Abstract
Heterosis, or hybrid vigor, has contributed substantially to genetic improvements in crops and trees and its physiological basis involves multiple processes. Four associations with the phytohormone gibberellin (GA) indicate its involvement in the regulation of heterosis for shoot growth in maize, sorghum, wheat, rice, tomato and poplar. (1) Inbreds somewhat resemble GA-deficient dwarfs and are often highly responsive to exogenous GA3. (2) Levels of endogenous GAs, including the bioeffector GA1, its precursors GA19 and GA20, and/or its metabolite GA8, are higher in some fast-growing hybrids than parental genotypes. (3) Oxidative metabolism of applied [3H]GAs is more rapid in vigorous hybrids than inbreds, and (4) heterotic hybrids have displayed increased expression of GA biosynthetic genes including GA 20-oxidase and GA 3-oxidase. We further investigated Brassica rapa, an oilseed rape, by comparing two inbreds (AO533 and AO539) and their F1 hybrid. Seedling emergence was faster in the hybrid and potence ratios indicated dominance for increased leaf number, area and mass, and stem mass. Overdominance (heterosis) was displayed for root mass, leading to slight heterosis for total plant mass. Stem contents of GA19,20,1 were similar across the Brassica genotypes and increased prior to bolting; elongation was correlated with endogenous GA but heterosis for shoot growth was modest. The collective studies support a physiological role for GAs in the regulation of heterosis for shoot growth in crops and trees, and the Brassica study encourages further investigation of heterosis for root growth.
Highlights
IntroductionGibberellins and Heterosis in Crops and Trees
[3 H]GA1 to [2 H]GA8 or its glucosyl conjugates. These findings indicate faster oxidative metabolism4 in fast-growing hybrids and provide the third line of evidence for a role of GAs in the regulation of heterosis for shoot growth
To further explore the growth components that contribute to heterosis, and to investigate their association with endogenous GAs, Brassica was used as another study system
Summary
Gibberellins and Heterosis in Crops and Trees. Heterosis is common in crop plants and in some trees and has been responsible for a substantial proportion of the genetic improvement through plant breeding over the past century [2,3]. While heterosis is widely utilized for agriculture, horticulture and silviculture, and despite a century of investigation, its physiological basis is poorly understood [4]. Studies indicate that there are multiple contributing genetic and physiological factors [5,6,7]. Phytohormones should be involved since these provide master regulators of plant form and function [8,9]. There is substantial evidence that the promotive regulator gibberellin (GA) is especially involved in heterosis for shoot growth. There have been preliminary reviews of this association [10,11] and we extend the analyses to incorporate recent findings, expand the range of study approaches and increase the number of plant species
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