Leaf Positioning of Arabidopsis in Response to Blue Light

Abstract

Appropriate leaf positioning is essential for optimizing photosynthesis and plant growth. However, it has not been elucidated how green leaves reach and maintain their position for capturing light. We show here the regulation of leaf positioning under blue light stimuli. When 1-week-old Arabidopsis seedlings grown under white light were transferred to red light (25 micromol m(-2) s(-1)) for 5 d, new petioles that appeared were almost horizontal and their leaves were curled and slanted downward. However, when a weak blue light from above (0.1 micromol m(-2) s(-1)) was superimposed on red light, the new petioles grew obliquely upward and the leaves were flat and horizontal. The leaf positioning required both phototropin1 (phot1) and nonphototropic hypocotyl 3 (NPH3), and resulted in enhanced plant growth. In an nph3 mutant, neither optimal leaf positioning nor leaf flattening by blue light was found, and blue light-induced growth enhancement was drastically reduced. When blue light was increased from 0.1 to 5 micromol m(-2) s(-1), normal leaf positioning and leaf flattening were induced in both phot1 and nph3 mutants, suggesting that phot2 signaling became functional and that the signaling was independent of phot1 and NPH3 in these responses. When plants were irradiated with blue light (0.1 micromol m(-2) s(-1)) from the side and red light from above, the new leaves became oriented toward the source of blue light. When we transferred these plants to both blue light and red light from above, the leaf surface changed its orientation to the new blue light source within a few hours, whereas the petioles initially were unchanged but then gradually rotated, suggesting the plasticity of leaf positioning in response to blue light. We showed the tissue expression of NPH3 and its plasma membrane localization via the coiled-coil domain and the C-terminal region. We conclude that NPH3-mediated phototropin signaling optimizes the efficiency of light perception by inducing both optimal leaf positioning and leaf flattening, and enhances plant growth.

The Arabidopsis rcn1-1 Mutation Impairs Dephosphorylation of Phot2, Resulting in Enhanced Blue Light Responses

“…The phototropin family of plant blue light receptors, phot1 and phot2, activates light responses that serve in general either to maximize capture of PAR (e.g., phototropism, leaf expansion, stomatal opening, chloroplast accumulation, rapid inhibition of the growth of etiolated hypocotyls [Christie, 2007], and leaf solar tracking [Inoue et al, 2008b]) or protect against damage under excessive light conditions (chloroplast avoidance) (Christie, 2007;Suetsugu and Wada, 2007). These photoreceptors have two highly conserved chromophore domains, designated LOV1 and LOV2 (because of their similarity to domains in otherwise different proteins that serve as sensors for light, oxygen, or voltage) (Huala et al, 1997).…”

Blue light-induced autophosphorylation of phototropin is a primary step for signaling

“…protein kinase ͉ stomata ͉ photoreceptor ͉ light signaling P hototropins (phot1 and phot2) mediate multiple blue light responses in Arabidopsis, including phototropism, chloroplast movements, leaf flattening, leaf positioning, stomatal opening, and rapid inhibition of hypocotyl growth (1)(2)(3)(4)(5). These responses enhance photosynthesis and optimize plant growth, particularly under weak light (6).…”

“…The gl1 leaves were flattened and suitable for capturing light (5,6), but the double-mutant leaves curled downward (Fig. 4 Band C).…”

Turning heads: The biology of solar tracking in sunflower

“…For instance, the leaves, petioles, and inflorescences of phototropin 1 and phototropin 2 double mutant Arabidopsis thaliana plants fail to reorient when a unilateral blue light source is moved to a new fixed position [37,38]. Notably, in wild-type Arabidopsis plants, these movements are reversible, and a subset of these movements occur with angular velocities comparable to those seen for leaf heliotropism in other plant species in field environments [5].…”