OsWRKY5 Promotes Rice Leaf Senescence via Senescence-Associated NAC and Abscisic Acid Biosynthesis Pathway.

Kim Kim,Paek Paek,An An,Kang Kang

doi:10.3390/ijms20184437

Kim Kim, Paek Paek + Show 2 more

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https://doi.org/10.3390/ijms20184437

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Abstract

The onset of leaf senescence is triggered by external cues and internal factors such as phytohormones and signaling pathways involving transcription factors (TFs). Abscisic acid (ABA) strongly induces senescence and endogenous ABA levels are finely tuned by many senescence-associated TFs. Here, we report on the regulatory function of the senescence-induced TF OsWRKY5 TF in rice (Oryza sativa). OsWRKY5 expression was rapidly upregulated in senescing leaves, especially in yellowing sectors initiated by aging or dark treatment. A T-DNA insertion activation-tagged OsWRKY5-overexpressing mutant (termed oswrky5-D) promoted leaf senescence under natural and dark-induced senescence (DIS) conditions. By contrast, a T-DNA insertion oswrky5-knockdown mutant (termed oswrky5) retained leaf greenness during DIS. Reverse-transcription quantitative PCR (RT-qPCR) showed that OsWRKY5 upregulates the expression of genes controlling chlorophyll degradation and leaf senescence. Furthermore, RT-qPCR and yeast one-hybrid analysis demonstrated that OsWRKY5 indirectly upregulates the expression of senescence-associated NAM/ATAF1/2/CUC2 (NAC) genes including OsNAP and OsNAC2. Precocious leaf yellowing in the oswrky5-D mutant might be caused by elevated endogenous ABA concentrations resulting from upregulated expression of ABA biosynthesis genes OsNCED3, OsNCED4, and OsNCED5, indicating that OsWRKY is a positive regulator of ABA biosynthesis during leaf senescence. Furthermore, OsWRKY5 expression was suppressed by ABA treatment. Taken together, OsWRKY5 is a positive regulator of leaf senescence that upregulates senescence-induced NAC, ABA biosynthesis, and chlorophyll degradation genes.

Highlights

Leaf senescence is the final stage of plant development and involves diverse molecular and cellular processes such as degradation of chlorophylls and macromolecules, and remobilization of nutrients into newly developing or storage organs through expression of senescence-associated genes (SAGs)
The WRKY domain of OsWRKY5 has a single consensus motif (WRKYGQK) and a zinc-finger C2H2 motif (Cx5Cx23HxH), indicating that OsWRKY5 belongs to the group II WRKY transcription factors (TFs) family [37]
Sci. 2019, 20, 4437 proteins, we found that the domain sequences of OsWRKY5 are quite similar to those of AtWRKY6 and AtWRKY47, members of the subgroup IIb Arabidopsis thaliana WRKY (AtWRKY) TF family (Figure S1)

Summary

Introduction

Leaf senescence is the final stage of plant development and involves diverse molecular and cellular processes such as degradation of chlorophylls and macromolecules, and remobilization of nutrients into newly developing or storage organs through expression of senescence-associated genes (SAGs). The STAY-GREEN (SGR) protein is a magnesium (Mg)-dechelatase, which produces pheophytin a by removing Mg from chlorophyll a [4,5]. Functional deficiency of SGR orthologs leads to a strong stay-green phenotype in diverse plant species including Arabidopsis thaliana [6], rice (Oryza sativa) [4], pea (Pisum sativum) [7], tomato (Solanum lycopersicum), bell pepper (Capsicum annuum) [8], and soybean (Glycine max) [9]. Knockdown of rice pheophorbide a oxygenase (OsPAO) leads to accumulation of pheide a and prolongs leaf greenness during dark incubation [11]. SAGs identified during leaf senescence in rice encode putative proteins involved in metabolic programing [12]; Osh and Osl encode an aminotransferase and isocitrate lyase, which participate in amino acid and fatty acid metabolism, respectively

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