Differential Transcript Expression of Wall-loosening Candidates in Leaves of Maize Cultivars Differing in Salt Resistance

Salt-sensitive crop plants such as maize (Zea mays L.) exhibit a strong and rapid growth reduction in response to NaCl stress. The unique salt-resistant maize hybrid SR03 and the salt-sensitive maize hybrid Lector provide good tools to characterize various genotypic responses to salinity in terms of shoot growth, shoot extensibility, and the expression pattern of wall-loosening candidates. The mRNA transcript levels of wall-loosening candidates such as xyloglucan endotransglucosylase (XET), endo-1,4-β-D-endoglucanase (EGase), α-expansins (EXPA), and the plasma membrane proton pump (PM-H⁺-ATPase) are correlated with cell-wall extensibility and with shoot growth under NaCl stress. We have found for the salt-sensitive maize that a decrease in the relative transcript abundance of ZmXET1, ZmEXPA1, and the composite PM-H⁺-ATPase mRNAs correlates with a decrease in wall extensibility and shoot growth. We suggest that this downregulation of wall-loosening candidates contributes to a reduction in extensibility and consequently in growth. In contrast, the decrease in wall extensibility is less strong in the salt-sensitive hybrid SR03. In the salt-resistant maize genotype, an upregulation of ZmXET1, ZmEXPA1 and PM-H^+-ATPase transcripts possibly mitigates the salinity-induced decrease in wall extensibility and thus in shoot growth.