Gibberellins and Heterosis in Maize : II. Response to Gibberellic Acid and Metabolism of [H]Gibberellin A(20)

Gibberellins and Heterosis in Crops and Trees: An Integrative Review and Preliminary Study with Brassica

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 Brassicarapa, 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 GA_19,20,1 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.

Roles of gibberellins and cytokinins in regulation of morphological and physiological traits in Polygonum cuspidatum responding to light and nitrogen availabilities

We evaluated the roles of gibberellins (GAs) and cytokinins (CKs) in regulation of morphological traits such as biomass allocation and leaf mass per area (LMA). Seedlings of Polygonum cuspidatum Siebold & Zucc. were grown under various light and N availabilities. We exogenously sprayed solutions of gibberellin (GA3), benzyl adenine (BA), uniconazole (an inhibitor of GA biosynthesis) or their mixtures on the aboveground parts, and changes in morphological and physiological traits and relative growth rate (RGR) were analysed. Endogenous levels of GAs and CKs in the control plants were also quantified. The morphological traits were changed markedly by the spraying. Biomass allocation to leaves was increased by GA3 and BA, whereas it decreased by uniconazole. GA3 decreased LMA, whereas uniconazole increased it. We found close relationships among morphological and physiological traits such as photosynthetic rate and net assimilation rate, and RGR under all growth conditions. Seedlings with high levels of endogenous GAs or CKs and low levels of endogenous GAs or CKs showed morphologies similar to those sprayed with GA3 or BA, and those sprayed with uniconazole, respectively. Thus we concluded these phytohormones are involved in the regulation of biomass allocation responding to either light or N availability.

Gibberellins: a phytohormonal basis for heterosis in maize

Four commercially important maize parental inbreds and their 12 F(1) hybrids were studied to investigate the role of the phytohormone gibberellin (GA) in the regulation of heterosis (hybrid vigor). All hybrids grew faster than any inbred. In contrast, all inbreds showed a greater promotion of shoot growth after the exogenous application of GA(3). Concentrations of endogenous GA(1), the biological effector for shoot growth in maize, and GA(19), a precursor of GA(1), were measured in apical meristematic shoot cylinders for three of the inbreds and their hybrids by gas chromatography-mass spectrometry with selected ion monitoring; deuterated GAs were used as quantitative internal standards. In 34 of 36 comparisons, hybrids contained higher concentrations of endogenous GAs than their parental inbreds. Preferential growth acceleration of the inbreds by exogenous GA(3) indicates that a deficiency of endogenous GA limits the growth of the inbreds and is thus a cause of inbreeding depression. Conversely, the increased endogenous concentration of GA in the hybrids could provide a phytohormonal basis for heterosis for shoot growth.

Gibberellins and heterosis of plant height in wheat (Triticum aestivum L.)

Background

Heterosis in internode elongation and plant height are commonly observed in hybrid plants, and higher GAs contents were found to be correlated with the heterosis in plant height. However, the molecular basis for the increased internode elongation in hybrids is unknown.

Results

In this study, heterosis in plant height was determined in two wheat hybrids, and it was found that the increased elongation of the uppermost internode contributed mostly to the heterosis in plant height. Higher GA₄ level was also observed in a wheat hybrid. By using the uppermost internode tissues of wheat, we examined expression patterns of genes participating in both GA biosynthesis and GA response pathways between a hybrid and its parental inbreds. Our results indicated that among the 18 genes analyzed, genes encoding enzymes that promote synthesis of bioactive GAs, and genes that act as positive components in the GA response pathways were up-regulated in hybrid, whereas genes encoding enzymes that deactivate bioactive GAs, and genes that act as negative components of GA response pathways were down-regulated in hybrid. Moreover, the putative wheat GA receptor gene TaGID1, and two GA responsive genes participating in internode elongation, GIP and XET, were also up-regulated in hybrid. A model for GA and heterosis in wheat plant height was proposed.

Conclusion

Our results provided molecular evidences not only for the higher GA levels and more active GA biosynthesis in hybrid, but also for the heterosis in plant height of wheat and possibly other cereal crops.

