THE BEAN PGIP GENE FAMILY IS COMPOSED BY FOUR MEMBERS SPANNING A 50 KB REGION

Polygalacturonase-inhibiting protein (PGIP) is a plant cell wall protein that regulates the action of fungal endopolygalacturonases (PG) favouring the formation of oligogalacturonides active as elicitors of plant defense responses. On the bases of these properties, it has been proposed that the PGIP is involved in plant defence mechanisms. Genomic analysis showed that genes encoding PGIPs are organized in gene families and functional analysis of their encoded products demonstrated that they can have different recognition capabilities against fungal PG. In bean such specificity can be controlled by a single amino acid substitution within the predicted b-sheet/b-turn structure. In the attempt to understand better the role of PGIP in plant defence and to identify possible genetic features of the pgip locus, we have characterized the bean pgip gene family by isolating and shot-gun sequencing two overlapping BAC clones covering about 150 Kbp. Sequence analysis of this region revealed that the pgip gene family is composed by four different members spanning a 50 Kbp region. Nucleotide sequence comparison of coding regions show a good level of polymorphism between members of a gene family and a limited variability between corresponding members of gene family from different bean cultivars. In this latter case non synonymous substitutions or small deletions are responsible for sequence variability, and in most case they reside within the predicted b-sheet/b-turn structure, in position potentially involved in PG recognition. The genetic features of the pgip bean locus include the presence of additional ORFs, a novel 796 bp Mu-like element, a member of a novel family of Ty1-copia retroelement, Simple Sequence Repeat (SSR) motifs and Tandem Repeats. On a total of 50 Kbp, about 47 % is annotated and 53 % has to be investigated. The non-annotated DNA is very rich in AT sequences. To determine the extent of recognition specificities contained in a single bean genotype, the four bean pgip genes are being expressed in PVX to be tested against fungal Pgs.