Abstract
The shade avoidance response is mainly evident as increased plant elongation at the expense of leaf and root expansion. Despite the advances in understanding the mechanisms underlying shade-induced hypocotyl elongation, little is known about the responses to simulated shade in organs other than the hypocotyl. In Arabidopsis, there is evidence that shade rapidly and transiently reduces the frequency of cell division in young first and second leaf primordia through a non-cell-autonomous mechanism. However, the effects of canopy shade on leaf development are likely to be complex and need to be further investigated. Using combined methods of genetics, cell biology, and molecular biology, we uncovered an effect of prolonged canopy shade on leaf development. We show that persistent shade determines early exit from proliferation in the first and second leaves of Arabidopsis. Furthermore, we demonstrate that the early exit from proliferation in the first and second leaves under simulated shade depends at least in part on the action of the Homeodomain-leucine zipper II (HD-Zip II) transcription factors ARABIDOPSIS THALIANA HOMEOBOX2 (ATHB2) and ATHB4. Finally, we provide evidence that the ATHB2 and ATHB4 proteins work in concert. Together the data contribute new insights on the mechanisms controlling leaf development under canopy shade.
Highlights
In dicots, plant growth is controlled by internal pathways that are strongly influenced by the environment
We have previously shown that leaves grown in simulated shade (Low R/FRLow photosynthetically active radiation (PAR)) are significantly smaller than those grown in simulated sun (High R/FRHigh PAR) and that cell number, not cell size, contributes to the reduction of leaf area under Low R/FRLow PAR
We reasoned that if the only effect of shade on leaf development is to induce a transient arrest of cell division in young leaf primordia, at later developmental stages leaves of equal area grown in High R/FRHigh PAR and Low R/FRLow PAR environments should show no significant morphological difference
Summary
Plant growth is controlled by internal pathways that are strongly influenced by the environment. The reduction in the red/far red (R/FR) ratio of light acts as a signal that triggers the shade avoidance response, causing profound changes in stem and petiole elongation and leaf area growth in the angiosperms that have evolved the capacity to avoid shade. Shade-tolerant plants have adapted their photosynthesis to function optimally under low-light conditions. These plants are capable of long-term survival under a canopy shade. In Arabidopsis, a typical shade-avoiding plant, the shade avoidance response consists of hypocotyl and petiole elongation and reduction of leaf lamina growth at the early stage of seedling development, and of elevation (hyponasty) and elongation of leaf petioles in older plants (Franklin, 2008; Casal, 2012)
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