Thellungiella - an extremophile for post-modern molecular genetics
Modified: August 12 2014.
About Thellungiella (and why it is the up-and-coming tool for plant molecular genetics)
Higher plants have adapted to virtually all terrestrial environments. Extremophiles are those operating in the most challenging environments at the far end of the stress tolerance continuum. Now, due to population growth, overgrazing and inappropriate irrigation management, the impact of stresses on both natural and agronomic ecosystems is increasing at an alarming rate. Understanding plants endemic to extreme environments is the long overdue foundation on which to build understanding of how less well adapted plants respond when faced with lesser stresses.
Given the long history of Arabidopsis as a model system, the plants most immediately useful for such studies at the genomic level are those closely related to it, especially the Thellungiellas, in the 7 chromosome, diploid branch of the Brassicaceae (subclade Eutremeae; see What's new for a note on nomenclature). Based on fossil evidence, A. thaliana and the Eutremeae diverged about 43 million years ago. T. parvula and A. thaliana have similar genome sizes, thus providing unique opportunities for tracing evolutionary rearrangements between the two species. The genome of T. salsuginea is about twice as large.
Thellungiella salsuginea has been studied because its ability, in the natural world, to function in extreme salt, cold, and freezing conditions, and for its efficient mobilization of resources in poor or degraded soils. A comparative study of 11 Brassicas suggests that T. parvula may perform slightly better under salt and drought conditions, but is otherwise comparable in cold and freezing responses. It is also highly tolerant of high levels of other cations.
These extreme adaptations are appropriate to their natural habitats: of the several T. salsuginea ecotypes that have been collected, all are from stress-prone locations — from Xinjiang and Shandong Provinces in the Peoples Republic of China, the Yukon Territory of Canada, and the Rocky Mountains of the United States. Similarly, the T. parvula ecotype used in recent genome sequencing originates from a salt flat in central Anatolia, Turkey. Lake Tuz, shown in this Landsat photo (June 2009), is one of the world's largest inland hypersaline lakes, with an area more than 1600 sq. km. (The scale bar is 8 mi or 13 km.)
A number of tools for comparative molecular characterizations have been developed for T. salsuginea, including tagged mutants, EST collections, and full length cDNA collections. To date, while transcriptome-level characteristics appear broadly similar to those shown by Arabidopsis, some universally stress-responsive genes are highly expressed in Thellungiella even in the absence of challenges. Others are induced only at much higher levels of stress than their isologs in Arabidopsis. In addition, altered gene expression patterns appear to be more stress-specific in Thellungiella than in Arabidopsis.
The Thellungiellas are critical resources for understanding adaptive responses to salinity. Previously, genetic studies of halophytes were virtually non-existent. Instead, research depended on Arabidopsis for the purpose, even though it is poorly adapted. T. salsuginea shares many characteristics with Arabidopsis including, at least broadly, its appearance. It is a small rosette plant. Although a 3 week period of vernalization is needed for synchronized flowering, there is at least one EMS mutant — developed by Jian-Kang Zhu's lab — lacking this requirement. The mutant has, like Arabidopsis, the ability to complete its life cycle in 6-8 weeks. Flower and silique structures are also very similar in the two species, as are seed yields per plant. T. parvula, in contrast, lacks the requirement for vernalization, but like the other two species germinates more quickly and uniformly after stratification. As seen in the figure at the top of this page, T. parvula also has a significantly different leaf and shoot morphology: it is not a rosette plant, flowers are borne at the ends of side shoots, and the flowers lack petals.
Both Thellungiella spp. continue to grow at salinities up to 500 mM NaCl, even if the salinization is done rapidly. Moreover, if after a period of growth, they are given a down-shock, even from 500 mM NaCl to 0 mM, the plants resume more rapid growth immediately. For reasons that are unclear, however, neither species germinates well in the presence of salt, probably indicating adaptation the the fluctuating salinity environments of their native habitats.
Although both species of Thellungiella are classic halophytes, they are also tolerant of related stresses, like drought. Their stomatal morphology and function are significantly different from Arabidopsis; this likely forms a basis for their more efficient responses to water supply. They have already proven useful in decoding genetic controls of regulation of osmotic potential, osmolyte accumulation, and growth. They have the required genetic features for generation and rapid screening of mutants. They have sufficient similarity to Arabidopsis that microarray tools need not be re-developed, but at the same time, they are easily subjected to the latest techniques in RNAseq. And finally, the species are adapted not only to salinity and drought, but also to low temperatures, toxic metals, desiccation and flooding.
So, welcome to thellungiella.org and to this exciting new tool for plant genomic research.
Bibliography for the back story Inan G, Zhang Q, Li PH, Wang ZL, Cao ZY, Zhang H, Zhang CQ, Quist TM, Goodwin SM, Zhu JH, Shi HH, Damsz B, Charbaji T, Gong QQ, Ma SS, Fredricksen M, Galbraith DW, Jenks MA, Rhodes D, Hasegawa PM, Bohnert HJ, Joly RJ, Bressan RA, Zhu JK. 2004. Salt cress. A halophyte and cryophyte Arabidopsis relative model system and its applicability to molecular genetic analyses of growth and development of extremophiles. Plant Physiology 135: 1718-1737. 10.1104/pp.104.041723
Orsini F, D'Urzo MP, Inan G, Serra S, Oh DH, Mickelbart MV, Consiglio F, Li X, Jeong JC, Yun DJ, Bohnert HJ, Bressan RA, Maggio A. 2010. A comparative study of salt tolerance parameters in 11 wild relatives of Arabidopsis thaliana. Journal of Experimental Botany 61: 3787-3798. 10.1093/jxb/erq188