Functional screening of a cDNA library from the desiccation-tolerant plant Selaginella lepidophylla in yeast mutants identifies trehalose biosynthesis genes of plant and microbial origin.

Trehalose is a non-reducing disaccharide that accumulates to large quantities in microbial cells, but in plants it is generally present in very low, barely-detectible levels. A notable exception is the desiccation-tolerant plant Selaginella lepidophylla, which accumulates very high levels of trehalose in both the hydrated and dehydrated state. As trehalose is known to protect membranes, proteins, and whole cells against dehydration stress, we have been interested in the characterization of the trehalose biosynthesis enzymes of S. lepidophylla; they could assist in engineering crop plants towards better stress tolerance. We previously isolated and characterized trehalose-6-phosphate synthases from Arabidopsis thaliana (desiccation sensitive) and S. lepidophylla (desiccation tolerant) and found that they had similar enzymatic characteristics. In this paper, we describe the isolation and characterization of trehalose-6-phosphate phosphatase from S. lepidophylla and show that its catalytic activities are also similar to those of its homolog in A. thaliana. Screening of an S. lepidophylla cDNA library using yeast trehalose biosynthesis mutants resulted in the isolation of a large number of trehalose biosynthesis genes that were of microbial rather than plant origin. Thus, we suggest that the high trehalose levels observed in S. lepidophylla are not the product of the plant but that of endophytes, which are known to be present in this plant. Additionally, the high trehalose levels in S. lepidophylla are unlikely to account for its desiccation tolerance, because its drought-stress-sensitive relative, S. moellendorffii, also accumulated high levels of trehalose.

The desiccation tolerant secrets of Selaginella lepidophylla: what we have learned so far?

Selaginella lepidophylla is a desiccation tolerant plant able to survive complete vegetative tissue dehydration and revive ('resurrect') in water conditions. Vegetative desiccation tolerance is an adaptive feature acquired by S. lepidophylla to withstand the long dry periods in its natural habitat, the Chihuahuan desert. Understanding the molecular basis of its drought stress tolerance may be of great benefit to help in developing novel strategies for improvement of drought stress tolerance in crops. Cell biological (e.g. gene discovery, comparative EST analysis, proteomics, metabolite profiling), ultrastructural and physiological studies have brought modest but already important insights in the desiccation tolerance mechanisms adapted by S. lepidophylla. Until recently, the desiccation tolerant mechanism of S. lepidophylla was related to its high trehalose levels. However, large-scale comparative metabolic analysis between S. lepidophylla and its desiccation susceptible relative Selaginella moellendorffii, unexpectedly revealed that S. moellendorffii contains higher trehalose levels than S. lepidophylla. Interestingly, polyols, such as sorbitol and xylitol are 100× more abundant in S. lepidophylla compared to S. moellendorffii. Whether this is linked to the higher stress tolerance remains to be established. Apart from these metabolites, we will also discuss the ultrastructural features that seem to play an important role in the desiccation tolerance of S. lepidophylla. Finally we discuss desiccation tolerance mechanism in other plant species.

Structure and regulation of the Asr gene family in banana.

Abscisic acid, stress, ripening proteins (ASR) are a family of plant-specific small hydrophilic proteins. Studies in various plant species have highlighted their role in increased resistance to abiotic stress, including drought, but their specific function remains unknown. As a first step toward their potential use in crop improvement, we investigated the structure and regulation of the Asr gene family in Musa species (bananas and plantains). We determined that the Musa Asr gene family contained at least four members, all of which exhibited the typical two exons, one intron structure of Asr genes and the "ABA/WDS" (abscisic acid/water deficit stress) domain characteristic of Asr genes. Phylogenetic analyses determined that the Musa Asr genes were closely related to each other, probably as the product of recent duplication events. For two of the four members, two versions corresponding to the two sub-genomes of Musa, acuminata and balbisiana were identified. Gene expression and protein analyses were performed and Asr expression could be detected in meristem cultures, root, pseudostem, leaf and cormus. In meristem cultures, mAsr1 and mAsr3 were induced by osmotic stress and wounding, while mAsr3 and mAsr4 were induced by exposure to ABA. mASR3 exhibited the most variation both in terms of amino acid sequence and expression pattern, making it the most promising candidate for further functional study and use in crop improvement.