The role of SDG8i from the resurrection grass Sporobolus stapfianus in ectopic expression system of Arabidopsis

Isolation of gene transcripts from desiccated leaf tissue of the resurrection grass Sporobolus stapfianus Gandoger, resulted in the identification of the gene, SDG8i, encoding a Group 1 glycosyltransferase (UGT). Glycolsyltransferases transfer a sugar to a number of acceptor molecules, including hormones and secondary metabolites, changing the solubility, stability and biological activity of these compounds. Functional analysis of the SDG8i was undertaken in Arabidopsis thaliana because no protocol for transformation of resurrection grasses exists. The phenotype of transgenic A. thaliana plants constitutively over-expressing the SDG8i UDP-glucosyltransferase under the control of the CaMV 35S promoter indicates that the glucosyltransferase may modulate the activity of both growth- and stress-related hormones. Plants overexpressing the UGT show elevated auxin levels and results suggest that the enzyme acts downstream of ABA to reduce drought-induced senescence. In vitro analysis of the activity of the UGT recombinant protein product indicates that SDG8i encodes a glucosyltransferase (SL-UGT) that potentially can glycosylate the synthetic analogue GR24, suggesting a link with strigolactone-related processes. As well as mediating the interactions of host plants with symbiotic fungi or parasitic plants, the well-conserved strigolactone hormone signalling system contributes to environmental regulation of plant growth. The phenotypes of SDG8i transgenic plants exhibit hyperbranching and a reduced ability to stimulate strigolactone-dependent germination of Orobanchaceae (Phelipanche ramosa) seed. The results indicate that SDG8i expression negatively affects the bioactivity of a strigolactone-like compound/s produced by Arabidopsis that mediates environmentally-induced repression of cell division and expansion, both during normal development and in response to stress. This leads to enhanced growth, reduced senescence, and a substantial improvement in protoplasmic drought tolerance in SDG8i transgenic plants. Therefore, overexpression of SDG8i confers substantial increases in growth rate and seed yield under non-stress conditions as well as large improvements in survival under cold-, salt- and drought-stress, thus providing a novel means of improving productivity in crop plants. Two putative transcription factors, SDG7y and SDG10y that may regulate SDG8i expression have previously been identified by the yeast-one hybrid system. The transgenic plants over-expressing SDG7y and SDG10y displayed similar phenotypes to that of SDG8i transgenics with substantial increases in shoot growth and shoot branching, suggesting they are driving expression of an Arabidopsis SDG8i orthologue.