Exposure of Lemna minor to arsenite: Expression levels of the components and intermediates of the ubiquitin/proteasome pathway

In animal cells, arsenite has been reported to cause sulfhydryl depletion, generate reactive oxygen species and increase the level of large ubiquitin-protein conjugates. Plant viability tests and DNA laddering experiments have shown that Lemna minor remains viable after exposure to 50 μM NaAsO₂ for periods of at least 6 h. However, protein metabolism is affected in two major ways: the synthesis of an array of stress proteins, which confer thermotolerance; and an increase in the amount of large ubiquitin-protein conjugates, particularly evident after 2-3 h of stress, indicative of a role for the ubiquitin/proteasome pathway. This outcome is primarily attributed to an increased availability of protein substrates during arsenite treatment for three main reasons: an increase in protein carbonyl content after 1-2 h of stress; moderate increments in the transcript levels of the sequences coding for the ubiquitin pathway components chosen as markers (polyubiquitin, E1 and E2, and the β subunit and the ATPase subunits of the 26S proteasome); the observed increase in ubiquitin conjugates does not depend on de novo protein synthesis. This study is the first report on the involvement of the ubiquitin/proteasome pathway in response to arsenite in plants. In addition, it addresses the simultaneous expression of selected genes encoding the various components of the pathway. The results suggest that in plants, unlike in animals, the response to a relatively low level of arsenite does not induce apoptotic cell death. As a whole, the response to arsenite apparently involves a conjugation of salvage and proteolytic machineries, including heat shock protein synthesis and the ubiquitin/proteasome pathway.