Effect of simultaneous down-regulation of pectate lyase and endo-B-1,4-glucanase genes on strawberry fruit softening

Abstract

Strawberry is a soft fruit with a short postharvest shelf life. The loss of fruit firmness during ripening is mainly due to the disassembly of parenchyma cell walls mediated by the expression of genes encoding enzymes acting on pectins, such as pectate lyase, or hemicellulose, e.g. endo--1,4-glucanase. To determine if the simultaneous down-regulation of FaplC and FaEG3 genes, encoding a pectate lyase and a endo--1,4-glucanase, respectively, exerted an additive effect on strawberry softening, transgenic plants expressing tandem antisense sequences of both genes under the control of the constitutive promoter CaMV35S were generated. Fifteen independent transgenic lines were obtained and fruit yields and several quality parameters of transgenic ripe fruit were recorded during two consecutive years. Fruit yield was reduced in most of the lines, especially in the first evaluation period, and 5 out of 15 lines, a 33%, did not set fruit. The expression of FaplC and FaEG3 genes was measured in ripe fruits from six selected lines showing the highest fruit yields. All selected lines showed a high level of FaplC gene silencing, ranging from 97 to 71%; however, FaEG3 gene expression was only significantly down-regulated in two lines. Fruit color and soluble solids contents were similar in control and transgenic ripe fruits, while fruit weight was slightly lower than control in some of the lines. In all lines, transgenic fruits were significantly firmer than control, with an increase on firmness ranging from 19 to 32%. The reduction of fruit softening in transgenic fruits was not correlated with the suppression of FaEG3 gene expression, and lines with the highest simultaneous down-regulation of FaplC and FaEG3 showed similar fruit firmness than lines where only FaplC was suppressed. These results indicate that pectate lyase and endo--1,4-glucanase do not act in an additive or synergistic way during strawberry softening, and question the role of glucanases in this process.