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PP2 participates in protection
Higher organisms, including plants, devote a surprisingly large proportion of their genome to encoding enzymes that modify other proteins by adding or removing phosphate groups. Some of these reactions are involved in regulating enzyme activity; others are involved in signalling pathways, responding to developmental and/or environmental cues, and it is one of these that is discussed here. Of the multiplicity of protein phosphatases, Xu et al. (Beijing, China, pp. 439-450) have focused on PP2A. This specifically removes phosphate from serine or threonine residues (and of course there are protein kinases that specifically transfer phosphate from ATP to these residues). A gene encoding the catalytic subunit of PP2A in wheat, Triticum aestivum (TaPP2Ac-1) had been previously identified as an incomplete cDNA (or EST, expressed sequence tag) in a library from seedlings of a drought-tolerant cultivar exposed to water deficit (-0.5 MPa) for 12 h. The authors have now obtained a full-length cDNA and used it as a probe to study the time-course of gene expression at the same water potential. Expression peaks 6-12 h after the start of water deficit and then declines (but not to baseline levels). The question then is whether this response has any role in the increased drought tolerance of this cultivar. To test this possibility, the gene was inserted into a binary vector based on the bi-partite genome of an RNA virus and delivered to tobacco leaves by Agrobacterium tumefaciens. This vector system allowed high levels of expression, as demonstrated by detection of the TaPP2Ac-1 mRNA and the protein itself. The transgenic plants showed a significantly increased drought tolerance, as shown by increased ability to retain water and to maintain relative water content, increased water use efficiency and greater leaf cell-membrane stability during drought conditions. Thus, although TaPP2Ac-1 is the first of the PP2A gene family to be isolated from wheat, there is already strong evidence for its involvement in responses to drought.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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