Genetic analysis of plant salt tolerance using Arabidopsis.

Soil salinity is one of the most significant abiotic stresses for plant agriculture. Apart from the practical goal of genetically improving the salt tolerance of crop plants, salt tolerance research represents an important part of basic plant biology, contributing to our understanding of subjects ranging from gene regulation, signal transduction to ion transport, and mineral nutrition. Research on two other major abiotic stresses, drought and cold, is intimately linked with salt stress work. For example, many genes that are regulated by salt stress are also responsive to drought or cold stress (Zhu et al., 1997). Because salt stress can be applied accurately and reproducibly, many “drought” stress studies in the laboratory use salt stress instead of actual drought. The widely known Hog pathway for osmotic stress perception and signaling in yeast was discovered by using NaCl stress (Brewster et al., 1993). Salt tolerance is a complex trait involving responses to cellular osmotic and ionic stresses and their consequent secondary stresses (e.g. oxidative stress) and whole plant coordination. The complexity and polygenic nature of salt stress tolerance are important factors contributing to the difficulties in breeding salt-tolerant crop varieties. Breeding efforts have been hampered by a lack of understanding of salt tolerance mechanisms as well as a lack of field and laboratory screening tests, including physiological and molecular markers. There was much optimism when molecular approaches began to be applied to salt stress research. Nearly 2 decades later, a long but incomplete list of salt stress-responsive genes has been produced by the molecular studies. No clear salt tolerance mechanism has emerged from the expression studies (Zhu et al., 1997). The limited success of the molecular approach in elucidating salt tolerance mechanisms is primarily due to two factors. First, the approach is only correlative. It is now widely recognized that many salt-responsive genes do not contribute to tolerance, rather, their induction reflects salt stress damage. Second, so far the molecular approach has mostly identified genes or gene products based only on their expression, but many genes that are important for salt tolerance may not be induced by salt stress. One notable success of molecular studies has been the identification of promoter elements and transcription factors that control the expression of protective proteins such as RD29A/ COR78 (Kasuga et al., 1999). Traditional differential screening/hybridization approaches are being replaced by more powerful methods, such as DNA microarray analysis, that provide a profile of gene expression at the genome level. Profiling at the genome level, when combined with systematic genetic analysis, promises to reveal much of the signaling networks that control stress tolerance. This review describes some recent developments and prospects in genetic analysis of salt stress tolerance. The focus of the review is on mutational analysis in the model plant Arabidopsis and saltspecific responses, i.e. ion homeostasis aspects of salt tolerance.