopectin, which indicates that cassava SBE2 catalyzes short chain formation during amylopectin biosynthesis. Transition from A- to B-type crystallinity was also detected in the starches. Our study showed that CRISPR/Cas9-mediated mutagenesis of starch biosynthetic genes in cassava is an effective approach for generating novel varieties with valuable starch properties for food and industrial applications.

A unique miR775-GALT9 module regulates leaf senescence in Arabidopsis during post-submergence recovery by modulating ethylene and the abscisic acid pathway

The submergence-induced hypoxic condition negatively affects the plant growth and development, and causes early onset of senescence. Hypoxia alters the expression of a number of microRNAs (miRNAs). However, the molecular function of submergence stress-induced miRNAs in physiological or developmental changes and recovery remains poorly understood. Here, we show that miR775 is an Arabidopsis thaliana-specific young and unique miRNA that possibly evolved non-canonically. miR775 post-transcriptionally regulates GALACTOSYLTRANSFERASE 9 (GALT9) and their expression is inversely affected at 24 h of complete submergence stress. The overexpression of miR775 (miR775-Oe) confers enhanced recovery from submergence stress and reduced accumulation of RBOHD and ROS, in contrast to wild-type and MIM775 Arabidopsis shoot. A similar recovery phenotype in the galt9 mutant indicates the role of the miR775-GALT9 module in post-submergence recovery. We predicted that Golgi-localized GALT9 is potentially involved in protein glycosylation. The altered expression of senescence-associated genes (SAG12, SAG29 and ORE1), ethylene signalling (EIN2 and EIN3) and abscisic acid (ABA) biosynthesis (NCED3) pathway genes occurs in miR775-Oe, galt9 and MIM775 plants. Thus, our results indicate the role for the miR775-GALT9 module in post-submergence recovery through a crosstalk between the ethylene signalling and ABA biosynthesis pathways.

To Fight or to Grow: The Balancing Role of Ethylene in Plant Abiotic Stress Responses

Plants often live in adverse environmental conditions and are exposed to various stresses, such as heat, cold, heavy metals, salt, radiation, poor lighting, nutrient deficiency, drought, or flooding. To adapt to unfavorable environments, plants have evolved specialized molecular mechanisms that serve to balance the trade-off between abiotic stress responses and growth. These mechanisms enable plants to continue to develop and reproduce even under adverse conditions. Ethylene, as a key growth regulator, is leveraged by plants to mitigate the negative effects of some of these stresses on plant development and growth. By cooperating with other hormones, such as jasmonic acid (JA), abscisic acid (ABA), brassinosteroids (BR), auxin, gibberellic acid (GA), salicylic acid (SA), and cytokinin (CK), ethylene triggers defense and survival mechanisms thereby coordinating plant growth and development in response to abiotic stresses. This review describes the crosstalk between ethylene and other plant hormones in tipping the balance between plant growth and abiotic stress responses.

2-Hydroxymelatonin, Rather Than Melatonin, Is Responsible for RBOH-Dependent Reactive Oxygen Species Production Leading to Premature Senescence in Plants

Unlike animals, plants amply convert melatonin into 2-hydroxymelatonin (2-OHM) and cyclic 3-hydroxymelatonin (3-OHM) through the action of melatonin 2-hydroxylase (M2H) and melatonin 3-hydroxylase (M3H), respectively. Thus, the effects of exogenous melatonin treatment in plants may be caused by melatonin, 2-OHM, or 3-OHM, or some combination of these compounds. Indeed, studies of melatonin's effects on reactive oxygen species (ROS) production have reported conflicting results. In this study, we demonstrated that 2-OHM treatment induced ROS production, whereas melatonin did not. ROS production from 2-OHM treatment occurred in old arabidopsis leaves in darkness, consistent with an ethylene-mediated senescence mechanism. Transgenic tobacco plants containing overexpressed rice M2H exhibited dwarfism and leaf necrosis of the upper leaves and early senescence of the lower leaves. We also demonstrated that 2-OHM-mediated ROS production is respiratory burst NADPH oxidase (RBOH)-dependent and that 2-OHM-induced senescence genes require ethylene and the abscisic acid (ABA) signaling pathway in arabidopsis. In contrast to melatonin, 2-OHM treatment induced senescence symptoms such as leaf chlorosis and increased ion leakage in arabidopsis. Senescence induction is known to begin with decreased levels of proteins involved in chloroplast maintenance, including Lhcb1 and ClpR1. Together, these results show that 2-OHM acts as a senescence-inducing factor by inducing ROS production in plants.

