Endogenous gibberellins in Arabidopsis thaliana and possible steps blocked in the biosynthetic pathways of the semidwarf ga4 and ga5 mutants.

Abstract

Twenty gibberellins (GAs) have been identified in extracts from shoots of the Landsberg erecta line of Arabidopsis thiliana by full-scan gas chromatography-mass spectrometry and Kovats retention indices. Eight of them are members of the early-13-hydroxylation pathway (GA'3, GA44, GA,9, GA17, GA209 GA,, GA29, and GA8), six are members of the early-3-hydroxylation pathway (GA37, GA27, GA36, GA,3, GA4, and GA34), and the remaining six are members of the non-3,13-hydroxylation pathway (GA,2, GA15, GA24, GA25, GA9, and GA51). Seven of these GAs were quantified in the Landsberg erecta line ofArabidopsis and in the semidwarf ga4 and gaS mutants by gas chromatography-selected ion monitoring (SIM) using internal standards. The relative levels of the remaining 13 GAs were compared by the use of ion intensities only. In comparison with the Landsberg erecta line, the ga4 mutant had reduced levels of the 3-hydroxy-and 3,13-dihydroxy-GAs, and it accumulated the 13-hydroxy-GAs, except GA53, and the non-3,13-hydroxy-GAs, except GA,2. The GA4 gene encodes, therefore, a protein with 3(3-hydroxylation activity. The gaS mutant had reduced levels of the C,9-GAs, which indicates that the product of the GAS gene catalyzes the elimination of C-20 at the aldehyde level. The gas mutant also had increased levels of certain C20-GAs, which indicates existence of an additional control, possibly hydroxylation of C-20.The growth-response data, as well as the accumulation of GA9 in thega4 mutant, indicate that GA9 is not active inArabidopsis, but it must be 3,B-hydroxylated to GA4 to become bioactive. It is concluded that the reduced levels of the 3.-hydroxy-GAs, GA, and GA4, are the cause of the semidwarf growth habit of both mutants.The biosynthesis of gibberellins (GAs) after GA12-aldehyde involves a series of oxidative steps that leads to the formation of bioactive GAs (1). Several GA pathways differing in hydroxylation pattern have been detected in different species. In many higher plants, such as maize (2, 3) and pea (4), the early-13-hydroxylation pathway (GA53, GA44, GA19, GA17, GA20, GA29, GA1, and GA8; for structures see Fig. 1) leads to GA1, which is probably the active member of this series in the control of stem growth (2, 5). In addition, there is a parallel non-3,13-hydroxylation pathway (GA12, GA15, GA24, GA25, GA9, and GA51), as found in pea embryos (4).Hydroxylation at the /3,-position may also occur at intermediate levels in this pathway, generating 3-hydroxylated GAs (GA37, GA36, GA13, and GA4), as found in bean embryos (6) and pumpkin endosperm (7). Although a number of cell-free systems for GA conversions have been described (1), purification of GA hydroxylases has turned out to be difficult, because they are low-abundance proteins and very unstable (6,8,9). Consequently, nothing is known at present about the mechanisms by which these enzymes are regulated at the molecular level.In Arabidopsis thaliana, a plant species suitable for molecular genetic studies (10, 11), a number of GA-responsive dwarf mutants have bee...

The Metabolism of Gibberellin A20 to Gibberellin A1 by Tall and Dwarf Mutants of Oryza sativa and Arabidopsis thaliana

“…The presence of low levels of the metabolite suggests that the mutant is leaky. These interpretations are in agreement with the data of Talon et al (1990), which showed that the endogenous level of GAl in ga4 plants is 30% of that of tall plants.…”

“…The level of GA1 in the mutant is less than 60% of that found in tall plants (Kobayashi et al, 1989). The ga5 mutant of Arabidopsis shows a similar growth response to both GA20 and GA, (Zeevaart and Talon, 1992), and the levels of the C19 GAS, GAzo, GAI, GAB, GAs, and G&, in mutant plants are from 7 to 29% of those in tall plants (Talon et al, 1990). The ga4 mutant shows a lower response to GAzo than to CAI (Zeevaart and Talon, 1992), and the level of endogenous GAl in mutant plants is onethird that in tall plants (Talon et al, 1990).…”

“…* Corresuondina author; fax 1-310-825-3177. ga4 mutant (Arabidopsis) the step GAZ0 to GAl (Murakami, 1972;Kobayashi et al, 1989;Talon et al, 1990). Thus, the dx mutant of rice gives a growth response when treated with intermediates early in the pathway, e.g.…”

“…Unpublished metabolic data (Zeevaart and Talon, 1992) indicate the presence of this step in Arabidopsis thaliana (L.) Heynh. GA20, GAl, and GA,, have been shown to be endogenous to rice (Kobayashi et al, 1988) and to Arabidopsis (Talon et al, 1990). These GAS are members of the early-13-hydroxylation pathway for GA biosynthesis, a pathway that is of widespread occurrence in higher plants (Phinney, 1984).…”

Extragenic Suppressors of the Arabidopsis gaiMutation Alter the Dose-Response Relationship of Diverse Gibberellin Responses1

“…2). As expected, wild-type plants showed an effect of GA 3 on growth, ga4 (GA deficient; Talon et al, 1990b) plants exhibited a marked growth response, and gai plants exhibited no response. gai gar2-1 plants, although taller than gai plants, did not respond to exogenous GA.…”