Involvement of gibberellins in the stem elongation of sun and shade ecotypes of Stellaria longipes that is induced by low light irradiance

Plants from two ecotypes of Stellaria longipes, alpine (an open, sunny habitat) and prairie (where adjacent plants provide a shaded habitat), were grown under normal and reduced levels of photosynthetically active radiation (PAR). Growth under low PAR is significantly promoted in both ecotypes. When quantified by the stable isotope dilution method, endogenous gibberellins (GAs) (GA1, GA8, GA20, GA19) were significantly elevated under low PAR in both 'sun' and 'shade' ecotypes, as was GA53 in the shade ecotype. Changes in endogenous GA1 levels were significantly correlated with stem growth during a 28 d growth cycle and with relative growth rate (RGR) for height under low PAR for both ecotypes. Interestingly, under low irradiance PAR, changes (both increases and decreases) in GA8, the 2beta-hydroxylated 'inactive' catabolite of GA1, closely parallel bidaily stem growth changes for both ecotypes. Because the significantly greater stem elongation of both ecotypes in response to low irradiance PAR is associated with significant increases in the endogenous levels of five GAs (GA53, GA19, GA1, GA8) in the early 13-hydroxylation GA biosynthesis pathway (measured at days 7,14 and 21), we conclude that the low irradiance PAR has very likely induced an overall increase in GA biosynthesis.

A genetic test of bioactive gibberellins as regulators of heterosis in maize

This study tested the hypothesis that gibberellin levels were responsible for the superior growth habit of hybrids (i.e., heterosis). If this were true, plants reduced in their capacity to produce gibberellin, such as maize plants homozygous for dwarf1 (d1), should display a lesser heterotic response. The d1 mutation was introgressed into two inbred lines of maize, B73 and Mo17, for seven generations. Plants segregating for the dwarf phenotype were produced both by self-fertilizing the introgressed inbred lines and by making reciprocal crosses between them to produce hybrids. Measurements were made of several physical traits. The results indicated that the hybrid dwarf plants experienced no loss of heterosis relative to their normal siblings. These results exclude the possibility that modulation of bioactive gibberellins is a major underlying basis of the heterotic response.

Gibberellins and the Legume-Rhizobium Symbiosis : III. Quantification of Gibberellins from Stems and Nodules of Lima Bean and Cowpea

Lima bean (Phaseolus lunatus L.) plants inoculated with Bradyrhizobium sp. strain 127E14 displayed a period of marked internode elongation that was not observed in plants inoculated with other compatible bradyrhizobia, including strain 127E15. When strain 127E14 nodulated an alternate host, cowpea (Vigna unguiculata L. Walp), a similar, although less dramatic growth response induced by the bacteria was observed. It has been speculated that the elongative growth promotion brought about by inoculation with strain 127E14 is mediated by gibberellins (GAs). Using deuterated internal standards and gas chromatography-mass spectroscopy analysis, we have quantified the levels of GA(1), GA(20), GA(19), and GA(44) in nodules and stems of two varieties of lima bean (bush and pole) and one variety of cowpea that were inoculated with either strain 127E14 or 127E15. In nodules formed by strain 127E14 on lima bean, endogenous levels of GA(20) and GA(19) were 10 to 40 times higher (35-88 ng/g dry weight) than amounts found in nodules formed by strain 127E15 (2.2-3.9 ng/g dry weight). Relative amounts of GA(44) were also higher (4- to 11-fold) in 127E14 nodules, but this increase was less pronounced. The rhizobial-induced increase of these GAs in the nodule occurred in both pole and bush varieties and seemed to be independent of host morphology. Regardless of rhizobial inoculum, levels of the "bioactive" GA(1) in the nodule (0.3-1.1 ng/g dry weight) were similar. In cowpea nodules, a similar, although smaller, difference in GA content due to rhizobial strain was observed. The concentration of GA(1) in lima bean stems was generally higher than that observed in the nodule, whereas concentrations of the other GAs measured were lower. In contrast with the nodule, GA concentrations in lima bean stems were not greater in plants inoculated with strain 127E14, and in some cases the slower growing plants inoculated with strain 127E15 actually had higher levels of GA(20), GA(19), and GA(44). Thus, there were major differences in concentrations of the precursors to GA(1) in nodules formed by the two bacterial strains, which were positively correlated with the observed elongation growth. These results support the hypothesis that the rhizobial strain modifies the endogenous GA status of the symbiotic system. This alteration in GA balance within the plant, presumably, underlies the observed growth response.

Genetic Regulation of Development in Sorghum bicolor: V. The ma(3) Allele Results in Gibberellin Enrichment

