A direct stimulatory role of mobile gibberellin in Arabidopsis hypocotyl xylem expansion

Background Can Arabidopsis research contribute to our understanding about wood development? Does the function of vascular cambium in a herbaceous weed resemble that of a tree? Despite its diminutive size as compared to a tree, Arabidopsis still displays cambial driven secondary thickening in several organs, including the hypocotyl. Hypocotyl is a good model organ for wood development studies, as in this organ the radial secondary development can be uncoupled from the apical primary growth. This is due to the fact that the hypocotyl elongates only for five days after germination; thus, the radial secondary growth starts only after the elongation has ceased. This is in contrast to the other Arabidopsis organs displaying cambial growth, where it is accompanied by the simultaneous activity of the shoot and root meristems. Two phases can be identified in the hypocotyl secondary development: 1) an early phase of proportional radial growth, where the cambium produces both xylem and phloem at a similar rate, and 2) a later xylem expansion phase, where more xylem than phloem is produced (Fig 1A) [1] . Notably, the composition of xylem is different between these two phases: the xylem produced during the first phase consists of xylem vessels and parenchyma cells, and of xylem vessels and fibers during the second phase. Especially the later phase, characterized by extensive wood formation, resembles the secondary growth in tree species. We have previously shown that in Arabidopsis hypocotyl the transition from the first to the second phase is triggered through the onset of flowering, when the identity of shoot apical meristem changes from vegetative to reproductive [1] . Upon this transition, instead of new leaves, an inflorescence stem emerges from the middle of rosette leaves. What could be the nature of this signal [2]?