Xyloglucan Endotransglycosylase Activity Increases during Kiwifruit (Actinidia deliciosa) Ripening (Implications for Fruit Softening)

The activity of xyloglucan endotransglycosylase (XET) was as-sayed in three tissue zones of kiwifruit (Actinidia deliciosa [A. Chev.] C.F. Liang et A.R. Ferguson var deliciosa cv Hayward) at harvest and at several softening stages following a postharvest ethylene treatment. At harvest, extractable XET activity per unit fresh weight in the inner pericarp (IP) and core tissue was 4.5 and 42 times higher, respectively, than in the outer pericarp (OP). Within 24 h of ethylene treatment there was an increase in the activity and specific activity of XET in all tissues that continued throughout softening. Activity increased most in the OP, where it showed a 12-fold rise 6 d after ethylene treatment compared with 4.5- and 2.5-fold increases in the IP and core tissues, respectively. Visible swelling of the cell wall in each tissue was observed 24 h after the first detectable rise in XET activity and was most pronounced in the OP, which showed the greatest percentage increase in XET activity. Xyloglucan, galactoglucomannan, and cell wall materials isolated and purified from kiwifruit OP were tested as donor substrates for kiwifruit XET. The enzyme showed activity against xyloglucan but was inactive against galactoglucomannan. XET was active against cell wall materials from unripe and ripe fruit, with swollen walls from the latter being the better substrate. The results indicate that XET may have a key role early in fruit ripening, loosening the cell wall in preparation for further modification by other cell wall-associated enzymes.