The Involvement of Ubiquitin in Vegetative Desiccation Tolerance

We have isolated a polyubiquitin cDNA from the modified desiccation-tolerant grass Sporobolus stapfianus. This cDNA, along with a commercially available polyclonal ubiquitin antibody, was used to characterize desiccation/rehydrated-associated changes in ubiquitin-mediated protein degradation in S. stapfianus and the fully desiccation-tolerant moss Tortula ruralis. Northern analysis demonstrated that in S. stapfianus leaves two ubiquitin transcripts, of ca. 1.4 and 1.2 kb, accumulated above control levels during drying and rehydration but were barely detectable in desiccated tissue. The peak in rehydration-associated transcript accumulation coincided with a depletion in ubiquitin monomer levels indicating an increase in protein degradation. Analysis of T. ruralis revealed three ubiquitin transcripts of ca. 1.9, 1.3 and 0.65 kb, with only the 1.3 kb transcript level varying in response to drying and rehydration and all transcripts being stable in dried tissue. Western analysis revealed that conjugated ubiquitin, indicative of proteins targeted for removal, was evident in all samples of Sporobolus but detectable only in slow-drying Tortula which also displayed reduced levels of ubiquitin monomer. These results demonstrate that desiccated T. ruralis gametophyte possesses stable ubiquitin transcripts which can be translated upon rehydration enabling rapid initiation of cellular repair through degradation of certain proteins. This is in contrast to S. stapfianus which requires several hours to replenish depleted ubiquitin transcripts. The ubiquitin response to drying and rehydration in evolutionarily diverse systems is characterized, and the role of repair mechanisms such as ubiquitin-mediated protein degradation in desiccation tolerance is assessed.