Multiple steps of leaf thickening during sun-leaf formation in Arabidopsis.

Rina Hoshino,Yuki Yoshida,Hirokazu Tsukaya

doi:10.1111/tpj.14467

Abstract

SummaryPlant morphological and physiological traits exhibit plasticity in response to light intensity. Leaf thickness is enhanced under high light (HL) conditions compared with low light (LL) conditions through increases in both cell number and size in the dorsoventral direction; however, the regulation of such phenotypic plasticity in leaf thickness (namely, sun‐ or shade‐leaf formation) during the developmental process remains largely unclear. By modifying observation techniques for tiny leaf primordia in Arabidopsis thaliana, we analysed sun‐ and shade‐leaf development in a time‐course manner and found that the process of leaf thickening can be divided into early and late phases. In the early phase, anisotropic cell elongation and periclinal cell division on the adaxial side of mesophyll tissue occurred under the HL conditions used, which resulted in the dorsoventral growth of sun leaves. Anisotropic cell elongation in the palisade tissue is triggered by blue‐light irradiation. We discovered that anisotropic cell elongation processes before or after periclinal cell division were differentially regulated independent of or dependent upon signalling through blue‐light receptors. In contrast, during the late phase, isotropic cell expansion associated with the endocycle, which determined the final leaf thickness, occurred irrespective of the light conditions. Sucrose production was high under HL conditions, and we found that sucrose promoted isotropic cell expansion and the endocycle even under LL conditions. Our analyses based on this method of time‐course observation addressed the developmental framework of sun‐ and shade‐leaf formation.

Highlights

Light levels directly affect plant morphology and physiology
In sun leaves induced under high light (HL) conditions, multiple layers of palisade cells were anisotropically elongated along the adaxial–abaxial axis or in the dorsoventral direction of leaves (Figure 1a0,b0,e0), whereas shade leaves induced under low light (LL) conditions had a single layer of palisade tissue composed of small round cells (Figure 1a,b,e)
The cell height of sun leaves was almost twice that of shade leaves (Figure 1e,e0,g), whereas there were no significant differences in cell area in the paradermal plane between the two leaf types (Figure 1d,d0,h)

Summary

Introduction

Light levels directly affect plant morphology and physiology. Plants can adapt to a wide range of light conditions by changing their photosynthetic apparatus (Gauhl, 1976; Boardman, 1977; Yano and Terashima, 2001). In ecological and anatomical terms, thick leaves induced under high light (HL) conditions are called ‘sun leaves’, whereas thin leaves formed under low light (LL) conditions are called ‘shade leaves’ (Haberlandt, 1914) These leaf types are thought to be determined by unknown systemic light signals transduced from mature leaves to developing leaves: i.e. mature leaves sense the surrounding light intensity and convert light information to systemic signals that determine the leaf type of developing primordia (Uemura et al, 2000; Yano and Terashima, 2001). Such regulation is important because adjusting the thickness of the leaf modifies the proportion of photosynthetic tissue per leaf area for the efficient absorption of light energy (Boardman, 1977; Terashima et al, 2005)

Results

Discussion

Conclusion