Abstract
As a result of salt (NaCl)-stress, sensitive varieties of maize (Zea mays L.) respond with a strong inhibition of organ growth. The reduction of leaf elongation investigated here has several causes, including a modification of the mechanical properties of the cell wall. Among the various tissues that form the leaf, the epidermis plays a special role in controlling organ growth, because it is thought to form a rigid outer leaf coat that can restrict elongation by interacting with the inner cell layers. This study was designed to determine whether growth-related changes in the leaf epidermis and its cell wall correspond to the overall reduction in cell expansion of maize leaves during an osmotic stress-phase induced by salt treatment. Two different maize varieties contrasting in their degree of salt resistance (i.e., the hybrids Lector vs. SR03) were compared in order to identify physiological features contributing to resistance towards salinity. Wall loosening-related parameters, such as the capacity of the epidermal cell wall to expand, β-expansin abundance and apoplastic pH values, were analysed. Our data demonstrate that, in the salt-tolerant maize hybrid which maintained leaf growth under salinity, the epidermal cell wall was more extensible under salt stress. This was associated with a shift of the epidermal apoplastic pH into a range more favourable for acid growth. The more sensitive hybrid that displayed a pronounced leaf growth-reduction was shown to have stiffer epidermal cell walls under stress. This may be attributable to the reduced abundance of cell wall-loosening β-expansin proteins following a high salinity-treatment in the nutrient solution (100 mM NaCl, 8 days). This study clearly documents that salt stress impairs epidermal wall-loosening in growth-reduced maize leaves.
Highlights
Salinity worsens soil fertility, which is detrimental for the cultivation of most agricultural crop plants such as maize (Zea mays L.)
This hypothesis is supported by wall creep measurements that demonstrate reduced cell wall extensibility in expanding maize leaves during the first phase of salt stress, a phase that is dominated by negative osmotic effects [10]
Salt-sensitive crops such as maize react with strong growth reduction upon salt stress[9, 32,33,34,35,36]
Summary
Salinity worsens soil fertility, which is detrimental for the cultivation of most agricultural crop plants such as maize (Zea mays L.). The decline in biomass formation in response to a salt-induced osmotic-stress phase is caused by several factors including reduced leaf growth, PLOS ONE | DOI:10.1371/journal.pone.0118406. Cramer and Bowman [9] have suggested that the modified capacity of cell walls irreversibly to expand is a major growth-limiting factor that causes growth inhibition under the osmotic phase of salt stress. This hypothesis is supported by wall creep measurements that demonstrate reduced cell wall extensibility in expanding maize leaves during the first phase of salt stress, a phase that is dominated by negative osmotic effects [10]
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