Sorghum bicolor genotypes, near isogenic with different alleles at the third maturity locus, were compared for development, for responsiveness to GA(3) and a GA synthesis inhibitor, and occurrence and concentrations of endogenous GAs, IAA, and ABA. At 14 days the genotype 58M (ma(3) (R)ma(3) (R)) exhibited 2.5-fold greater culm height, 1.75-fold greater total height, and 1.38-fold greater dry weight than 90M (ma(3)ma(3)) or 100M (Ma(3)Ma(3)). All three genotypes exhibited similar shoot elongation in response to GA(3), and 58M showed GA(3)-mediated hastening of floral initiation when harvested at day 18 or 21. Both 90M and 100M had exhibited hastening of floral initiation by GA(3) previously, at later application dates. Tetcyclacis reduced height, promoted tillering, and delayed flowering of 58M resulting in plants which were near phenocopies of 90M and 100M. Based on bioassay activity, HPLC retention times, cochromatography with (2)H(2)-labeled standards on capillary column GC and matching mass spectrometer fragmentation patterns (ions [m/z] and relative abundances), GA(1), GA(19), GA(20), GA(53), and GA(3) were identified in extracts of all three genotypes. In addition, based on published Kovats retention index values and correspondence in ion masses and relative abundances, GA(44) and GA(17) were detected. Quantitation was based on recovery of coinjected, (2)H(2)-labeled standards. In 14 day-old-plants, total GA-like bioactivity and GA(1) concentrations (nanograms GA/gram dry weight) were two- to six-fold higher in 58M than 90M and 100M in leaf blades, apex samples, and whole plants while concentrations in culms were similar. Similar trends occurred if data were expressed on a per plant basis. GA(1) concentrations for whole plants were about two-fold higher in 58M than 90M and 100M from day 7 to day 14. Concentrations of ABA and IAA did not vary between the genotypes. The results indicate the mutant allele ma(3) (R) causes a two- to six-fold increase in GA(1) concentrations, does not result in a GA-receptor or transduction mutation and is associated with phenotypic characteristics that can be enhanced by GA(3) and reduced by GA synthesis inhibitor. These observations support the hypothesis that the allele ma(3) (R) causes an overproduction of GAs which results in altered leaf morphology, reduced tillering, earlier flowering, and other phenotypic differences between 58M and 90M or 100M.

Relation between relative growth rate, endogenous gibberellins, and the response to applied gibberellic acid for Plantago major

Relationships between relative growth rate (RGR), endogenous gibberellin (GA) concentration and the response to application of gibberellic acid (GA(3) ) were studied for two inbred lines of Plantago major L., which differed in RGR. A4, the fast-growing inbred line, had a higher free GA concentration than the slow-growing W9, as analyzed by enzyme immunoassay. GA(3) application increased total plant weight and RGR(3) particularly for the slow-growing line. Chlorophyll a content and photosynthetic activity per unit leaf area were decreased, while transpiration rate was unaffected by GA(3) application. The increase in RGR by GA(3) application was associated with an increased leaf weight ratio; specific leaf area and percentage of dry matter in the leaves were only temporarily affected. Root respiration rate per unit dry weight was unaffected. The correlation between low RGR, low GA concentration and high responsiveness to applied GA(3) supports the contention that gibberellins are involved in the regulation of RGR. However, the transient influence of GA(3) application on some growth components suggests the involvement of other regulatory factors in addition to GA.

Growth and development of Brassica genotypes differing in endogenous gibberellin content. II. Gibberellin content, growth analyses and cell size

Three rapid cycling Brassica rapa genotypes were grown in greenhouse conditions to investigate the possible relationships between endogenous gibberellin (GA) content and shoot growth. Endogenous GA(1) GA(3) and GA(20) were extracted from stem samples harvested at 3 weekly intervals and analyzed by gas chromatography-mass spectrometry with selected ion monitoring, using [(2) H(2) ]-GA(1) and [(2) H(2) ]-GA(20) as quantitative internal standards. During the first 2 weeks, GA levels of the dwarf, rosette (ros), averaged 36% of levels in normal plants (on a per stem basis). Levels in the tall mutant, elongated internode (ein), were consistently higher, averaging 305% of levels in normal plants. Differences in shoot height across the genotypes resulted from varying internode length which resulted from epidermal cell length and number being increased in ein and decreased in ros relative to the normal genotype. The exogenous application of GA(3) to normal plants increased cell length while the application of paclobutrazol (PP333), a triazole plant growth retardant, reduced cell size. Thus, exogenous GA manipulations mimicked the influence of the mutant genes ros and ein. The dwarf, ros, had reduced shoot dry weights and relative growth rates compared to the other genotypes. Total dry weights were similar in ein and the normal genotype but stem weights were increased in ein, compensating for decreased leaf weights. Thus, the gibberellin-deficiency of ros resulted in generally reduced shoot growth. The overproduction of endogenous GA by ein did not result in enhanced shoot growth but rather a specific enhancement of internode elongation and stem growth at the expense of leaf size.

