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

Abstract

We previously demonstrated that maintenance of cell elongation in the apical region of maize primary roots at low water potentials (psi(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 psi(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 alpha-expansins (Exp1 and Exp5) and two beta-expansins (ExpB2 and ExpB8) are expressed specifically in the growing region, whereas expression of beta-expansin ExpB6 is shifted basipetally. After seedlings were transplanted to vermiculite with a psi(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 beta-expansins ExpB2 and ExpB6 showed distinctive patterns of expression and responses to low psi(w,) indicative of distinctive functions. Inhibition of abscisic acid (ABA) accumulation at low psi(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 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 not appear to be mediated by changes in ABA content.

Figure 1

Northern-blot analysis of five expansin genes in maize primary roots. Root tips from inbred and hybrid lines were cut into three sections from the apex, A (0–5 mm), B (5–10 mm), and C (10–20 mm), 10 and 20 h after seedlings were transplanted to wet vermiculite (WW condition, ψ_w = −0.03 MPa) and 10 and 48 h after transplanting to vermiculite of low ψ_w (WS condition, ψ_w = −1.6 MPa). Roots grown in low ψ_w vermiculite for 48 h reached the same length (50 mm) as those grown in high ψ_w vermiculite for 20 h. rRNA band on agarose gel visualized with ethidium bromide was used as loading control.

Figure 2

Time course for expression of five expansin genes in maize roots at high or low ψ_w by northern-blot analysis. A, Northern-blot images. B, Quantification of northern-blot signals in A. Roots were harvested at different times after transplanting into WW or WS conditions. The first 20 mm of the root tip was cut into three sections from the apex: A (0–5 mm), B (5–10 mm), and C (10–20 mm). Roots were shorter than 20 mm during the first 5 h after transplanting, so samples could not be collected for section C during this period.

Figure 3

Overall expansin gene expression at high or low ψ_w assessed by northern-blot analysis. Root tips were cut into three sections from the apex, A (0–5 mm), B (5–10 mm), and C (10–20 mm), after seedlings were grown for 10 and 20 h under WW conditions and for 10 and 48 h under WS conditions. Roots grown in low ψ_w vermiculite for 48 h reached the same length as those grown in high ψ_w vermiculite for 20 h. α-Expansins, RNA blot was probed with a mixture of five α-expansin cDNAs; β-expansins, RNA blot was probed with a mixture of seven β-expansin cDNAs.

Figure 4

Root length increase (A) and ABA content (B) of the 0- to 5-mm (A) and 5- to 10-mm (B) regions of the root tip 15 h after transplanting to WW or WS conditions. ABA accumulation under water stress was decreased by treatment with FLU, and ABA was added to FLU-treated roots to restore root elongation. Data are means ± se (root length increases, n = 102; ABA contents, n = 3). The experiment was repeated with similar results.

Figure 5

Northern-blot analysis of transcript levels of five expansin genes in the 0- to 5-mm (A) and 5- to 10-mm (B) regions of the root tip 15 h after transplanting to WW or WS conditions. Samples were analyzed from the same experiment for which elongation and ABA contents are presented in Figure 4. The experiment was repeated with similar results (statistical analysis of combined experiments shown in Table II).