Characterization of the harvest-induced expression of β-galactosidase in Asparagus officinalis

The harvest-induced senescence of asparagus spears is accompanied by both up-regulated and down-regulated gene expression. The expression of pTIP31, coding for asparagus β-galactosidase (EC 3.2.1.23) is temporally associated with removal of the asparagus spear from the main body of the plant — neither wounding or compression treatments induce up-regulation of transcripts corresponding to pTIP31. Harvest-induced pTIP31 transcripts appear initially in cells below the meristems of the side branches, and in inner bracts and developing flowers of the spear tip, and in the cells of the outer cortex and conjunctive tissue lower down the spear. Transcripts are also located in vascular tissues 24 h after harvest. Enhanced levels of β-galactosidase activity are found in the branches and bracts of the spear tip within 12 h of harvest, while the rate of increase in activity in the middle zone of the spear was comparable to that of the branches and bracts after 24 h. The proportion of galactose in spear cell walls decreases after harvest. We propose that galactosidase activity releases galactose residues that may be used as respiratory substrate for the rapidly deteriorating asparagus spears.