Arabidopsis WRKY71 regulates ethylene-mediated leaf senescence by directly activating EIN2, ORE1 and ACS2 genes

Leaf senescence is a pivotal step in the last stage of the plant life cycle and is influenced by various external and endogenous cues. A series of reports have indicated the involvement of the WRKY transcription factors in regulating leaf senescence, but the molecular mechanisms and signaling pathways remain largely unclear. Here we provide evidence demonstrating that WRKY71 acts as a positive regulator of leaf senescence in Arabidopsis. WRKY71-1D, an overexpressor of WRKY71, exhibited early leaf senescence, while wrky71-1, the WRKY71 loss-of-function mutant, displayed delayed leaf senescence. Accordingly, a set of senescence-associated genes (SAGs) were substantially elevated in WRKY71-1D but markedly decreased in wrky71-1. Chromatin immunoprecipitation assays indicated that WRKY71 can bind directly to the promoters of SAG13 and SAG201. Transcriptome analysis suggested that WRKY71 might mediate multiple cues to accelerate leaf senescence, such as abiotic stresses, dark and ethylene. WRKY71 was ethylene inducible, and treatment with the ethylene precursor 1-amino-cyclopropane-1-carboxylic acid enhanced leaf senescence in WRKY71-1D but caused only a marginal delay in leaf senescence in wrky71-1. In vitro and in vivo assays demonstrated that WRKY71 can directly regulate ETHYLENE INSENSITIVE2 (EIN2) and ORESARA1 (ORE1), genes of the ethylene signaling pathway. Consistently, leaf senescence of WRKY71-1D was obviously retarded in the ein2-5 and nac2-1 mutants. Moreover, WRKY71 was also proved to interact with ACS2 in vitro and in vivo. Treatment with AgNO₃ and aminoethoxyvinylglycine and acs2-1 could greatly arrest the leaf senescence of WRKY71-1D. In conclusion, our data revealed that WRKY71 mediates ethylene signaling and synthesis to hasten leaf senescence in Arabidopsis.

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

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

Highly pleiotropic functions of CYP78As and AMP1 are regulated in non-cell-autonomous/organ-specific manners

The cytochrome P450 CYP78A5/KLUH in Arabidopsis thaliana is predicted to be involved in the synthesis of a mobile signal molecule that has a pleiotropic function that is distinct from classical phytohormones. CYP78A5 has five close relatives in Arabidopsis. We first investigated their functions, focusing on the plastochron, leaf size, and leaf senescence. Our analyses revealed that CYP78A5 and CYP78A7 are involved in the plastochron and leaf size, and CYP78A6 and CYP78A9 are involved in leaf senescence. Complementation analyses using heterologous promoters and expression analyses suggested that CYP78A isoforms have a common biochemical function and are functionally differentiated via organ-specific expression. The altered meristem program1 (amp1) carboxypeptidase mutant shows a phenotype very similar to that of the cyp78a5 mutant. Complementation analyses using boundary and organizing center-specific promoters suggested that both CYP78A5 and AMP1 act in a non-cell-autonomous manner. Analyses of multiple cyp78a mutants and crosses between cyp78a and amp1 mutants revealed that AMP1/LIKE AMP1 (LAMP1) and CYP78A isoforms regulate plastochron length and leaf senescence in the same genetic pathway, whereas leaf size is independently regulated. Furthermore, we detected feedback regulation between CYP78A6/CYP78A9 and AMP1 at the gene expression level. These observations raise the possibility that AMP1 and CYP78A isoforms are involved in the synthesis of the same mobile signal molecule, and suggest that AMP1 and CYP78A signaling pathways have a very close, albeit complex, functional relationship.

A Plant Based Modified Biostimulant (Copper Chlorophyllin), Mediates Defense Response in Arabidopsis thaliana under Salinity Stress

To date, managing salinity stress in agriculture relies heavily on development of salt tolerant plant varieties, a time-consuming process particularly challenging for many crops. Plant based biostimulants (PBs) that enhance plant defenses under stress can potentially address this drawback, as they are not crop specific and are easy to apply in the field. Unfortunately, limited knowledge about their modes of action makes it harder to utilize them on a broader scale. Understanding how PBs enhance plant defenses at cellular and molecular levels, is a prerequisite for the development of sustainable management practices utilizing biostimulants to improve crop health. In this study we elucidated the protective mechanism of copper chlorophyllin (Cu-chl), a PB, under salinity stress. Our results indicate that Cu-chl exerts protective effects primarily by decreasing oxidative stress through modulating cellular H₂O₂ levels. Cu-chl treated plants increased tolerance to oxidative stress imposed by an herbicide, methyl viologen dichloride hydrate as well, suggesting a protective role against various sources of reactive oxygen species (ROS). RNA-Seq analysis of Cu-chl treated Arabidopsis thaliana seedlings subjected to salt stress identified genes involved in ROS detoxification, and cellular growth.

Basic Protein Modules Combining Abscisic Acid and Light Signaling in Arabidopsis

It is generally accepted that plants use the complex signaling system regulated by light and abscisic acid (ABA) signaling components to optimize growth and development in different situations. The role of ABA-light interactions is evident in the coupling of stress defense reactions with seed germination and root development, maintaining of stem cell identity and stem cell specification, stem elongation and leaf development, flowering and fruit formation, senescence, and shade avoidance. All these processes are regulated jointly by the ABA-light signaling system. Although a lot of work has been devoted to ABA-light signal interactions, there is still no systematic description of central signaling components and protein modules, which jointly regulate plant development. New data have emerged to promote understanding of how ABA and light signals are integrated at the molecular level, representing an extensively growing area of research. This work is intended to fill existing gaps by using literature data combined with bioinformatics analysis.