Functional characterization of ZmGRP5, a glycine-rich protein specifically expressed in the cell wall of maize silk tissue

Abstract

Silk tissue is a specialized reproductive tissue of the maize plant, equivalent to the stigma and style portion of the female inflorescence. The moist and nutrient rich properties of maize silk tissue that facilitate pollen reception and the support of pollen tube growth also make maize silk a preferred site of infection by fungal pathogens such as Fusarium graminearum. The cDNA clone zmgrp5 was isolated in a previous study to identify silk tissue-specific genes. ZmGRP5, the encoded protein, was predicted to be a cell wall glycine-rich protein (GRP) and was experimentally characterized in this study. Using polyclonal antiserum, immunoblot analysis confirmed the silk tissue specificity of the protein. Additionally, subcellular fractionation studies confirmed ZmGRP5 localization in the cell wall fraction, and not in any other subcellular fractions. Interaction of ZmGRP5 with the cell wall matrix was observed to be disrupted by the addition of the reducing agent beta-ME. The reversible nature of disulfide bond formation and disruption under different redox conditions suggest that ZmGRP5 could potentially be important in the regulation of cell wall structural properties such as elasticity and rigidity in accordance with environmental and developmental changes. The variable immobilization of ZmGRP5 to the cell wall matrix could also serve as a potential mechanism of activation or inactivation of any non-structural functions. The identification of potential post-translational modifications such as phosphorylation and glycosylation, which are rarely observed in other cell wall GRPs, suggest that the functional significance of these modifications in ZmGRP5 is worthy of further study.