Class I TCP proteins TCP14 and TCP15 are required for elongation and gene expression responses to auxin

Key Message Two class I TCP transcription factors are required for an efficient elongation of hypocotyls in response to auxin and for the correct expression of a subset of auxin-inducible genes In this work, we analyzed the response to auxin of plants with altered function of the class I TEOSINTE BRANCHED 1, CYCLOIDEA, PCF (TCP) transcription factors TCP14 and TCP15. SeveralSMALL AUXIN UP RNA(SAUR) genes showed decreased expression in mutant plants defective in these TCPs after an increase in ambient temperature to 29 degrees C, a condition that causes an increase in endogenous auxin levels. Overexpression ofSAUR63caused a more pronounced elongation response in the mutant than in the wild-type at 29 degrees C, suggesting that the decreased expression ofSAURgenes is partly responsible for the defective elongation at warm temperature. Notably, severalSAURgenes and the auxin response geneIAA19also showed reduced expression in the mutant after auxin treatment, while the expression of otherSAURgenes and ofIAA29was not affected or was even higher. Expression of the auxin reporterDR5::GUSwas also higher in atcp15mutant than in a wild-type background after auxin treatment. However, the elongation of hypocotyls in response to auxin was impaired in the mutant. Remarkably, a significant proportion of auxin inducible genes and of targets of the AUXIN RESPONSE FACTOR 6 are regulated by TCP15 and often contain putative TCP recognition motifs in their promoters. Furthermore, we demonstrated that several among them are recognized by TCP15 in vivo. Our results indicate that TCP14 and TCP15 are required for an efficient elongation response to auxin, most likely by regulating a subset of auxin inducible genes related to cell expansion.

Automated summary

The TCP transcription factors TCP14 and TCP15 are essential for efficient elongation response to auxin in plants. Mutant plants with altered function of TCP14 and TCP15 show decreased expression of SAUR genes and the auxin response gene IAA19, leading to impaired elongation in response to auxin treatment. TCP15 regulates a subset of auxin inducible genes related to cell expansion, with a significant proportion of them containing putative TCP recognition motifs in their promoters.