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N-glycans: The time is ripe

Mary O'Reilly

In addition to outward signs such as softening and color change, fruit ripening can be measured on a molecular level by analyzing the expression of plant hormones and cell wall-degrading enzymes. With some success, genetic engineering has been applied to exploit these pathways in an effort to improve the global fruit trade by extending the expiration date. Like some plant hormones and enzymes, the composition of free N-glycans changes as fruit ripens, suggesting that these sugars and the enzymes that process them also modulate ripening. To test this possibility, Asis Datta and colleagues explored the role of two N-glycoprotein modifying enzymes, α-mannosidase (α-man) and β-D-N-acetylhexosaminidase (β-hex), in the cell-signaling network that controls tomato ripening.

As they mature, tomatoes pass through four stages: mature green, breaker, pink and red ripe. The authors found that α-man and β-hex expression levels peaked during the breaker and pink stages of ripening. They also noticed that these enzymes were expressed in the tomato's cell wall, but not in any other part of the plant. Further experiments showed that α-man and β-hex degrade cell wall glycans, producing free glycans that further augment ripening, thus indicating that N-glycan processing by these enzymes hastens the softening process.

Next, the authors looked at mutant tomatoes that are slow to ripen because of impaired production and recognition of ethylene, a plant hormone that promotes cell wall softening. Given that α-man and β-hex levels were also generally reduced in these fruits, they concluded that these enzymes are regulated by ethylene. In turn, α-man and β-hex expression was shown to boost expression of other enzymes that enhance ripening by degrading cell wall components such as cellulose and pectin.

When expression of α-man or β-hex was specifically inhibited, tomatoes developed normally and then continued to retain their texture and firmness on the vine longer than wild-type controls. Once picked from the plant, the α-man- or β-hex-deficient tomatoes remained red and firm for several weeks after the wild-type fruits had wrinkled and turned brown. Microscopy experiments attributed the enhanced firmness to less cell separation and more carbohydrate build-up in the cell wall of transgenic tomatoes as compared with wild-type fruits.

This study outlines a cell-signaling pathway in which a hormone triggers glycan processing enzymes that degrade sugars and activate the breakdown of several other cell wall components in fruit; all of this accelerates ripening. At the crossroads of this pathway lie the enzymes α-man and β-hex, ideal targets for the genetic modification of longer-lasting fruits.

Author: Heather Buschman