Metabolism of tritiated gibberellin a(20) in maize

Hydrolysis and reconjugation of gibberellin A20 glucosyl ester by seedlings of Zea mays L

The [6-2H]glucosyl ester of [17-13C,3H]gibberellin A20 (GA20) was injected into light-grown 14-day-old seedlings of normal, dwarf-1, and dwarf-5 maize (Zea mays L.). The plant material was extracted 24 h later, and the extracts were purified by solvent partitioning, column chromatography, and HPLC. 13C-labeled metabolites were identified from the purified extracts by full-scan gas chromatography/mass spectrometry and selected ion current monitoring in conjunction with Kovats retention indices. The metabolites, [13C]GA20, [13C]GA29, [13C]GA20-13-O-glucoside, and [13C]GA29-2-O-glucoside, were identified from normal, dwarf-1, and dwarf-5 seedlings. [13C]GA8 and [13C]GA8-2-O-glucoside were also identified from normal and dwarf-5 seedlings but not from dwarf-1 seedlings. The data provide definitive evidence for the endogenous hydrolysis by the seedlings of the introduced conjugate and its reconjugation to three glucosides.

Gibberellin A(3) Is Biosynthesized from Gibberellin A(20) via Gibberellin A(5) in Shoots of Zea mays L

[17-(13)C,(3)H]-Labeled gibberellin A(20) (GA(20)), GA(5), and GA(1) were fed to homozygous normal (+/+), heterozygous dominant dwarf (D8/+), and homozygous dominant dwarf (D8/D8) seedlings of Zea mays L. (maize). (13)C-Labeled GA(29), GA(8), GA(5), GA(1), and 3-epi-GA(1), as well as unmetabolized [(13)C]GA(20), were identified by gas chromatography-selected ion monitoring (GC-SIM) from feeds of [17-(13)C, (3)H]GA(20) to all three genotypes. (13)C-Labeled GA(8) and 3-epi-G(1), as well as unmetabolized [(13)C]GA(1), were identified by GC-SIM from feeds of [17-(13)C, (3)H]GA(1) to all three genotypes. From feeds of [17-(13)C, (3)H]GA(5), (13)C-labeled GA(3) and the GA(3)-isolactone, as well as unmetabolized [(13)C]GA(5), were identified by GC-SIM from +/+ and D8/D8, and by full scan GC-MS from D8/+. No evidence was found for the metabolism of [17-(13)C, (3)H]GA(5) to [(13)C]GA(1), either by full scan GC-mass spectrometry or by GC-SIM. The results demonstrate the presence in maize seedlings of three separate branches from GA(20), as follows: (a) GA(20) --> GA(1) --> GA(8); (b) GA(20) --> GA(5) --> GA(3); and (c) GA(20) --> GA(29). The in vivo biogenesis of GA(3) from GA(5), as well as the origin of GA(5) from GA(20), are conclusively established for the first time in a higher plant (maize shoots).

Endogenous Gibberellins in Elongating Shoots of Clones of Salix dasyclados and Salix viminalis

Elongating shoots of rapidly growing clones of Salix viminalis L. (clone 683-4) and Salix dasyclados Wimm. (clone 908) harvested in early August were analyzed for endogenous gibberellins (GA). Distribution of GA-like activity, determined by Tan-ginbozu dwarf rice microdrop bioassay after reverse phase C(18) high performance chromatography, was similar for both species. For S. dasyclados, combined gas chromatography-selected ion monotoring (GC-SIM) yielded identifications of GA(1), GA(8), GA(19), GA(20), and GA(29). Identifications of GA(4) and GA(9) were also made using co-injections of known amounts of [17, 17-(2)H(2)]GAs. By bioassay, the main activity was GA(19)-like in both species. Gibberellin A(1), GA(19), and GA(20) concentrations were approximated by GC-SIM using co-injections of known amounts of [17,17-(2)H(2)]GAs. Both bioassay and GC-SIM results indicated very high concentrations of GA(19) and GA(20) (about 6000 nanograms per kilogram fresh weight shoot tissue using GC-SIM: 800 ng using bioassay), compared to the concentration of GA(1) (about 130 nanograms per kilogram fresh weight using either GC-SIM or bioassay).

