The leaf senescence-promoting transcription factor AtNAP activates its direct target gene\xa0CYTOKININ OXIDASE\xa03 to facilitate senescence processes by degrading cytokinins

Youzhen Hu,Huazhong Ren,Liping Chen,Bin Liu,Christopher B Watkins,Su-Sheng Gan

doi:10.1186/s43897-021-00017-6

Abstract

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

Highlights

Cytokinins (CKs) have pervasive roles in regulating many aspects of plant growth and development, and is involved in the response to stresses in plants (Gan and Amasino, 1996; Kieber and Schaller, 2014)
Leaf senescence is delayed in the atckx3 knockout To investigate the biological function of AtCKX3 in leaf senescence, we obtained an Arabidopsis line named CS308578 from the Arabidopsis Biological Resource Center; this line contained a T-DNA inserted in the first exon of AtCKX3 that abolished its expression in the homozygous mutant (Fig. 1D and E)
The atckx3 knockout plants exhibited a delayed leaf senescence phenotype (Fig. 1F), and there were no obvious differences in growth and development prior to senescence between the null mutants and wild type (WT) (Fig. 1G)

Summary

Introduction

Cytokinins (CKs) have pervasive roles in regulating many aspects of plant growth and development, and is involved in the response to stresses in plants (Gan and Amasino, 1996; Kieber and Schaller, 2014). The role of CK in inhibiting plant senescence has been investigated (2021) 1:12 for decades (Gan 2010). Exogenous CK and senescencespecific production of endogenous CK biosynthesis led to the retardation of leaf senescence (Gan and Amasino, 1995; Gan and Amasino 1996). CK concentrations decrease with progression of senescence in many plant species (Gan and Amasino, 1996; Noodén, 2012). It is known that during leaf senescence the expression of genes involved in CK biosynthesis in Arabidopsis decreases (Breeze et al, 2011) and the transcript levels of genes of CK-degrading enzymes increase (Guo et al, 2004; Breeze et al, 2011). The involvement of CKXs in leaf senescence and their regulatory mechanisms remain to be deciphered

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