Mobile Gibberellin Directly StimulatesArabidopsisHypocotyl Xylem Expansion  

Abstract

Secondary growth of the vasculature results in the thickening of plant structures and continuously produces xylem tissue, the major biological carbon sink. Little is known about the developmental control of this quantitative trait, which displays two distinct phases in Arabidopsis thaliana hypocotyls. The later phase of accelerated xylem expansion resembles the secondary growth of trees and is triggered upon flowering by an unknown, shoot-derived signal. We found that flowering-dependent hypocotyl xylem expansion is a general feature of herbaceous plants with a rosette growth habit. Flowering induction is sufficient to trigger xylem expansion in Arabidopsis. By contrast, neither flower formation nor elongation of the main inflorescence is required. Xylem expansion also does not depend on any particular flowering time pathway or absolute age. Through analyses of natural genetic variation, we found that ERECTA acts locally to restrict xylem expansion downstream of the gibberellin (GA) pathway. Investigations of mutant and transgenic plants indicate that GA and its signaling pathway are both necessary and sufficient to directly trigger enhanced xylogenesis. Impaired GA signaling did not affect xylem expansion systemically, suggesting that it acts downstream of the mobile cue. By contrast, the GA effect was graft transmissible, suggesting that GA itself is the mobile shoot-derived signal.

Vascular Cambium Development

“…Lateral growth of the hypocotyl has two distinct phases: phase I, where equal amounts of secondary xylem and phloem are produced, and phase II, called xylem expansion, where more xylem than phloem tissue is produced (Sibout et al, 2008;Ragni et al, 2011). The cell-type composition of the xylem tissue also differs between the two phases; xylem produced during the first phase consists of xylem vessels and parenchyma cells, whereas xylem vessels and fibers form during the second phase.…”

Class I KNOX transcription factors promote differentiation of cambial derivatives into xylem fibers in the Arabidopsis hypocotyl

“…Class I KNOX genes have previously been suggested to regulate the biosynthesis of the plant hormones cytokinin and GA in the SAM (Jasinski et al, 2005). Furthermore, graft-transmissible GA promotes fiber formation in the hypocotyl (Ragni et al, 2011). To address the issue of whether class I KNOX genes act locally in the hypocotyl/root to influence fiber differentiation, we used reciprocal grafting experiments.…”

Primary root protophloem differentiation requires balanced phosphatidylinositol-4,5-biphosphate levels and systemically affects root branching

“…Grafting between hypocotyls of cvp2 cvl1 mutant scions and Col-0 wildtype stocks (or vice versa) using 5-day-old seedlings grown on plates at 25°C in 16 h light period was performed in tissue culture as previously described (Ragni et al, 2011). Four days after grafting, root growth of successfully grafted seedlings was measured.…”