Identification of endogenous gibberellins from oilseed rape

Oilseed rape (Brassica napus, canola variety ;Westar') plants were grown in greenhouse conditions and shoots were harvested during the final stages of shoot elongation. Leaves and immature pods were removed and the remaining stem tissue was extracted and purified. The extract was chromatographed on sequential, step-eluted silica gel partition and reverse-phase C(18) HPLC columns, and gibberellin (GA)-like substances were detected using the ;Tan-ginbozu' dwarf rice microdrop assay. Purified fractions showing GA-like activity were analyzed by capillary gas chromatography-mass spectrometry (GC-MS) and GC-selected ion monitoring (GC-SIM). Gibberellins A(1), A(3), and iso-A(3) were identified by full spectrum GC-MS with GA(1) being the most abundant GA in the stem tissue. Gibberellins A(19) and A(20) were identified by GC-SIM and are logical precursors of the GA(1).

Gibberellins and gravitropism in maize shoots: endogenous gibberellin-like substances and movement and metabolism of [3H]Gibberellin A20

[3H]Gibberellin A20 (GA20) of high specific radioactivity (49.9 gigabecquerel per millimole) was applied equilaterally in a ring of microdrops to the internodal pulvinus of shoots of 3-week-old gravistimulated and vertical normal maize (Zea mays L.), and to a pleiogravitropic (prostrate) maize mutant, lazy (la). All plants converted the [3H]GA20 to [3H]GA1- and [3H]GA29-like metabolites as well as to several metabolites with the partitioning and chromatographic behavior of glucosyl conjugates of [3H]GA1, [3H]GA29, and [3H]GA8. The tentative identification of these putative [3H]GA glucosyl conjugates was further supported by the release of the free [3H]GA moiety after cleavage with cellulase. Within 12 hours of the [3H]GA20 feed, there was a significantly higher proportion of total radioactivity in lower than in upper halves of internode and leaf sheath pulvini in gravistimulated normal maize. Further, there was a significantly higher proportion of putative free GA metabolites of [3H]GA20, especially [3H]GA1, in the lower halves of normal maize relative to upper halves. The differential localization of the metabolites between upper and lower halves was not apparent in the pleiogravitropic mutant, la. Endogenous GA-like substances were also examined in gravistimulated maize shoots. Forty-eight hours after gravistimulation of 3-week-old maize seedlings, endogenous free GA-like substances in upper and lower leaf sheath and internode pulvini halves were extracted, chromatographed, and bioassayed using the "Tanginbozu" dwarf rice microdrop assay. Lower halves contained consistently higher total levels of GA-like activity. The qualitative elution profile of GA-like substances differed consistently, upper halves containing principally a GA20-like substance and lower halves containing principally a GA20-like substance and lower halves containing mainly GA1-like and GA19-like substances. Gibberellins A1 (10 nanograms per gram) and A20 (5 nanograms per gram) were identified from these lower leaf sheath pulvini by capillary gas chromatography-selected ion monitoring. Results from all of these experiments are consistent with a role for GAs in the differential shoot growth that follows gravitropism, although the results do not eliminate the possibility that the redistribution of GAs results from the gravitropic response.

Identification of endogenous gibberellins from sorghum

Gibberellins (GA) A(1), A(19), and A(20) were identified in shoot cylinders containing the apical meristems from sorghum (Sorghum bicolor L.). Extracts were purified by sequential SiO(2) partition chromatography and reversed-phase C(18) high performance liquid chromatography and biologically active (dwarf rice cv Tan-ginbozu microdrop assay) fractions were subjected to gas chromatography-selected ion monitoring. Based on the use of [(3)H]GA and [(2)H](d(2))GA internal standards, amounts of GA(1), GA(19), and GA(20) were estimated to be 0.7, 8.8, and 1.5 namograms per gram dry weight of tissue, respectively.

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.

Internode length in Zea mays L. : The dwarf-1 mutation controls the 3β-hydroxylation of gibberellin A20 to gibberellin A 1

[(13)C, (3)H]Gibberellin A20 (GA20) has been fed to seedlings of normal (tall) and dwarf-5 and dwarf-1 mutants of maize (Zea mays L.). The metabolites from these feeds were identified by combined gas chromatography-mass spectrometry. [(13)C, (3)H]Gibberellin A20 was metabolized to [(13)C, (3)H]GA29-catabolite and [(13)C, (3)H]GA1 by the normal, and to [(13)C, (3)H]GA29 and [(13)C, (3)H]GA1 by the dwarf-5 mutant. In the dwarf-1 mutant, [(13)C, (3)H]GA20 was metabolized to [(13)C, (3)H]GA29 and [(13)C, (3)H]GA29-catabolite; no evidence was found for the metabolism of [(13)C, (3)H]GA20 to [(13)C, (3)H]GA1. [(13)C, (3)H]Gibberellin A8 was not found in any of the feeds. In all feeds no dilution of (13)C in recovered [(13)C, (3)H]GA20 was observed. Also in the dwarf-5 mutant, the [(13)C]label in the metabolites was apparently undiluted by endogenous [(13)C]GAs. However, dilution of the [(13)C]label in metabolites from [(13)C, (3)H]GA20 was observed in normal and dwarf-1 seedlings. The results from the feeding studies provide evidence that the dwarf-1 mutation of maize blocks the conversion of GA20 to GA1.

