Submitted to: Plant Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 30, 1999
Publication Date: N/A
Interpretive Summary: This paper represents part of a study focused on genes involved in repair of cellular damage, specifically the ubiquitin gene that is involved in the turnover of damaged proteins, expressed indesiccation-tolerant plants. These studies are part of our ongoing effort to understand how some plants tolerate extreme water stress or drought. In this report we investigate the eexpression of ubiquitin genes and proteins in Sporobolus stapfianus and compare it to the ubiquitin response in a desiccation tolerant plant where cellular repair is the main mechanism for tolerance, Tortula ruralis and a desiccation sensitive plant, Sporobolus pyrimidalis. In all three plants ubiquitin responds to desiccation but only in the desiccation tolerant plants is there a rehydration response. In S. stapfianus all ubiquitin genes respond to the stress but in T. ruralis there is one specific ubiquitin gene that responds to desiccation and rehydration. Also T. ruralis can store ubiquitin transcripts in the dried state unlike S. stapfianus. The rate of protein turnover is much faster in T. ruralis than in S. Stapfianus implying a more efficient damage repair system in T. ruralis. The results from this work allow us to both assess damage and when it occurs and to target certain genes for possible use in a biotechnological approach to drought resistance.
Technical Abstract: We have isolated a polyubiquitin cDNA from the desiccation tolerant grass Sporobolus stapfianus. This cDNA, along with a polyclonal ubiquitin antibody was used to characterize desiccation/rehydration-associated changes in ubiquitin mediated protein turnover in S. stapfianus and the desiccation tolerant moss Tortula ruralis. Northern analysis demonstrated that in S. stapfianus leaves two ubiquitin transcripts, 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 turnover. Analysis of T. ruralis revealed three ubiquitin transcripts of approximately 1.9, 1.3 and o.65 Kb, with only the 1.3 Kb transcript level varying in response to drying and rehydration and all transcripts were stable in dried tissue. Western analysis revealed that in S. stapfianus conjugated ubiquitin, indicative o 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 posesses stable ubiquitin transcripts which can be translated upon rehydration enabling rapid initiation of cellular repair through protein turnover. 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 turnover in desiccation tolerance is assessed.