Expression of multiple forms of polygalacturonase gene during ripening in banana fruit

The activity of polygalacturonase (PG, E.C 3.2.1.15) during ripening in climacteric fruits has been positively correlated with softening of the fruit tissue and differential expression of its gene is suspected to be regulated by the plant hormone ethylene. We have cloned four partial cDNAs, MAPG1 (acc. no. AF311881), MAPG2 (acc. no. AF311882), MAPG3 (acc. no. AF542382) and MAPG4 (acc. no. AY603341) for PG genes and studied their differential expression during ripening in banana. MAPG3 and MAPG4 are believed to be ripening related and regulated by ethylene whereas MAPG2 is associated more with senescence. MAPG1 shows constitutive expression and is not significantly expressed in fruit tissue. The genomic clone MAGPG (acc. No. AY603340) includes the complete MAPG3 gene, which consists of four exons and three introns. The structure of the gene has more similarity to tomato abscission PG rather than tomato fruit PG. It is concluded that softening during ripening in banana fruit results from the concerted action of at least four PG genes, which are differentially expressed during ripening.