Abiotic stress – a challenge for the master enzyme plasma membrane H+-ATPase

The plasma membrane (PM) H+-ATPase is the powerhouse of the plant. The enzyme establishes an electrochemical proton gradient across the PM. This gradient is responsible for cell-wall acidification which is a requisite for cell-extension growth and it energizes transport processes of symporters and antiporters at the PM. Abiotic stresses often have a negative impact on the functions of enzymes which can disturb major physiological processes and interfere with plant development. Therefore, the effect of abiotic stress on the PM H+-ATPase´s two main functions were investigated in this thesis. The effect of Mg deficiency on apoplastic acidification was studied in developing maize leaves and the effect of salt stress on energization of transport processes was examined in developing maize kernels. Magnesium (Mg) deficiency is often observed in agricultural crops with an insufficient Mg fertilization regime, finally resulting in yield depression. One goal was to identify the process that primarily limits maize (Zea mays L.) growth and yield under insufficient Mg supply. A sufficient (500 µM) and a low (25 µM) Mg concentration were used in time-course hydroponic experiments to investigate parameters which are considered to be important for the characterization of the key processes of growth, namely cell division and cell-extension growth. It was shown that cell division was not susceptible to Mg deficiency, since neither DNA replication nor sugar or protein availability limited this growth process. However, PM H+-ATPase activity was inhibited by a lack of the enzyme´s co-substrate Mg under Mg deficiency. The inhibition of the PM H+-ATPase reduced apoplastic acidification causing a reduced cell-extension growth. This primarily limited maize growth under Mg deficiency. Salt stress affects yield formation of maize at various physiological levels resulting in an overall decrease of grain yield. In this thesis it was investigated how salt stress affects kernel development at and shortly after pollination. Maize kernels grown under control and salt stress were harvested 0 and 2 d after pollination (DAP) and at kernel maturity. Kernel development was not inhibited 0 DAP, but it was inhibited 2 DAP under salt stress. On this day, kernel PM H+-ATPase activity was reduced which caused a lower pH gradient across the plasma membranes of endosperm and embryo tissue. The lower pH gradient supposedly resulted in a decreased energization of transporters responsible for hexose import into the cytoplasm of the kernels. A lack of hexoses reduced the energy status of the cells which impaired cell division. The impaired cell division probably caused the observed growth reduction of kernels 2 DAP, which resulted in an observed lower kernel number and grain yield at maize maturity under salt stress. It is concluded that the impairment of PM H+-ATPase by Mg deficiency and salt stress in maize is a major reason for growth and yield reduction under these environmental constraints. However, while the impairment of the cell-wall acidification is the reason for a reduced extension growth under Mg deficiency in leaves, the decrease in pH gradient at the PM caused kernel abortion under salt stress.