Role of Endogenous Plant Growth Regulators in Seed Dormancy of Avena fatua: II. Gibberellins

Gibberellin A(1) (GA(1)) was identified by combined gas chromatographymass spectrometry as the major biologically active gibberellin (GA) in seeds of wild oat (Avena fatua L.) regardless of the depth of dormany or stage of imbibition. Both unimbibed dormant and nondromant seeds contained similar amounts of GA(1) as estimated by the d5-maize bioassay. During imbibition, the level of GA(1) declined in both dormant and non-dormant seeds, although the decline was more rapid in dormant seeds. Only in imbibing nondormant seeds did the GA biosynthesis inhibitor, 2-chloroethyltrimethyl ammonium chloride (CCC), cause a reduction in the level of GA(1) from that observed in control seeds. These results are interpreted as an indication that while afterripening does not cause a direct change in the levels of GAs during dry storage, it does induce a greater capacity for GA biosynthesis during imbibition.Nondormant seeds imbibed in the presence of 50 millimolar CCC germinated equally as well as untreated seeds. When wild oat plants were fed CCC throughout the entire life cycle, viable seeds were produced that lacked detectable GA-like substances. These seeds afterripened at a slightly slower rate than the controls. Moreover, completely afterripened (nondormant) seeds from plants fed CCC continuously contained no detectable GA-like substances, and when these seeds germinated, dwarf seedlings were produced, indicating GA biosynthesis was inhibited during and after germination. In total, these results suggest that the increased capacity for GA biosynthesis observed in imbibing nondormant seeds is not a necessary prerequisite for germination. It is therefore possible that GA biosynthesis in imbibing nondormant seeds is one of many coordinated biochemical events that occur during germination rather than an initiator of the processes leading to germination.

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.

Gibberellins and heterosis in maize : I. Endogenous gibberellin-like substances

Under controlled environment and/or field conditions, vegetative growth (height, internode length, leaf area, shoot dry weight, grain yield) was greater in an F(1) maize hybrid than in either parental inbred. Endogenous gibberellin (GA)-like substances in apical meristem cylinders were also higher in the hybrid than in either inbred, both on a per plant and per gram dry weight basis. There were no apparent qualitative differences in GA-like substances, however. Levels of GA-like substances in all genotypes were highest prior to tassel initiation. Chromatographic comparisons of the GA-like substances and authentic standards of GA native to maize on gradient-eluted SiO(2) partition and reverse-phase C(18) high-pressure liquid chromatography columns are described. No consistent differences in abscisic acid levels of the three genotypes were observed. This correlation of heterosis for endogenous GA-like substances with heterosis for growth suggests that amounts of endogenous GA may be related to hybrid vigor in maize.

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

Two maize inbreds, CM7 and CM49, and CM7 x CM49, their F(1) hybrid (which displayed significant heterosis), were examined with regard to response to exogenous gibberellin A(3) (GA(3)), and in their ability to metabolize GA(20), a native GA of maize. The leaf sheath elongation response to GA(3) was far greater for the imbreds than for their hybrid. The inbreds also displayed significant elongation of the leaf blades in response to GA(3), whereas the hybrid was unaffected. Promotion of cell division in the leaf sheath of CM7 and the hybrid was effected by GA(3), but no promotion of cell elongation was observed in CM49, even though significant leaf sheath elongation occurred. Shoot dry weight of both inbreds was significantly increased by GA(3), but response by the hybrid in this parameter was slight and variable. Root dry weight of CM7 was significantly increased by GA(3), but was unchanged in CM49 and the hybrid. Thus, inbred shoot dry weight increases effected by GA(3) were not at the expense of the root system. Rapid metabolism of [2,3-(3)H]GA(20) occurred in all genotypes, although genotypic differences were observed. The hybrid had the highest rates of metabolism to GA glucosyl conjugate-like substances. Oxidative metabolism was also fastest in the hybrid, followed by CM7, and slowest in CM49, the slowest-growing inbred. Thus, rate of GA(20) metabolism is under genetic control in normal (i.e. not dwarfed) maize genotypes. These results, taken together with previous reports that the hybrid has significantly enhanced levels of endogenous GA-like substances, suggest that GA play a role in the expression of heterosis in maize.