Abstract
The ability for plant growth to be optimized, either in the light or dark, depends on the intricate balance between cell division and differentiation in specialized regions called meristems. When Arabidopsis (Arabidopsis thaliana) seedlings are grown in the dark, hypocotyl elongation is promoted, whereas root growth is greatly reduced as a result of changes in hormone transport and a reduction in meristematic cell proliferation. Previous work showed that the microtubule-associated protein CLASP sustains root apical meristem size by influencing microtubule organization and by modulating the brassinosteroid signaling pathway. Here, we investigated whether CLASP is involved in light-dependent root growth promotion, since dark-grown seedlings have reduced root apical meristem activity, as observed in the clasp-1 null mutant. We showed that CLASP protein levels were greatly reduced in the root tips of dark-grown seedlings, which could be reversed by exposing plants to light. We confirmed that removing seedlings from the light led to a discernible shift in microtubule organization from bundled arrays, which are prominent in dividing cells, to transverse orientations typically observed in cells that have exited the meristem. Brassinosteroid receptors and auxin transporters, both of which are sustained by CLASP, were largely degraded in the dark. Interestingly, we found that despite the lack of protein, CLASP transcript levels were higher in dark-grown root tips. Together, these findings uncover a mechanism that sustains meristem homeostasis through CLASP, and they advance our understanding of how roots modulate their growth according to the amount of light and nutrients perceived by the plant.
Highlights
At the beginning of their lives, plants face a precarious situation
It is well known that hypocotyl expansion is stimulated and that root growth is inhibited in the dark, we noted that most previous dark-growth investigations with the model system Arabidopsis thaliana included some sucrose in the media
Cytoplasmic Linker Associated Protein (CLASP) is required for increased cell proliferation in response to light and/or sucrose
Summary
At the beginning of their lives, plants face a precarious situation. Resources stored in the seed will only sustain life for a limited time, and it is critical that plants quickly emerge from the soil to begin the process of photosynthesis and sugar production. Plants must use their initial resources strategically during the first days of development to ensure survival. It is known that when seedlings are germinated and grown in complete darkness (skotomorphogenesis), the hypocotyl grows rapidly, whereas root elongation is inhibited. When grown in light conditions (photomorphogenesis), sucrose derived from aerial organs is sufficient to promote root elongation in Arabidopsis (Kircher and Schopfer, 2012), and to reduce hypocotyl growth. One fundamental question is how growth is promoted in some organs and inhibited in others to meet the needs of a plant during developmental transitions
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