Overexpression of OsEXPA8, a Root-Specific Gene, Improves Rice Growth and Root System Architecture by Facilitating Cell Extension

Nana Ma,Junli Huang,Shichun Qiu,Guixue Wang,Zhenhui Kang,Ying Wang,Shugang Che,Malcolm Bennett

doi:10.1371/journal.pone.0075997

Abstract

Expansins are unique plant cell wall proteins that are involved in cell wall modifications underlying many plant developmental processes. In this work, we investigated the possible biological role of the root-specific α-expansin gene OsEXPA8 in rice growth and development by generating transgenic plants. Overexpression of OsEXPA8 in rice plants yielded pleiotropic phenotypes of improved root system architecture (longer primary roots, more lateral roots and root hairs), increased plant height, enhanced leaf number and enlarged leaf size. Further study indicated that the average cell length in both leaf and root vascular bundles was enhanced, and the cell growth in suspension cultures was increased, which revealed the cellular basis for OsEXPA8-mediated rice plant growth acceleration. Expansins are thought to be a key factor required for cell enlargement and wall loosening. Atomic force microscopy (AFM) technology revealed that average wall stiffness values for 35S::OsEXPA8 transgenic suspension-cultured cells decreased over six-fold compared to wild-type counterparts during different growth phases. Moreover, a prominent change in the wall polymer composition of suspension cells was observed, and Fourier-transform infrared (FTIR) spectra revealed a relative increase in the ratios of the polysaccharide/lignin content in cell wall compositions of OsEXPA8 overexpressors. These results support a role for expansins in cell expansion and plant growth.

Highlights

Plant growth and development depends on the regulation of cell extension and growth through cell wall modifications such as loosening and reassembly
The expression levels of the transgene were evaluated by quantitative real-time PCR performed with total RNA extracted from the mature leaves of eight transgenic plants
As an exclusively expressed gene in rice root, OsEXPA8 does not express in leaf and other tissues [17]; the leaf is suitable for determining the expression of the introduced OsEXPA8 construct

Summary

Introduction

Plant growth and development depends on the regulation of cell extension and growth through cell wall modifications such as loosening and reassembly. Proteins in the cell wall are believed to play important roles in the modulation of cell wall extensibility, a crucial parameter in determining cell expansion [1]. Expansins are thought to be a key wall-loosening factor that have long been implicated in the control of plant growth processes via their role as modulators of cell wall extensibility [2], the exact mechanism of expansin activity remains unclear. Experimental evidence suggests that they may disrupt the non-covalent bonding between the cellulose microfibrils and matrix polysaccharides, permitting the loosening and extension of the cell wall and the turgor-driven growth of the cell [3,4]. Transient local microinduction of CsEXP1 in the shoot apical meristem of Nicotiana tabacum induces the leaf development process, leading to altered local lamina and abnormal leaves [7]

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Conclusion