In order to reduce the pool of bioactive GA, GAs can either be deactivated by 2ȕ-hydroxylation and further catabolism, as described above, or they can be converted into conjugates (60). Conjugation to glucose is found most commonly for GAs and this can occur either via a hydroxyl group to give a GA-O-glucosyl ether (GA-O-Glc), or via the 7-carboxyl group to give a GAglucosyl ester (GA-Glc ester). The most common sites within the GA molecule for –O-glc conjugation are C-2, C-3 and C-13. When applied to bioassay plants GA-O-Glcs show little or no activity, whereas GA-glc esters can exhibit bioactivity in certain assays, although this is unlikely to be activity of the conjugate per se. Instead it appears that if a bioassay plant, or microbial contaminant of the plant, possesses the requisite hydrolytic enzyme to cleave the glucose moiety and if the resulting aglycone is a potentially active GA, then the GA-conjugate will appear to have bioactivity. Feeding studies suggest that GA-Glc-esters sequester bioactive GAs, often quite rapidly, and release the free GA as required. On the other hand, the fate of GA-O-Glcs appears to be determined by the nature of the parent GA. GA-2O-glcs, upon hydrolysis, will yield inactive GAs, whereas GA-3-O-glcs are hydrolyzed to bioactive GAs. Thus the enzymes for the synthesis and hydrolysis of GA-3-O-glcs have higher specificity than those catalyzing the hydrolysis of GA 2-O-glcs, reflecting their more direct role in maintaining the pool size of active GA.
In order to reduce the pool of bioactive GA, GAs can either be deactivated by 2ȕ-hydroxylation and further catabolism, as described above, or they can be converted into conjugates (60). Conjugation to glucose is found most commonly for GAs and this can occur either via a hydroxyl group to give a GA-O-glucosyl ether (GA-O-Glc), or via the 7-carboxyl group to give a GAglucosyl ester (GA-Glc ester). The most common sites within the GA molecule for –O-glc conjugation are C-2, C-3 and C-13. When applied to bioassay plants GA-O-Glcs show little or no activity, whereas GA-glc esters can exhibit bioactivity in certain assays, although this is unlikely to be activity of the conjugate per se. Instead it appears that if a bioassay plant, or microbial contaminant of the plant, possesses the requisite hydrolytic enzyme to cleave the glucose moiety and if the resulting aglycone is a potentially active GA, then the GA-conjugate will appear to have bioactivity. Feeding studies suggest that GA-Glc-esters sequester bioactive GAs, often quite rapidly, and release the free GA as required. On the other hand, the fate of GA-O-Glcs appears to be determined by the nature of the parent GA. GA-2O-glcs, upon hydrolysis, will yield inactive GAs, whereas GA-3-O-glcs are hydrolyzed to bioactive GAs. Thus the enzymes for the synthesis and hydrolysis of GA-3-O-glcs have higher specificity than those catalyzing the hydrolysis of GA 2-O-glcs, reflecting their more direct role in maintaining the pool size of active GA.
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