Alpha- and Beta-Expansins Expressed in Different Zones of the Growing Root of Maize

Abstract

Expansins are small proteins that play a key role in modifying the structure of the cell wall during various physiological processes, in particular cell growth by elongation. Expansins are encoded by a large multigene family and are divided into four subfamilies, the main ones being alpha- and beta-expansins; the latter have received special development in cereals. Expansins are thought to modify the interactions of cellulose with xyloglucan (alpha-expansins) or with arabinoxylan (beta-expansins). At the same time, expansins do not have catalytic activity, the specific mechanism of their action is unclear, same as the physiological significance of such a wide variety of isoforms. To study the expression patterns of individual expansins, we conducted a transcriptomic analysis of all expansin genes identified in the maize genome using a convenient model system—zones of the maize growing primary root that differ in the stage of cell development and the composition of their cell walls. Of the 91 maize expansin genes, 67 were expressed in the root, and most expansin genes were characterized by a narrow range of zones with maximum transcript levels. Using a glycoarray containing 183 polysaccharides from the cell walls of plants of various species, we demonstrated that recombinant expansins AtEXPA1 and AtEXPB1 are able to bind to arabinogalactans and rhamnogalacturonans I and a number of other cell wall polysaccharides, which expands the list of their potential carbohydrate targets. Differences in the specificity of the interaction of alpha- and beta-expansins with various polysaccharides, both quantitatively and qualitatively, have been demonstrated. It has been hypothesized that the abundance of expansins in one plant organism and the fine regulation of their expression can be explained, at least in part, by the specific binding of individual expansins to specific cell wall polysaccharides.