Involvement of initial pectin branching and role of ?well known? loosening proteins on fruit texture

VICENTE AR; SALATO G; PONCE NMA; RAFFO MD; POWELL ALT; BENNET AB; SRORTZ CA; LABAVITCH JM

Lemessos

Congreso; Postharvest Unlimited; 2014

ISHS

Fruit softening has been largely associated with cell wall degradation by a number of relatively well characterized loosening proteins. Firmness is a major quality attribute of fresh cherries and also an important factor affecting the susceptibility to postharvest rots. By analyzing the solubilization, depolymerization and composition of pectins and hemicelluloses in two cultivars with highly contrasting firmness (Sweetheart, firm and Newstar, soft) at four developmental stages we found that the pattern and extent of their wall disassembly was quite similar. No marked pectin or hemicellulose depolymerization was observed and a similar reduction in tightly-bound pectins and hemicellulose was detected in both varieties during ripening. However, firm Sweetheart cherries showed higher cell wall yields and presented pectic polymers with lower neutral contents. This suggests that the variation in total wall polysaccharide and the branching of pectins assembled early in development, may contribute to the differences in firmness between cultivars. Dismantling of the cell wall by the action of relatively ?well known? loosening agents is involved in the progressive softening occurring during ripening. Two of these proteins include polygalacturonases (PG) which are known to hydrolyze homogalacturonans and expansins (Exp) believed participate in the relaxation of the cell wall by reducing H bonding between cellulose microfibrils and xyloglucan. We investigated the in vivo roles of these wall-disassembling proteins, by overexpressing PG and Exp1 both alone and in combination in a non-ripening rin tomato background. The simultaneous overexpression of PG and Exp1 in rin fruit restored softening in these non-ripening fruit. Interestingly, PG overexpression resulted in higher hemicellulose depolymerization and increased levels of Exp1 accelerated pectin turnover. This shows that besides their ?well known? in vitro functions these proteins act in muro by facilitating the degradation of non-directly targeted wall components, likely by increasing the accessibility of pre-existing wall-degrading proteins to their polysaccharide substrates.