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.