Functional screening of plant stress-related cDNAs by random over-expression in Escherichia coli

A functional screening, based on random over-expression of a plant cDNA library in Escherichia coli, was used to identify plant genes associated to salt tolerance. E. coli cells, transformed with phagemids derived from a cDNA library from Solanum commersonii plantlets, were selected on medium supplemented with 0.8 M NaCl. Sequence analysis of seven cDNAs from salt tolerant bacterial colonies revealed homology either with known plant stress-related genes (dhn2, cpn60b, epoxide hydrolase and others) or with Arabidopsis thaliana putative proteins with unknown functions. Expression of these two novel plant genes was induced by NaCl in S. commersonii and S. tuberosum cells as well as in A. thaliana seedlings, suggesting that the function of their encoded proteins in the cellular response to high salt are conserved across prokaryotes and plant kingdom. N2.1 and N2.2 transcripts were also induced by cold and water stress, but not ABA, revealing that they belong to the ABA- independent stress-responsive gene category. The two unknown cDNAs, as well as dhn2 and cpn60b, were constitutively expressed at higher level in plants of the cold tolerant potato species S. commersonii than in plants of S. tuberosum, the cold susceptible species. These findings support previous evidence that a pre-committed high constitutive transcript level of a set of stress-responsive genes is crucial for plant stress tolerance.