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

We previously demonstrated that maintenance of cell elongation in the apical region of maize primary roots at low water potentials (ψ_w) was associated with an increase in expansin activity and extractable expansin protein. Here, we characterized the spatial pattern of expansin gene expression along the growing maize root and studied the effect of low ψ_w on expansin gene expression. Roots were divided into three segments: apical 0 to 5 mm, subapical 5 to 10 mm, and non-growing 10 to 20 mm. Of the five expansin genes expressed in control roots, two α-expansins (Exp1 and Exp5) and two β-expansins (ExpB2 and ExpB8) are expressed specifically in the growing region, whereas expression of β-expansin ExpB6 is shifted basipetally. After seedlings were transplanted to vermiculite with a ψ_w of -1.6 MPa, transcripts for Exp1, Exp5, and ExpB8 rapidly accumulated in the apical region of the root. These mRNA changes correlated with the maintenance of root elongation and increases in wall extensibility found previously. The β-expansins ExpB2 and ExpB6 showed distinctive patterns of expression and responses to low ψ_w, indicative of distinctive functions. Inhibition of abscisic acid (ABA) accumulation at low ψ_w (by fluridone treatment) had no effect on expansin expression, except that ExpB2 transcript level showed a minor dependence on ABA. Gene-specific regulation of α- and β-expansin mRNA pools likely contributes to growth alterations of the maize (Zea mays) root as it adapts to a low ψ_w, but these changes do not appear to be mediated by changes in ABA content.