Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2007
Publication Date: 2/18/2008
Citation: Klotz, K.L., Haagenson, D.M. 2008. Wounding, anoxia and cold induce sugarbeet sucrose synthase transcriptional changes that are unrelated to protein expression and activity. Journal of Plant Physiology. 165:423-434. Interpretive Summary: Sugarbeet roots are often subjected to cold and anaerobic conditions during production and postharvest storage, and they are inevitably wounded by harvest and piling operations. The impact of these stresses on sugarbeet metabolism, however, is largely unexplored. In this study, the impact of anaerobic conditions, cold temperature and wounding on the expression of sucrose synthase was examined. Sucrose synthase, as the principal sucrose degrading enzyme throughout most of production, at harvest and during storage, is a key enzyme in sugarbeet root metabolism and has been implicated as a factor in root growth, sugar partitioning to the root, and storage loss. Anaerobic conditions, cold temperature and wounding all affected sucrose synthase RNA levels, but had little impact on sucrose synthase protein levels or enzyme activity. The differences between RNA levels, protein levels and enzyme activity suggest that sucrose synthase expression is largely regulated by post-transcriptional mechanisms in response to these stresses. The unresponsiveness of sucrose synthase protein levels or enzyme activity to anaerobic conditions, cold temperature and wounding suggests that this enzyme may not be actively involved in the metabolic changes that occur upon exposure to these stresses.
Technical Abstract: Wounding, anoxia, and cold are often encountered during production and storage of sugarbeet (Beta vulgaris L.) root. The effect of these stresses on the expression of sucrose synthase, a key enzyme in sugarbeet root metabolism implicated in root growth, carbon partitioning and postharvest storage loss, however, is largely unknown. Transcript and protein levels for the two sucrose synthase genes expressed in sugarbeet root (SBSS1 and SBSS2) and sucrose synthase enzyme activity were determined after wounding, anoxia or exposure to low temperature during 24 h and 7 d time course experiments. Transcript levels for both genes were altered several fold in injured, anoxic and cold-treated roots with SBSS1 transcript levels elevated in response to all three stresses, and SBSS2 transcript levels elevated in response to wounding, cold and short exposures (3 to 12 h) to anaerobic conditions, but reduced in roots exposed to anaerobic conditions for more than 24 h. SBSS1 and SBSS2 protein levels exhibited little change in stressed roots, even after 7 d. Enzyme activity was also relatively unchanged in stressed roots, except for small activity differences of 1 - 2 d duration that were unrelated to transcriptional changes. The disparity between transcript levels, protein abundance and enzyme activity indicate that SBSS1 and SBSS2 expression in response to wounding, anoxia and cold is largely regulated by post-transcriptional mechanisms. The unresponsiveness of sucrose synthase protein levels or enzyme activity to wounding, anoxia and cold questions the importance of this enzyme to stress responses in sugarbeet root.