The plant hormone auxin directs timing of xylem development by inhibition of secondary cell wall deposition through repression of secondary wall NAC-domain transcription factors
Physiol Plant. 2018 May 28 [Epub ahead of print]
Johnsson C, Jin X, Xue W, Dubreuil C, Lezhneva L, Fischer U

Abstract
Wood formation in higher plants is a complex and costly developmental process regulated by a complex network of transcription factors, short peptide signals and hormones. Correct spatiotemporal initiation of differentiation and downstream developmental stages is vital for proper wood formation. Members of the NAC (NAM, ATAF1/2, and CUC) family of transcription factors are described as top level regulators of xylem cell fate and secondary cell wall (SCW) deposition, but the signals initiating their transcription have yet to be elucidated. We found that treatment of Populus stems with auxin repressed transcription of NAC transcription factors associated with fibre and SCW formation, and induced vessel-specific NACs, whereas gibberellic acid induced the expression of both classes of NAC domain transcription factors involved in wood formation. These transcriptional changes were reflected in alterations of stem anatomy, i.e. auxin treatment reduced cell wall thickness, whereas gibberellic acid had a promotive effect on SCW deposition and the rate of wood formation. Similar changes were observed on treatment of Arabidopsis thaliana stems with gibberellic acid (GA) or the synthetic auxin NAA. We also observed corresponding changes in PIN5 over-expressing lines, where interference with auxin transport leads to premature SCW deposition and formation of additional fibre bundles. Together, this suggests wood formation is regulated by an integrated readout of both auxin and gibberellic acid which, in turn controls expression of fibre and vessel specific NACs.

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