Regulation of xylem fiber differentiation by gibberellins through DELLA-KNAT1 interaction

Abstract

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.

SOBIR1/EVR prevents precocious initiation of fiber differentiation during wood development through a mechanism involving BP and ERECTA

“…The regulation of fiber formation by ER and ERL1 was proposed to be BP-dependent and to involve down-regulation of GA signaling (12,33). Recently, DELLA proteins were shown to limit the BP activity through physical interaction (11). Considering such reports and that the sobir1/evr er and er erl1 phenotypes are very similar, we suggest that the interaction of ER with SOBIR1/EVR or with ERL1 leads to the same or very similar signaling outcomes.…”

“…All plants were grown in LD conditions (16 h light/8 h dark). All phenotyping was performed by using toluidine blue or phloroglucinol-stained micrographs as described previously (1,11). GWAS was performed by using the GWAPP application (16).…”

“…We performed qRT-PCR with such primers on DNA obtained from a ChIP assay on pBP::BP-CFP plants (30) by using anti-GFP antibodies. We included the promoters of AT1G77530 and AT4G35160, 2 previously reported BP targets (11), as control in our experiment. Our results clearly demonstrated that BP binds the promoter of SOBIR1/EVR (Fig.…”

“…BP positively regulates the expression of the SEC-ONDARY WALL THICKENING PROMOTING FACTOR/ SECONDARY WALL-ASSOCIATED NAC DOMAIN PRO-TEIN (NST/SND) NST1 and NST3/SND1 transcription factors, 2 master regulators of xylem fibers differentiation, in a GA-dependent manner (7)(8)(9)(10). Indeed, BP has recently been demonstrated to be limited in function by the DELLA proteins through physical interaction (11). The leucine-rich repeat receptor-like kinases (LRR-RLKs) ERECTA (ER) and ER-LIKE 1 (ERL1) redundantly prevent the excessive radial growth of the hypocotyl by controlling the amount of fiber production during phase II of xylem expansion, presumably in a BP-dependent manner (12).…”

Plant vascular development: mechanisms and environmental regulation

“…In order for the cambium and developing xylem cells to become sensitive to GA and, therefore, differentiate fibers instead of parenchyma, the activity of the class I KNOX transcription factor KNAT1/BREVIPEDICELLUS (KNAT1/BP) is needed [72,73]. Indeed, KNAT1/BP activity has been shown to be negatively regulated by DELLA through physical interaction [74], and bp mutants do not display fiber formation [75]. Such a defect cannot be rescued by GA application [74].…”