Some fruits remain relatively firm as they ripen, whereas others — such as strawberries and avocados — soften until they melt in the mouth. Such fruits quickly become over-ripe, even after picking, giving them a very short 'shelf life'. Such softening was thought to occur mainly from hydrolytic enzymes, such as polygalacturonase, breaking down the fruit's cell walls. Now, José Mercado and colleagues report in Plant Physiology that they have identified a new player in the process — pectate lyase.

Pectate lyases have been extensively studied in pathogenic bacteria, where they aid infection of higher plants by breaking down the hemicellulose pectin in their cell walls. They have a highly regular β-helical structure, and attack pectins by a β-elimination reaction rather than the hydrolytic approach of most polysaccharide-degrading enzymes. Pectate lyases are also present in pollen, where they aid germination and growth of the pollen tube through the tissues of the stigma and style.

A pectate lyase gene had previously been isolated from ripe strawberries, so Mercado and colleagues decided to investigate its function by reducing its expression in transgenic plants using an antisense sequence of the gene under the control of a 35S promoter.

Three of the 41 independent transgenic lines they produced showed no significant decrease in the size and amount of fruit produced and yet had no detectable pectate lyase expression. However, the fruit that they produced were much firmer than those from control plants, and far fewer had become over-ripe and 'squashy' four days after picking.

The researchers do not comment on the taste of their transgenic strawberries, although less subjective measures of fruit quality, such as the size, colour and concentration of soluble solids, were not altered. Along with improved shelf life, such transgenic plants may prove a boon to jam makers, who require fruit with high levels of pectin as, when boiled, this causes jam to set. This approach might also be applicable to other fruits, as a pectate lyase has already been found in ripening bananas.