Transient alkalinization of the leaf apoplast stiffens the cell wall during onset of chloride salinity in corn leaves

During chloride salinity, the pH of the leaf apoplast (pH(apo)) transiently alkalizes. There is an ongoing debate about the physiological relevance of these stress-induced pH(apo) changes. Using proteomic analyses of expanding leaves of corn (Zea mays L.), we show that this transition in pH(apo) conveys functionality by (i) adjusting protein abundances and (ii) affecting the rheological properties of the cell wall. pH(apo) was monitored in planta via microscopy-based ratio imaging, and the leaf-proteomic response to the transient leaf apoplastic alkalinization was analyzed via ultra-high performance liquid chromatography-MS. This analysis identified 1459 proteins, of which 44 exhibited increased abundance specifically through the chloride-induced transient rise in pH(apo). These elevated protein abundances did not directly arise from high tissue concentrations of Cl- or Na+ but were due to changes in the pH(apo). Most of these proteins functioned in growth-relevant processes and in the synthesis of cell wall-building components such as arabinose. Measurements with a linear-variable differential transducer revealed that the transient alkalinization rigidified (i.e. stiffened) the cell wall during the onset of chloride salinity. A decrease in t-coumaric and t-ferulic acids indicates that the wall stiffening arises from cross-linkage to cell wall polymers. We conclude that the pH of the apoplast represents a dynamic factor that is mechanistically coupled to cellular responses to chloride stress. By hardening the wall, the increased pH abrogates wall loosening required for cell expansion and growth. We conclude that the transient alkalinization of the leaf apoplast is related to salinity-induced growth reduction.