The Roles of Germin Gene Products in Plants Under Salt Stress

The members of plants response various internal and external signals differently. The responses of plants to biotic and abiotic stress factors involve biochemical, physiological, morphological and developmental changes. Among the various circumstances salt stress is particularly under extensive studies due to low salt tolerance of crop plants. Germin and germin-like gene products were previously announced to be involved in various aspects of plant development such as plant defence, embryonic development and they are responsive to biotic and abiotic stress including salt. The responses of germin and germin like genes to salt stress are found to be various in different plants. The salinity of soil is an important problem in agriculture, particularly since the majority of crop plants have low salt tolerance. The response of plants to salt stress is a complex phenomenon that involves biochemical and physiological processes as well as morphological and developmental changes (Flowers et al., 1977; Greenway & Munns, 1980). The identification of genes whose expression enables plants to adapt to or tolerate to salt stress is essential for breeding programs, but little is known about the genetic mechanisms for salt tolerance. One approach in clarifying the molecular mechanisms involved in salt stress is to identify the genes whose levels change as a result of salt stress. In this aspect, Hurkman et al. (1989) reported that in barley, gene regulation is altered by salt stress and the levels of translatable mRNAs change with salt treatment. Among the salt stress responsive gene products, germin and germin-like proteins (GLP) were identified (Caliskan, 1997; Hurkman et al., 1989). Cereal germin protein is a homopentameric apoplastic glycoprotein whose synthesis is associated with the onset of growth in germinating wheat embryos (Lane, 1991). Germin genes and their proteins were first detected in germinating cereals (Grzelczak et al., 1985), but subsequently, germin-like proteins were also identified in dicotyledonous angiosperms (Michalowski and Bohnerd, 1992), gymnosperms (Domon et al., 1995) and mosses (Yamahara et al., 1999). Germins are suggested to be a member of “superfamily” which comprises various growth-related genes (Dunwell et al., 2000). Cereal germin proteins display strong oxalate oxidase activity (Lane et al., 1993), an activity that generates one mole of H2O2 and two moles of CO2 from the degradation of oxalic acid. It is reported that H2O2 might act as a signaling molecule at low concentration (Luthell, 1993) or a component of cell wall modifications at high concentrations (Showalter, 1993). Another germin-like protein isolated from the cells of a moss, Barbula unguiculata, was