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

Stress-induced senescence and plant tolerance to abiotic stress.

Although abscisic acid (ABA) is an important hormone that regulates seed dormancy, stomatal closure, plant development, as well as responses to environmental stimuli, the physiological mechanisms of ABA response to multiple stress in rice remain poorly understood. In the ABA biosynthetic pathway, 9-cis-epoxycarotenoid dioxygenase (NCED) is the key rate-limiting enzyme. Here, we report important functions of OsNCED3 in multi-abiotic stress tolerance in rice. The OsNCED3 is constitutively expressed in various tissues under normal condition, Its expression is highly induced by NaCl, PEG, and H2O2 stress, suggesting the roles for OsNCED3 in response to the multi-abiotic stress tolerance in rice. Compared with wild-type plants, nced3 mutants had earlier seed germination, longer post-germination seedling growth, increased sensitivity to water stress and H2O2 stress and increased stomata aperture under water stress and delayed leaf senescence. Further analysis found that nced3 mutants contained lower ABA content compared with wild-type plants, overexpression of OsNCED3 in transgenic plants could enhance water stress tolerance, promote leaf senescence and increase ABA content. We conclude that OsNCED3 mediates seed dormancy, plant growth, abiotic stress tolerance, and leaf senescence by regulating ABA biosynthesis in rice; and may provide a new strategy for improving the quality of crop.

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9-cis-Epoxycarotenoid Dioxygenase 3 Regulates Plant Growth and Enhances Multi-Abiotic Stress Tolerance in Rice.

Leaf senescence is an essential physiological process in plants that supports the recycling of nitrogen and other nutrients to support the growth of developing organs, including young leaves, seeds, and fruits. Thus, the regulation of senescence is crucial for evolutionary success in wild populations and for increasing yield in crops. Here, we describe the influence of a NAC transcription factor, SlNAP2 (Solanum lycopersicum NAC-like, activated by Apetala3/Pistillata), that controls both leaf senescence and fruit yield in tomato (S. lycopersicum). SlNAP2 expression increases during age-dependent and dark-induced leaf senescence. We demonstrate that SlNAP2 activates SlSAG113 (S. lycopersicum SENESCENCE-ASSOCIATED GENE113), a homolog of Arabidopsis (Arabidopsis thaliana) SAG113, chlorophyll degradation genes such as SlSGR1 (S. lycopersicum senescence-inducible chloroplast stay-green protein 1) and SlPAO (S. lycopersicum pheide a oxygenase), and other downstream targets by directly binding to their promoters, thereby promoting leaf senescence. Furthermore, SlNAP2 directly controls the expression of genes important for abscisic acid (ABA) biosynthesis, S. lycopersicum 9-cis-epoxycarotenoid dioxygenase 1 (SlNCED1); transport, S. lycopersicum ABC transporter G family member 40 (SlABCG40); and degradation, S. lycopersicum ABA 8′-hydroxylase (SlCYP707A2), indicating that SlNAP2 has a complex role in establishing ABA homeostasis during leaf senescence. Inhibiting SlNAP2 expression in transgenic tomato plants impedes leaf senescence but enhances fruit yield and sugar content likely due to prolonged leaf photosynthesis in aging tomato plants. Our data indicate that SlNAP2 has a central role in controlling leaf senescence and fruit yield in tomato.

