| A significant portion of the energy produced by cells is used to maintain the membrane potential which is fundamental for various physiological processes. In the root cell of plants, the membrane potential is the driving force for ionic traffic across the membrane, either absorbing necessary inorganic/organic nutrition from soils or extruding solutes from the cytoplasm.; In Chapter One, it was genetically demonstrated that both absorbing K+ from the surrounding environment and keeping Na + out of the cytoplasm are essential for plants to survive in salt stress condition. Both K+ transport mutants akt1 (A&barbelow;rabidopsis K&barbelow;+ t&barbelow;ransporter) and Na+-H+ antiporter mutant sos1 (s&barbelow;alt o&barbelow;verly s&barbelow;ensitive) showed salt sensitive phenotypes, though to different extents. Meanwhile, two pieces of evidence supported the idea that over accumulation of cytoplasmic Na+ due to mutation in the Na+-H+ antiporter affects the activity of K+ transporters: (1) mild salt stress strongly inhibited the sos1 root-cell permeability to K +; (2) introducing 10 mM NaCl into the cytoplasm of a patch-clamped wild-type root cell completely abolished the AKT1 K+ channel activity. The underlying mechanism of the homeostasis of the two ions is discussed in this Chapter.; Experiments presented in Chapter Two and Three demonstrate that six amino acids, besides serving as major organic form of nitrogen, rapidly and significantly depolarize the plasma membrane of root cells and evoke a spike in cytoplasmic Ca2+. The effects of the six amino acids on the membrane potential were mediated by Arabidopsis Glutamate Receptor 3.3 and 3.5 in a Ca2+-dependent manner. These Arabidopsis receptor channels are homologous to the ionotropic glutamate receptors in the mammalian central nervous system. It was also demonstrated that as low as 10 muM Glu can promote Ca 2+ permeability of the root cells. Glutathione (GSH), the most abundant tripeptide in plants, composed of Glu-Cys-Gly, can induce significant cytoplasmic Ca2+ spike and transient, large peaked depolarization. The GSH-induced large depolarization in the root cells was largely abolished in the atglr3.3 and atglr3.5 mutants. These data not only shed new light on the potential regulatory roles of amino acids in plant physiology, but also firstly suggested that AtGLRs be the receptor of GSH. |
