Salt Cress. A Halophyte and Cryophyte Arabidopsis Relative Model System and Its Applicability to Molecular Genetic Analyses of Growth and Development of Extremophiles

Abstract

Salt cress (Thellungiella halophila) is a small winter annual crucifer with a short life cycle. It has a small genome (about 2 3 Arabidopsis) with high sequence identity (average 92%) with Arabidopsis, and can be genetically transformed by the simple floral dip procedure. It is capable of copious seed production. Salt cress is an extremophile native to harsh environments and can reproduce after exposure to extreme salinity (500 mM NaCl) or cold to 215°C. It is a typical halophyte that accumulates NaCl at controlled rates and also dramatic levels of Pro (.150 mM) during exposure to high salinity. Stomata of salt cress are distributed on the leaf surface at higher density, but are less open than the stomata of Arabidopsis and respond to salt stress by closing more tightly. Leaves of salt cress are more succulent-like, have a second layer of palisade mesophyll cells, and are frequently shed during extreme salt stress. Roots of salt cress develop both an extra endodermis and cortex cell layer compared to Arabidopsis. Salt cress, although salt and cold tolerant, is not exceptionally tolerant of soil desiccation. We have isolated several ethyl methanesulfonate mutants of salt cress that have reduced salinity tolerance, which provide evidence that salt tolerance in this halophyte can be significantly affected by individual genetic loci. Analysis of salt cress expressed sequence tags provides evidence for the presence of paralogs, missing in the Arabidopsis genome, and for genes with abiotic stressrelevant functions. Hybridizations of salt cress RNA targets to an Arabidopsis whole-genome oligonucleotide array indicate that commonly stress-associated transcripts are expressed at a noticeably higher level in unstressed salt cress plants and are induced rapidly under stress. Efficient transformation of salt cress allows for simple gene exchange between Arabidopsis and salt cress. In addition, the generation of T-DNA-tagged mutant collections of salt cress, already in progress, will open the door to a new era of forward and reverse genetic studies of extremophile plant biology.Salinity is a severe and increasing constraint on the productivity of agricultural crops. High concentrations of salts in the soil have a strong inhibitory effect on the growth and harvestable yield of all crop species. Secondary salinization significantly impairs crop production on at least 20% of irrigated land worldwide (Ghassemi et al., 1995), and irrigated agriculture contributes more than 30% of global agricultural production (Hillel, 2000). Salinization of arable land arising from poor water management has led to the decline of past civilizations, and it threatens the long-term sustainability of many current large-scale irrigation systems, especially those in Asia (Sharma and Goyal, 2003). Soil salinity almost always originates from previous exposure to seawater (Flowers et al., 1986). Although it is believed that, for most of the Earth's history, the salt level of the oceans was much lower than at present (Serrano et al., 1997), all plant spec...

Genome Structures and Halophyte-Specific Gene Expression of the Extremophile Thellungiella parvula in Comparison with Thellungiella salsuginea (Thellungiella halophila) and Arabidopsis

“…in order to derive a molecular-level understanding of adaptation to challenging environments appears particularly appropriate. Both Thellungiella species are comparable to Arabidopsis in terms of growth rates and size under similar growth conditions and in terms of fertility, seed number, and ease of transformation by flower dipping (Inan et al, 2004).…”

“…A number of tools for comparative molecular characterizations have been developed for T. salsuginea, including tagged mutants, EST collections, and fulllength cDNA collections (Inan et al, 2004;Taji et al, 2004Taji et al, , 2008Wang et al, 2004Wang et al, , 2006Gong et al, 2005;Wong et al, 2005). To date, results of stress challenge studies have identified transcriptome responses that are broadly similar to those shown by Arabidopsis.…”

“…Mb (Inan et al, 2004). We now estimate, based on flow cytometry, a diploid genome size of approximately 180 Mb for T. parvula.…”

Life at the extreme: lessons from the genome

“…The T. parvula genome is of particular interest because chromosomal assemblies that approach the coverage of A. thaliana are available. Moreover, because the Thellungiella species share many of the characteristics that led to the acceptance of Arabidopsis as a model (size, growth habit, seed amount, mutants, and transformation ability), they have been recognized as excellent candidates for comparative genomics studies [15,22]. …”

Salt Stress Signaling and Mechanisms of Plant Salt Tolerance

“…The casparian strip in the endodermis plays a crucial role in preventing apoplastic Na + influx into the root stele. Compared with Arabidopsis, halophytes such as salt cress (Thellungiella halophila) develop both an extra endodermis and a cortex cell layer in roots (36). In maize seedlings stressed at 200 mM NaCl, the casparian strip radial width was increased by 47% compared with control seedlings (37).…”