The NAC Transcription Factor SlNAP2 Regulates Leaf Senescence and Fruit Yield in Tomato

The rice root system is important for growth The crosstalk between auxin and cytokinin mediates root initiation and elongation However, it remains unclear how the transcriptional network upstream of the auxin and cytokinin signalling pathways determines root development Here, we observed that the knockdown of OsNAC2, which encodes a NAC transcription factor, increased the primary root length and the number of crown roots OsNAC2 predominantly expressed in primary root tips, crown roots and lateral root primordia, implying it influences root development Molecular analyses revealed that the expressions of auxin- and cytokinin-responsive genes were affected in OsNAC2-overexpressing (OsNAC2-OX; ON7 and ON11), RNA interference (OsNAC2-RNAi; RNAi25 and RNAi31) and CRISPR/Cas9 plants Additionally, OsNAC2 can directly bind to the promoters of IAA inactivation-related genes (GH36 and GH38), an IAA signalling-related gene (OsARF25), and a cytokinin oxidase gene (OsCKX4) Furthermore, genetic analysis of ON11/osgh36 and RNAi31/osckx4 homozygote confirmed that OsCKX4 and OsGH36 functioned downstream of OsNAC2 The mRNA levels of CROWN ROOTLESS (CRL) genes and cyclin-dependent protein kinase (CDK) genes increased in OsNAC2-RNAi and OsNAC2-cas9 lines while reduced in OsNAC2-OX lines Thus, we describe that OsNAC2 functions as an upstream integrator of auxin and cytokinin signals that affect CRL and CDK production to regulate cell division during root development This novel auxin-OsNAC2-cytokinin model should provide a new insight into the understanding of NAC TFs and crosstalk of auxin and cytokinin pathway, and can be potentially applied in agriculture to enhance rice yields by genetic approaches

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pbi.13209

OsNAC2 integrates auxin and cytokinin pathways to modulate rice root development

Melatonin and abscisic acid (ABA) play contrasting roles in regulating leaf senescence in plants. The molecular mechanism underlying the interaction between melatonin and ABA involved in leaf senescence, however, remains poorly defined. Herein, we found that exogenous application of melatonin delayed the senescence of Chinese flowering cabbage, accompanied by reduced expression of chlorophyll catabolic and ABA biosynthetic genes, and a lower endogenous ABA level. Significantly, three nucleus-localized transcriptional activators BrABF1, BrABF4, and BrABI5 were identified, and their expressions were repressed by melatonin. In vitro and in vivo binding experiments revealed that BrABF1, BrABF4, and BrABI5 activated the transcription of a series of ABA biosynthetic and chlorophyll catabolic genes by physically binding to their promoters. Moreover, transient over-expression of BrABF1, BrABF4, and BrABI5 in tobacco leaves induced ABA accumulation and promoted chlorophyll degradation by upregulating tobacco ABA biosynthetic and chlorophyll catabolic genes, resulting in the accelerated leaf senescence. These effects were significantly attenuated by melatonin treatment. Our findings suggest that melatonin-mediated inhibition of leaf senescence involves suppression of ABFs-mediated ABA biosynthesis and chlorophyll degradation. Unraveling of the molecular regulatory mechanism of leaf senescence controlled by ABA and melatonin expands our understanding of the regulation of this phenomenon and offers potentially more effective molecular breeding strategies for extending the shelf-life of Chinese flowering cabbage.

Melatonin delays leaf senescence of Chinese flowering cabbage by suppressing ABFs-mediated abscisic acid biosynthesis and chlorophyll degradation.

It is well known that abscisic acid (ABA)-induced leaf senescence and premature leaf senescence negatively affect the yield of rice (Oryza sativa). However, the molecular mechanism underlying this relationship, especially the upstream transcriptional network that modulates ABA level during leaf senescence, remains largely unknown. Here, we demonstrate a rice NAC transcription factor, OsNAC2, that participates in ABA-induced leaf senescence. Overexpression of OsNAC2 dramatically accelerated leaf senescence, whereas its knockdown lines showed a delay in leaf senescence. Chromatin immunoprecipitation-quantitative PCR, dual-luciferase, and yeast one-hybrid assays demonstrated that OsNAC2 directly activates expression of chlorophyll degradation genes, OsSGR and OsNYC3. Moreover, ectopic expression of OsNAC2 leads to an increase in ABA levels via directly up-regulating expression of ABA biosynthetic genes (OsNCED3 and OsZEP1) as well as down-regulating the ABA catabolic gene (OsABA8ox1). Interestingly, OsNAC2 is upregulated by a lower level of ABA but downregulated by a higher level of ABA, indicating a feedback repression of OsNAC2 by ABA. Additionally, reduced OsNAC2 expression leads to about 10% increase in the grain yield of RNAi lines. The novel ABA-NAC-SAGs regulatory module might provide a new insight into the molecular action of ABA to enhance leaf senescence and elucidates the transcriptional network of ABA production during leaf senescence in rice.

