Effect of down-regulation of polygalacturonase and expansin on tomato fruit softening and susceptibility to Botrytis cinerea

CANTU, D; VICENTE AR; DEWEY M; BENNETT, AB; LABAVITCH, JML; POWELL ALT.

California USA

Jornada; PGP Retreat; 2007

University of California Davis

Tomato fruit during ripening undergo a developmental transition from unripe, firm, green and resistant or tolerant of rotting pathogens such as Botrytis cinerea, to ripe, soft, red and susceptible. This transition is caused by changes in gene expression, physiology and structural texture.  Textural changes associated with softening are largely due to the digestion of polysaccharide components of the fruit cell walls (CWs) by endogenous hydrolases but softening also is assumed to facilitate susceptibility to rotting by pathogens.  While other aspects of ripening are important to provide quality products for human consumption, the management of softening is key to optimize the storage and distribution of palatable fruit.  However, little is known about how B. cinerea, utilizes the normal fruit ripening process during growth and how decomposition by the fungus is limited in unripe green fruit.  Our hypothesis is that green fruit respond uniquely to B. cinerea, perhaps differently from vegetative tissues, and that this response changes and is replaced by other functions in ripe fruit that encourage fungal decomposition of fruit and dispersing mature seeds.  To evaluate the significance of CW breakdown in fruit susceptibility, the fruit from transgenic tomato lines with suppressed expression of the fruit ripening-associated polygalacturonase (PG) and/or expansin (LeExp1) were evaluated. Fruit softening and susceptibility to B. cinerea was reduced by 50% in fruit from lines in which both PG and LeExp1 (-PG-Exp) were suppressed. Cell wall pectin solubililization and swelling, common changes observed during fruit softening, were reduced in the –PG-Exp fruits.  Total CW polysaccharides were prepared from red fruit from all lines and added as a carbon source for liquid culture growth of B. cinerea.  Growth of B. cinerea was much greater in on CWs from control wild type fruit than on CWs from –PG-Exp fruit suggesting a direct effect of the CW structure on vigor of fungal growth. Future work will be done to determine if plant responses also are involved in the restriction of B. cinerea development observed in the transgenic lines and in green fruit. These results show that the functions provided by the expression of PG and LeExp1 in fruit softening contribute to the susceptibility of ripe fruit to B. cinerea.