GENETIC AND MOLECULAR DISSECTION OF OSMOTIC AND ABA SIGNALS

Extreme environments such as high soil salinity, drought, high temperature or cold require special adaptation of plants. Moreover, environmental stress respresent serious limitation for agricultural productivity. Osmotic and oxidative stresses develop in plants during drought or when the soil has high salt content. Our group is studying the regulation of osmotic stress responses in the model organism, Arabidopsis thaliana. Using genetic approaches such as T-DNA insertion mutagenesis and random cDNA transfer, we have identified several Arabidopsis genes which influence resposes to salt, drought and oxidative stress or ABA signalling. Proline metabolism is used to study metabolic responses to such environmental contrains.

Development of genetic tools to identify and study novel regulatory genes

Several genetic technologies were developed in our laboratory which permit the identification of stress regulatory genes in plants (Papdi et al., 2009, Papdi et al., 2010). T-DNA insertion mutagenesis is an important tool for the identification and functional characterization of regulatory genes in Arabidopsis. We have established a tagged mutant collection with annotation of more than thousand insertion sites (Szabados et al., 2002). Using a promoter trap technique which employed the firefly luciferase as reporter gene, numerous in situ gene fusions have been identified where the T-DNA insert was inserted in stress-responsive genes (Alvarado et al., 2004). List of the tagged genes can be downloaded from our website. Mutant lines are available for research purposes.

List of T-DNA insertion lines

More recently the Conditional cDNA Overexpression System (COS) was developed, which allows random cDNA transfer and facile gene identification in transgenic Arabidopsis lines. The COS system is suitable for intra and interspecific gene transfer and generates conditional dominant phenotypes which is particularly suitable for the identification of stress regulatory genes (Papdi et al., 2008, Papdi et al., 2009, Rigó et al., 2012). The heat shock transcription factor HSFA4A was identified with the COS system. Overexpression of HSFA4A in transgenic plants enhanced tolerance to different stress conditions and diminished oxidative damage. HSFA4A is an important regulator of oxidative stress responses by controlling the expression of numerous stress induced genes.

Mitochondrial respiration and stress responses

Screening for stress hypersensitivity of our insertion mutant collection lead to the identification of the ppr40 mutant, which is more sensitive to salt, high osmotics, abscisic acid (ABA) and oxidative agents. The disrupted gene encodes the PPR40 protein, involved in the regulation of mitochondrial electron transport (Zsigmond et al., 2008). Impaired mitochondrial antioxidant homeostasis is responsible for the enhanced stress sensitivity of the ppr40 mutant (Zsigmond et al., 2011). Importance of mitochondrial electron transport stability in stress tolerance was confirmed by PPR40 overexpressing plants which showed enhanced salt tolerance (Zsigmonds et al., 2012).

Proline metabolism and stress regulation

Proline accumulation during drought or salt stress is a well-known phenomenon in plants. In order to characterize regulation of proline accumulation, the Arabidopsis P5CS1 and P5CS2 genes, which encode the rate-limiting enzyme of proline biosynthesis, the pyrroline-5-carboxylate synthase, were characterized. We showed that functional divergence of the two P5CS genes is manifested in differential transcriptional regulation of these genes during plant development and in responses to biotic and abiotic stress (Ábrahám et al., 2003, Fabro et al., 2004). The importance of proline accumulation in osmotic stress tolerance was confirmed with loss-of-function insertion p5cs1 mutants, which showed enhanced salt sensitivity and oxidative damage. The p5cs2 mutations lead to embryo lethality, and therefore confirmed the essential housekeeping function of the P5CS2 gene (Székely et al., 2008). Importance of proline metabolism in maintaining cellular homeostasis, redox balance, and metabolic signalling is studied in international collaborations (Szabados, Savouré, 2010, Lehman et al., 2010, Stein et al., 2011).