Polygalacturonase gene expression in kiwifruit: relationship to fruit softening and ethylene production

In kiwifruit, much of the softening process occurs prior to the respiratory climacteric and production of ethylene. This fruit therefore represents an excellent model system for dissecting the process of softening in the abse nce of endogenous ethylene production. We have characterized the expression of three polygalacturonase (PG) cDNA clones (CkPGA, B and C) isolated from fruit of Actinidia chinensis. Expression of CkPGA and B was detected by no rthern analysis only in fruit producing endogenous ethylene, and by RT-PCR in other tissues including flower buds, petals at anthesis, and senescent p etals. CkPGA promoter fragments of 1296, 860 and 467 bp fused to the beta-g lucuronidase (uidA) reporter gene directed fruit-specific gene expression d uring the climacteric in transgenic tomato. CkPGC gene expression was obser ved in softening fruit, and reached maximum levels (50-fold higher than for CkPGA and B) as fruit passed through the climacteric. However, expression of this gene was also readily detected during fruit development and in frui t harvested prior to the onset of softening. Using RT-PCR, expression of Ck PGC was also detected at low levels in root tips and in senescent petals. T hese results suggest that PG expression is required not only during periods of cell wall degeneration, but also during periods of cell wall turnover a nd expansion.