Modification of expansin transcript levels in the maize primary root at low water potentials

We previously demonstrated that maintenance of cell elongation in the apica l region of maize primary roots at low water potentials (psi (w)) was assoc iated with an increase in expansin activity and extractable expansin protei n. Here, we characterized the spatial pattern of expansin gene expression a long the growing maize root and studied the effect of low psi (w) on expans in gene expression. Roots were divided into three segments: apical 0 to 5 m m, subapical 5 to 10 mm, and non-growing 10 to 20 mm. Of the five expansin genes expressed in control roots, two alpha -expansins (Exp1 and Exp5) and two beta -expansins (ExpB2 and ExpB8) are expressed specifically in the gro wing region, whereas expression of beta -expansin ExpB6 is shifted basipeta lly. After seedlings were transplanted to vermiculite with a psi (w) of -1. 6 MPa, transcripts for Exp1, Exp5, and ExpB8 rapidly accumulated in the api cal region of the root. These mRNA changes correlated with the maintenance of root elongation and increases in wall extensibility found previously. Th e beta -expansins ExpB2 and ExpB6 showed distinctive patterns of expression and responses to low psi (w), indicative of distinctive functions, Inhibit ion of abscisic acid (ABA) accumulation at low psi (w) (by fluridone treatm ent) had no effect on expansin expression, except that ExpB2 transcript lev el showed a minor dependence on ABA. Gene-specific regulation of alpha- and beta -expansin mRNA pools likely contributes to growth alterations of the maize (Zea mays) root as it adapts to a low psi (w), but these changes do n ot appear to be mediated by changes in ABA content.