TCP Transcription Factors Regulate Shade Avoidance Via Promoting The Activity Of PIFs And The Expression Of Auxin Biosynthetic Genes

Light quality surrounding a plant is largely determined by the density of its neighboring vegetation.Plants are able to sense shade light signals and initiate a series of adaptation responses,which is known as shade avoidance syndrome(SAS).PHYTOCHROME INTERACTING FACTORS(PIFs)are key factors in SAS network by regulating the biosynthesis of multiple phytohormones and the expression of cell expansion genes.The transcriptional regulation of PIFs in response to shade remains poorly understood.Here we show that TCP17,TCP5,and TCP13 play important roles in mediating shade-induced hypocotyl elongation by up-regulating auxin biosynthesis via a PIFs-dependent and a PIFs-independent pathway.In constitutive white light(Wc),a tcp5 tcp13 tcp17 triple mutant(3tcp)showed a subtle hypocotyl defective phenotype.In shade,however,3tcp showed a significantly reduced hypocotyl elongation phenotype,indicating a positive role of TCPs in regulating SAS.Our in depth analyses indicated that TCP17 is significantly accumulated in shade.The biochemical and genetic analyses demonstrated that TCP17 promotes hypocotyl elongation in shade via PIF transcription factors by directly regulating the expression of PIF4 and PIF5,and forming a complex with PIF4 to elevate the transcription activity of PIF4.In addition to a PIF-dependen t manner,TCP17 binds to the promoters of YUC2,YUC5,and YUC8,key factors in auxin biosynthesis and promotes their expression.TCP transcription factors increase auxin level in shade through these two indirectly or directly pathway and subsequently promote hypocotyl growth.These data provide new insights into our better understanding of the regulatory mechanisms of SAS in plants.