Gibberellins, Amylase, and the Onset of Heterosis in Maize Seedlings

Abstract

Rood, S. B. and Larsen, K. M. 1988. Gibberellins, amylase, and the onset of heterosis in maize seedlings.—J. exp. Bot. 39: 223-233. The possible involvement of gibberellins and amylase in heterosis of maize seedlings was investigated in two parental inbreds, CM7 and CM49, and their single cross F, hybrid, CM7 x CM49. Germination of all three genotypes was complete within 36 h after the onset of imbibition. By 48 h, heterosis (hybrid vigour) for increased shoot and root length was consistently observed. The endogenous concentration of gibberellin (GA;) was measured in 48 h seedlings by gas chromatography-mass spectrometry with selected ion monitoring (GC-SIM) using pHJ-GAj as an internal standard. The GA3 concentration was highest in the hybrid (59 ng g~' dry wt.), intermediate in CM49 (9-Ongg-1), and lowest in CM7 (<50ngg_1). Amylase activities in all three genotypes were very low at 24 h, but increased during the next 24 h, after which time amylase activity in the hybrid was significantly higher than that of either parental inbred. Inhibitors of gibberellin (GA) biosynthesis, AMO-1618 or CCC, inhibited germination, shoot and root growth, and amylase activity in all three genotypes. Conversely, exogenous gibberellic acid (GA3) increased amylase activity, particularly in the inbred CM7. Amylase isozymes were separated through polyacrylamide gel electrophoresis and generally similar profiles of starch degrading enzymes were observed in the three genotypes. Since GA is known to control a-amylase biosynthesis in some cereals, these results are consistent with the hypothesis that GAs are involved in the regulation of heterosis in maize. A higher endogenous GAt concentration in the hybrid could result in increased amylase activity in the hybrid seedlings and consequently, more rapid starch hydrolysis which fuels heterosis for early growth.