Gibberellins, Endogenous and Applied, in Relation to Flower Induction in the Long-Day Plant Lolium temulentum

Changes in endogenous gibberellin-like substances (GAs) and related compounds in the shoot apices of Lolium temulentum during and after flower induction by one long day was examined for plants grown in three consecutive years. The total GA level in the shoot apical tissue was high (up to 42 micrograms per gram dry weight, or 3 x 10(-5) molar GA(3) equivalents), increasing several-fold on the day after the long day and then declining. Of the many GA-like substances present, the putative polyhydroxylated components-with HPLC retention times between those of GA(8) (three hydroxyls) and GA(32) (four hydroxyls), and accounting for about a quarter of the total GA activity-were most consistent and striking in their changes. Their level in the apices increased 3- to 5-fold on the day after the long day and then subsided. When various GAs were applied to plants in noninductive short days, flower initiation was induced by several, most notably by GA(32), GA(5), 2,2-dimethyl GA(4), GA(3), and GA(7). GA(32) was most like one long day in eliciting a strong flowering response while having little effect on stem growth, whereas GA(1) had the opposite effect. It is suggested that highly hydroxylated C-19 GAs may play a central role in the induction of flowering in this long-day plant.

Photocontrol of gibberellin metabolism in situ in maize

Mature maize seeds were labeled with 10 to 100 pg per seed of [(3)H] gibberellins (GA) and [(3)H]GA glucosyl conjugate-like substances by feeding [(3)H]GA(20) of high specific activity (2.3 Curies per millimole) during seed maturation. The dry seeds, which contained 14% [(3)H]GA(20), 7% putative [(3)H]GA(1) and 78% [(3)H]GA glucosyl conjugate-like metabolites, were imbibed and germinated in the dark and under incandescent light. In both light and dark the proportion of [(3)H]GA conjugate-like metabolities declined (relative to that in the mature dry seeds) during imbibition and up to germination at hour 36. This decline was accompanied by increases in the proportions of [(3)H]GA(20) and putative [(3)H]GA(1) thereby indicating hydrolysis, which was greater in the dark than in the light. The proportions of [(3)H]GA conjugate-like substances in light-grown germinants were higher (121 and 141% of dark-grown) at 24 and 48 hour harvests and this statistically significant pattern was sustained up to 120 hours after imbibition. Conversely, the proportions of [(3)H]GA(20) and putative [(3)H]GA(1) were lower in the light-grown seedlings. Thus, during imbibition, hydrolysis (de-conjugation) of [(3)H]GA glucosyl conjugate-like substances apparently occurred, and occurred more rapidly in the dark than in the light. Subsequently, during germination the reformation of [(3)H]GA conjugate-like substances was less rapid in the dark than in the light. The observation that dark-imbibed seeds and dark-grown seedlings have higher proportions of putative free [(3)H]GAs, relative to [(3)H]GA conjugate-like substances, is consistent with the increased shoot elongation (etiolation) that occurs in dark-grown maize seedlings, and may indicate a homeostatic role for GAs and their conjugates in shoot elongation of maize germinants.

Reversible conjugation of gibberellins in situ in maize

Gibberellins [(3)H]GA(4) (1.33 Curies per millimole) and [(3)H]GA(20) (2.36 Curies per millimole) were injected into the shanks of maize (Zea mays L.) cobs during rapid grain filling and mature seeds were subsequently harvested. Extracts of mature, dry seeds from 1980 feeds yielded only 20 to 30% of the (3)H radioactivity in acidic, ethyl acetate-soluble form, and this was principally associated with the precursor, with lesser amounts of the major metabolite, [(3)H]GA(1) (putative identification based on sequential SiO(2) partition, and gradient-eluted reverse-phase C(18) high performance liquid chromatography [HPLC]). Most of the radioactivity in the dry seeds was associated with compounds having partition characteristics of, and co-chromatographing on, sequential SiO(2) partition and reverse-phase HPLC with glucosyl conjugates of the precursors (GA(4) or GA(20)) and their probable major metabolite (GA(1)). The majority of conjugate associated with the precursor GA(4) eluted coincidental with GA(4) glucoside. Subsequent acid or enzymic hydrolysis (beta-glucosidase or cellulase) yielded the free GAs, putative identification being based on isocratic HPLC of each (3)H-labeled conjugate --> hydrolysis --> isocratic HPLC of the (3)H-labeled hydrolysate. Upon imbibition of the seeds, radioactivity associated with the conjugate fraction decreased; concomitantly, statistically significant increases in levels of free [(3)H]GA-like compounds were observed. Although the specific ratios of GA-like and GA-glucosyl conjugate-like substances varied substantially across years, hybrids, and even, in different plants from the same hybrid, this ;reversible conjugation' (i.e. apparent conjugation during seed maturation followed by release of the GA moiety during germination), was reproducible for [(3)H]GA(20) in seed from two maize hybrids produced over 2 years.