A Rice NAC Transcription Factor Promotes Leaf Senescence via ABA Biosynthesis

Auxin is an important plant hormone that is essential for growth and development due to its effects on organogenesis, morphogenesis, tropisms, and apical dominance. The functional diversity of auxin highlights the importance of its biosynthesis, transport, and associated responses. In this study, we show that a NAC transcription factor, ANAC092 (also named AtNAC2 and ORESARA1), known to positively regulate leaf senescence and contribute to abiotic stress responses, also affects primary root development. Plants overexpressing ANAC092 had altered root meristem lengths and shorter primary roots compared with the wild-type control. Additionally, expression of the proANAC092::GUS was strongly induced by indole-3-acetic acid. Quantitative real-time RT-PCR (qRT-PCR) analysis revealed that the YUCCA2, PIN, and ARF expression levels were downregulated in ANAC092-overexpressing plants. Moreover, yeast one-hybrid and chromatin immunoprecipitation assays confirmed that ANAC092 binds to the promoters of AUXIN RESPONSE FACTOR 8 (ARF8) and PIN-FORMED 4 (PIN4). Furthermore, a dual-luciferase assay indicated that ANAC092 decreases ARF8 and PIN4 promoter activities. We also applied a CRISPR/Cas9 system to mutate ANAC092. The roots of three of the analyzed mutants were longer than normal. Collectively, our findings indicate that ANAC092 negatively affects root development by controlling the auxin pathway.

Arabidopsis ANAC092 regulates auxin-mediated root development by binding to the ARF8 and PIN4 promoters.

The invention belongs to the technical field of gene engineering, and particularly relates to a coding sequence expressed in rice and belonging to a cytokinin oxidation/dehydrogenase gene (denoted asOsCKX4) as well as application of the gene. The coding sequence particularly comprises cloning of the gene OsCKX4, an expression mode of an OsCKX4 organ and the change of the expression quantity of the OsCKX4 treated by different hormones and different stress. The invention also discloses sequencing authentication of a Tilling mutant of the gene OsCKX4, which can influence the growth of the root of the rice and the yield of the rice. The gene OsCKX4 can be applied to plant variety improvement.

Coding sequence of rice cytokinin oxidation/dehydrogenase gene OsCKX4 as well as application of gene

The invention belongs to the technical field of gene engineering, and specifically relates to a coding sequence of an indole-3-acetic acid-amido synthetase gene OsGH3.6 expressed in paddy rice and applications thereof. The applications are as follows: the nucleotide coding sequence of the indole-3-acetic acid-amido synthetase gene OsGH3.6 is cloned; the spatial expression pattern of different endogenous organs and tissues of the gene of paddy rice is analyzed and identified; after treatments such as drought treatment, high salt stress, plant hormone treatment, and the like, the change of expression pattern is analyzed and identified; the tilling mutant of OsGH3.6 is detected, and the root length and output are analyzed.

He Jianmei

Papers

A Rice NAC Transcription Factor Promotes Leaf Senescence via ABA Biosynthesis

Arabidopsis ANAC092 regulates auxin-mediated root development by binding to the ARF8 and PIN4 promoters.

Coding sequence of rice cytokinin oxidation/dehydrogenase gene OsCKX4 as well as application of gene

Coding sequence of paddy rice indole-3-acetic acid-amido synthetase gene OsGH3.6 and applications thereof