Detection of endogenous gibberellins and their relationship to hypocotyl elongation in soybean seedlings.

Four gibberellins, GA53, Ga19, GA20, and GA1, were detected by bioassay, chromatography in two HPLC systems, and combined gas chromatography-mass spectroscopy-selected ion monitoring (GC-MS-SIM) in etiolated soybean (Glycine max [L.] Merr.) hypocotyls. GC-MS-SIM employed [2H2]-labeled standards for each endogenous gibberellin detected, and quantities estimated from bioassays and GC-MS-SIM were similar. This result plus the tentative detection of GA44 and GA8 (standards not available) indicates that the early-C-13-hydroxylation pathway for gibberellin biosynthesis predominates in soybean hypocotyls. Other gibberellins were not detected. Growth rates decreased after transfer to low water potential vermiculite and were completely arrested 24 hours after transfer. The GA1 content in the elongating region of hypocotyls had declined to 38% of the 0 time value at 24 hours after transfer to low water potential vermiculite, a level which was only 13% of the GA1 content in control seedlings at the same time (24 hours posttransfer). Rewatering seedlings following 24 hours growth in low water potential vermiculite resulted in a complete recovery in elongation rate, an increase in GA1 (20% at 2 hours, two-fold at 8 hours, eightfold at 24 hours), and a decrease in ABA levels (tenfold at 2 hours). Treatment of well-watered seedlings with the Ga-synthesis inhibitor tetcyclacis (TCY) resulted, in lowered GA1 levels and increased ABA levels. When seedlings grown 24 hours in low water potential vermiculite were rewatered with TCY, recovery of the elongation rate was delayed and reduced, and the decline in ABA levels was slowed. Addition of GA3 restored the elongation rate inhibited by TCY. Seedlings were growth responsive to exogenous GA3, and this Ga3-promoted growth was inhibited by exogenous ABA. The data are consistent with the hypothesis that changes in GA1 and ABA levels play a role in adjusting hypocotyl elongation rates. However, the changes observed are not of sufficient magnitude nor do they occur rapidly enough to suggest they are the primary regulators of elongation rate responses to rapidly changing plant water status.