The leaf senescence-promoting transcription factor AtNAP activates its direct target gene CYTOKININ OXIDASE 3 to facilitate senescence processes by degrading cytokinins

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.

Regulation of rose petal dehydration tolerance and senescence by RhNAP transcription factor via the modulation of cytokinin catabolism

“…3 and 6). This is consistent with the observation that leaf senescence is associated with a decrease in CK content and CK signaling suppression (Gan and Amasino 1995;Kim et al 2006;Hu et al 2021).…”

“…However, the molecular pathways that govern the antagonistic actions of CK and ABA in organ senescence and dehydration tolerance are still unclear, particularly in petals that have no stomata. Similarly, while NAP is involved in ABA responses and regulates senescenceassociated genes, and especially those that affect stomatal movement (Zhang and Gan 2012;Liang et al 2014;Hu et al 2021), the regulatory mechanisms and significance of NAP action in senescing astomatous petals is unclear (van Doorn 1997).…”

Recent progresses in molecular postharvest biology

“…AtNAP is one of the most important transcription factors discussed in the Review. Two research articles in the collection report the most recent findings that AtNAP promotes leaf senescence by activating the gene of cytokinin oxidase 3 (that degrades senescence-inhibiting plant hormone cytokinins) (Hu et al 2021) Flowers are "food for the mind, ' and senescence of flowers limits vase life and reduces aesthetic appreciation. In this special collection there are one review article (Sun et al 2021) and one research article (Zou et al 2021) representing the frontiers in flower senescence research.…”

A positive feedback regulatory loop, SA-AtNAP-SAG202/SARD1-ICS1-SA, in SA biosynthesis involved in leaf senescence but not defense response

“…Total RNA extractions from Arabidopsis leaves and realtime PCR analyses were performed according to (Hou et al, 2013). cDNA was synthesized from 3 μg of total RNA (treated with RNase-free DNase; New England Biolabs, USA) at 42 °C with MV-Reverse Transcriptase (Promega, USA) (Hu et al, 2021). For qPCR, 1 μL of each diluted sample (40 folds) was used as a template in a 25-μL reaction.…”

“…It is known that AtNAP TF directly binds to the promoter of SAG113 to activate the expression of a senescencespecific and Golgi-localized protein phosphatase 2C gene to promote senescence (Zhang and Gan, 2012;Zhang et al, 2012a). The TF also physically binds to the promoter of cytokinin oxidase genes in Arabidopsis, rice (Hu et al, 2021) and rose (Zou et al, 2021) to degrade the senescence-retardant cytokinins, which facilitates the senescence process.…”