David W. Pearce

Gibberellins: A Phytohormonal Basis for Heterosis in Maize

Four commercially important maize parental inbreds and their 12 F1 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 GA3. Concentrations of endogenous GA1, the biological effector for shoot growth in maize, and GA19, a precursor of GA1, 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 GA3 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.

Chromatography of gibberellins

Abstract The recent literature on the use of chromatography in the analysis of gibberellins in plants is reviewed. Particular emphasis is placed on the application of solid-phase purification techniques, immunoaffinity chromatography, HPLC, GC-MS and LC-MS.

Stem elongation and gibberellins in alpine and prairie ecotypes of Stellaria longipes

The potential for gibberellins (GAs) to control stem elongation and itsplasticity (range of phenotypic expression) was investigated inStellaria longipes grown in long warm days. Gibberellinmetabolism and sensitivity was compared between a slow-growing alpine dwarfwithlow stem elongation plasticity and a rapidly elongating, highly plastic prairieecotype. Both ecotypes elongated in response to exogenous GA1,GA4 or GA9, but surprisingly, the alpine dwarf wasrelatively unresponsive to GA3. Endogenous GA1,GA3, GA4, GA5, GA8, GA9and GA20 were identified and quantified in stem tissue harvested atcommencement, middle and end of the period of most rapid elongation. Theconcentration of GAs which might be expected to promote shoot elongation washigher during rapid elongation than toward its end for both ecotypes. Whilethere was a trend for certain GAs (GA3, GA4,GA9, GA20) to be higher in stems of the alpine ecotypeduring rapid elongation, that result does not explain the slower growth of thealpine ecotype and the faster growth of the prairie ecotype under a range ofconditions. To determine if the two ecotypes metabolized GA20differently, plants were fed [2H]- or[3H]-GA20. The metabolic products identified included[2H2]-GA1, -GA8, -GA29,-GA60, -3-epi-GA1, GA118(-1-epi-GA60) and -GA77. The concentration of[2H2]-GA1 also did not differ between the twoecotypes and metabolism of [2H2]- or[3H]-GA20 was also similar. In the same experiments thepresence of epi-GA1, GA29, GA60,GA118 and GA77 was indicated, suggesting that these GAsmay also occur naturally in S. longipes, in addition tothose described above. Collectively, these results suggest that while stemelongation within ecotypes is likely regulated by GAs, differences in GAcontent, sensitivity to GAs (GA3 excepted), or GA metabolism areunlikely to be the controlling factor in determining the differences seen ingrowth rate between the two ecotypes under the controlled environmentconditionsof this study. Nevertheless, further study is warranted especially underconditions where environmental factors may favour a GA:ethylene interaction.

Structure determination and synthesis of a new trihydroxy gibberellin from Brassica campestris cotyledons and a related tetrahydroxy derivative from immature seeds of prunus persica

The structures of two new gibberellins, GA85 from Brassica campestris, and GA86 (1,2-dihydro-GA32) from Prunus persica, have been established as the 3β,12α,13-trihydroxy derivative 2 and the 3β,12α,13,15β-tetrahydroxy analogue 3, respectively, by synthesis of their methyl esters from gibberellic acid (1).

Effect of 2-chloroethyltrimethyl ammonium chloride on tuberization and endogenous GA3 in roots of potato cuttings

Cuttings of potato shoots treated with the plant growth retardant 2-chloroethyltrimethyl ammonium chloride (CCC) form tubers earlier and have less biologically-active gibberellin (GA)-like substances in the roots than control cuttings. The major GA-like substance in roots of potato cuttings was identified as GA3 by gas-chromatography-mass spectrometry (GC-MS). The content of GA3 in roots of control cuttings, estimated by GC-MS-selected ion monitoring (SIM) using [17, 17-2H]GA3 as a quantitative internal standard, was 38.8 ng per g fresh weight (fw), and in roots of CCC-treated cuttings, in which tuberization was promoted, was 0.6 ng per g fw. Gibberellin A1, GA8 and GA20 were also indicated as minor components of roots from both control and CCC-treated cuttings. The comparatively high GA3 content in roots of control cuttings might be the ‘root factor’ responsible for delaying tuberization in potato.

Effect of exo-16,17-dihydro-gibberellin A5 on gibberellin A20 metabolism in seedlings of dwarf rice (Oryza sativa L. cv. Tan-ginbozu)

Several of the 16,17-dihydro gibberellins (GAs) inhibit elongation in a variety of species. In a study of their mechanism of action we have investigated the effect of exo-16,17-dihydro-Ga5 (diHGA5) on the metabolism of GA20 in dwarf rice (Oryza sativa cv. Tan-ginbozu). A mixture of [3H]- and [3H]-GA20 (100 ng per plant) was applied in microdrops to 4 d old seedlings which were harvested 72 h later. Concurrent treatment with diHGA5 at 100 ng or 333 ng per plant reduced GA20-induced elongation of the second leaf sheath by 41–66%. There was a concomitant reduction in the amount of [2H2]GA1 present at harvest, measured by gas chromatography-mass spectrometry-selected ion monitoring. The [2H2]GA29 content was also reduced. There was no clear effect of diHGA5 on the total radioactivity recovered, or on conversion of the [3H]GA20 to putative [3H]GA conjugates, or on the amount of [2H2]GA20 found. No free [2H2]GA8 was detected. In other experiments there was little effect of diHGA5 on elongation induced by treatment with GA1. We conclude that diHGA5 inhibited GA20-induced elongation in dwarf rice shoots at least partly by reducing the 3β-hydroxylation of GA20 to GA1.

Idenfication of gibberellin A89 from Brassica campestris

Abstract A Tetrahydroxy gibberellin, ent -2β,3α,12β,13-tetrahydroxy-20-nor-gibberell-16-en-7-oate 19,10-lactone, namely 2β-hydroxy GA 85 , was identified from an extract of the cotyledons of young seedlings of Brassica campestris cv. Tobin. Determination was based on GC-MS by comparison of its mass spectrum and R I with those of authentic 2β-hydroxy GA 85 (MeTMSi). The new GA has been assigned the trivial name gibberellin has been assigned the trivial name gibberellin A 89 (GA 89 ).