The rice genome encodes two vacuolar invertases with fructan exohydrolase activity but lacks the related fructan biosynthesis genes of the Pooideae.
Fructans are believed to contribute to cold and drought tolerance in several plant families (Poaceae, Asparagaceae and Asteraceae), but it is not clear why the ability to accumulate these polymers is found in some genera (e.g. Triticum) but not in others (e.g. Oryza). As fructan biosynthesis enzymes (FBEs) evolved from vacuolar invertases (VINs), we searched the rice genome sequence for genes related to both FBE and VIN genes of wheat and other members of the Pooideae. We compared them at the levels of exon-intron structure, protein sequence, and the enzymatic properties of recombinant proteins after expression in the yeast Pichia pastoris. We found that rice possesses two VIN genes (OsVIN1 and OsVIN2) and no FBE genes. FBE genes appear to have arisen in the Pooideae by a series of gene duplications from an ancestor of wheat TaVIN3. Recombinant TaVIN2, OsVIN1 and OsVIN2 behaved as invertases with no FBE activity, but possessed high fructan exohydrolase activity, especially OsVIN1. The engineering of fructan accumulation into rice for greater stress tolerance could founder on endogenous exohydrolases, but the fact that OsVIN1 transcripts are absent from peduncles of well watered and drought-stressed plants removes one potential obstacle to this endeavour.
New Phytologist (2007) 173 (1) 50-62 [doi: 10.1111/j.1469-8137.2006.01896.x]