Characterization of expressed Pgip genes in rice and wheat reveals similar extent of sequence variation to dicot PGIPs and identifies an active PGIP lacking an entire LRR repeat.

Polygalacturonase-inhibiting proteins (PGIPs) are leucine-rich repeat (LRR) proteins involved in plant defence. A number of PGIPs have been characterized from dicot species, whereas only a few data are available from monocots. Database searches and genome-specific cloning strategies allowed the identification of four rice (Oryza sativa L.) and two wheat (Triticum aestivum L.) Pgip genes. The rice Pgip genes (Ospgip1, Ospgip2, Ospgip3 and Ospgip4) are distributed over a 30 kbp region of the short arm of chromosome 5, whereas the wheat Pgip genes, Tapgip1 and Tapgip2, are localized on the short arm of chromosome 7B and 7D, respectively. Deduced amino acid sequences show the typical LRR modular organization and a conserved distribution of the eight cysteines at the N- and C-terminal regions. Sequence comparison suggests that monocot and dicot PGIPs form two separate clusters sharing about 40% identity and shows that this value is close to the extent of variability observed within each cluster. Gene-specific RT-PCR and biochemical analyses demonstrate that both Ospgips and Tapgips are expressed in the whole plant or in a tissue-specific manner, and that OsPGIP1, lacking an entire LRR repeat, is an active inhibitor of fungal polygalacturonases. This last finding can contribute to define the molecular features of PG-PGIP interactions and highlights that the genetic events that can generate variability at the Pgip locus are not only limited to substitutions or small insertions/deletions, as so far reported, but can also involve variation in the number of LRRs.