Drought tolerance through overexpression of monoubiquitin in transgenic tobacco.

Ubiquitin (Ub) is present in all eukaryotic species examined. It is a multifunctional protein and one of its main known functions is to tag proteins for selective degradation by the 26S proteasome. In this study, Ta-Ub2, a cDNA sequence containing a single Ub repeat and a 3' non-coding region of a polyubiquitin gene, was isolated from wheat (Triticum aestivum) by reverse transcription-polymerase chain reaction (RT-PCR). A PBI sense vector with Ta-Ub2 was constructed and transformed into tobacco plants. Ub expression in wheat leaves, monitored by semi-quantitative RT-PCR, responded to drought stress. In transgenic tobacco, determined by protein gel blot analysis, we found higher amounts of Ub-protein conjugates than in control (tobacco carrying a PBI GUS vector without Ta-Ub2) and wild-type (WT) lines. However, free Ub levels did not significantly differ in the 3 genotypes. Seeds from transgenic, Ub-overexpressing tobacco germinated faster and seedlings grew more vigorously than control and WT samples, both under drought and non-drought conditions. Furthermore, CO(2) assimilation of transgenic plants was significantly higher under drought stress. Our results indicate that Ub may be involved in the response of plants to drought stress and that overexpression of monoubiquitin might be an effective strategy for enhancing drought tolerance.