[EN] The thickening of plant organs is supported by secondary growth, a process by which new vascular tissues (xylem and phloem) are produced. Xylem is composed of several cell types, including xylary fibers, parenchyma and vessel elements. In Arabidopsis, it has been shown that fibers are promoted by the class-I KNOX gene KNAT1 and the plant hormones gibberellins, and are repressed by a small set of receptor-like kinases; however, we lack a mechanistic framework to integrate their relative contributions. Here, we show that DELLAs, negative elements of the gibberellin signaling pathway, physically interact with KNAT1 and impair its binding to KNAT1-binding sites. Our analysis also indicates that at least 37% of the transcriptome mobilized by KNAT1 is potentially dependent on this interaction, and includes genes involved in secondary cell wall modifications and phenylpropanoid biosynthesis. Moreover, the promotion by constitutive overexpression of KNAT1 of fiber formation and the expression of genes required for fiber differentiation were still reverted by DELLA accumulation, in agreement with post-translational regulation of KNAT1 by DELLA proteins. These results suggest that gibberellins enhance fiber development by promoting KNAT1 activity. This work was funded by the Ministerio de Ciencia y Tecnologia (BFU2016-80621-P, BIO2016-71933-P and BIO2016-79147-R to M.A.B., D.A. and J.A., respectively). A.F-B. and N.B-T. were recipients of a Ministerio de Economia y Competitividad FPI Fellowships (BES-2011-045689 and BES-2014-068868, respectively), and A.S.-M. and M.A.B. acknowledge funding from the European Union (H2020-MSCA-IF-2016-746396). J.A. is supported by a Ramon y Cajal contract from the Ministerio de Economia y Competitividad (RYC-2014-15752). Felipo-Benavent, A.; Urbez Lagunas, C.; Blanco-Touriñán, N.; Serrano Mislata, A.; Baumberger, N.; Achard, P.; Agustí, J.... (2018). Regulation of xylem fiber differentiation by gibberellins through DELLA-KNAT1 interaction. Development. 145(23). https://doi.org/10.1242/dev.164962