Abstract
Several lines of evidence have implicated the involvement of the phytohormone gibberellin (GA) in modulating leaf senescence in plants. However, upstream transcription factors (TFs) that regulate GA biosynthesis in association with GA-mediated leaf senescence remain elusive. In the current study, we report the possible involvement of a TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) TF BrTCP21 in GA-delayed leaf senescence in Chinese flowering cabbage. Exogenous GA3 treatment maintained a higher value of maximum PSII quantum yield (Fv/Fm) and total chlorophyll content, accompanied by the repression of the expression of senescence-associated genes and chlorophyll catabolic genes, which led to the delay of leaf senescence. A class I member of TCP TFs BrTCP21, was further isolated and characterized. The transcript level of BrTCP21 was low in senescing leaves, and decreased following leaf senescence, while GA3 could keep a higher expression level of BrTCP21. BrTCP21 was further found to be a nuclear protein and exhibit trans-activation ability through transient-expression analysis in tobacco leaves. Intriguingly, the electrophoretic mobility shift assay (EMSA) and transient expression assay illustrated that BrTCP21 bound to the promoter region of a GA biosynthetic gene BrGA20ox3, and activated its transcription. Collectively, these observations reveal that BrTCP21 is associated with GA-delayed leaf senescence, at least partly through the activation of the GA biosynthetic pathway. These findings expand our knowledge on the transcriptional mechanism of GA-mediated leaf senescence.
Highlights
Leaf senescence is a highly orchestrated biological process that is finely controlled by various intrinsic and external factors, such as developmental stage, natural plant hormones and stresses [1,2,3,4,5]
We identified and characterized several transcription factors (TFs) including BrWRKY65 [30], BrWRKY6 [31], BrERF72 [18] and BrNAC055 [34] in association with leaf senescence in Chinese flowering cabbage, among which BrWRKY6 was shown to suppress the expression of two GA biosynthetic genes (BrKAO2 and BrGA20ox2) during GA-delayed leaf senescence [31]
Previous studies indicate that the effect of GA on senescence seems contradictory, as GA treatment delays leaf senescence in nasturtium [7], rumex [8], alstromeria [9] and herbaceous perennial Paris polyphylla [10], while accelerates leaf senescence in Arabidopsis rosette leaves [12]
Summary
Leaf senescence is a highly orchestrated biological process that is finely controlled by various intrinsic and external factors, such as developmental stage, natural plant hormones and stresses [1,2,3,4,5]. Exogenous GA3 treatment has been recently reported to promote senescence of Arabidopsis rosette leaves [12], indicating the complicated effects of GA on leaf senescence. Increasing evidence demonstrates that transcription factors (TFs) are important regulatory proteins that control the onset and progression of leaf senescence by affecting the expression of senescence-associated genes (SAGs), hormone biosynthetic and signaling genes [5,6]. NAM/ATAF/CUC (NAC) TFs of Arabidopsis [15], rice [16] and foxtail millet [17] function in ABA-induced leaf senescence by regulating ABA biosynthetic and signaling genes, or CCGs. A Chinese flowering cabbage ERF TF BrERF72 is found to enhance JA accumulation by inducing the expression of three JA biosynthetic genes (BrLOX4, BrAOC3 and BrOPR3) during MeJA-promoted leaf senescence [18]. The transcriptional regulation of GA response in relation to leaf senescence is not clear
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