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.
Highlights
In higher land plants, any excess sugar provided by photosynthesis and not needed for maintenance of the general metabolism is invested into growth and transported from photosynthetic source organs toward sink organs (Ye, 2002; Thompson, 2006)
Since flowering triggers xylem expansion along Arabidopsis hypocotyls (Figures 1A to 1C) (Sibout et al, 2008), we investigated whether this phenomenon is conserved in other species and whether it correlates with the rosette growth habit
In all nonrosette plants analyzed (Arabis alpina, Aster alpinus, Nicotiana benthamiana, and Solanum lycopersicum), xylem started to expand during the vegetative phase (Figures 1D to 1G), as indicated by the appearance of fibers, whereas in rosette plants, (Arabidopsis, Cardamine hirsuta, Barberea verna, and Taraxacum officinalis), no xylem expansion was observed before flowering (Figures 1H to 1K)
Summary
Any excess sugar provided by photosynthesis and not needed for maintenance of the general metabolism is invested into growth and transported from photosynthetic source organs toward sink organs (Ye, 2002; Thompson, 2006) This transport occurs in the vascular phloem, a highly specialized tissue comprised of various cell types, such as the sieve elements, which perform the actual transport of the phloem sap; companion cells, which are responsible for loading and unloading of phloem sap cargo; phloem parenchyma cells, which transfer metabolites to and from companion cells; and phloem fibers, cells with thick secondary cell walls that provide structural support. The major group of extant plants, the dicotyledons, have solved this developmental problem by continuously expanding their vascular tissues throughout their life cycle, resulting in the radial expansion of stems and roots This so-called secondary growth offers the advantage that nonfunctional xylem vessels can be replaced, as they do not forever resist the strain of the negative pressure created by the transpiration stream. Because negative pressure and mechanical strain increase with height, secondary growth permits plants to grow taller and represents the key invention that permitted the evolution of trees (Spicer and Groover, 2010)
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