Early-13-hydroxylation pathway

The remainder of this article will provide a brief review of selected details of pathways subsequent to GA] 2 hy > concentrating on the "early-13-hydroxylation" pathway and its role in the control of shoot growth in maize (Zea mays). 

The gibberelllns that comprise this pathway are both unique to higher plants (l.e. not present in the fungus) and widespread among higher plants. The pathway Itself has been shown to be present In maize (Zea mays) (2 12, ) and pea (Plsum sativum) (27, ). This report will consider only the early-13-hydroxylation pathway for maize. 

Figure 6. The early-13-hydroxylation pathway for maize (Zea mays). The dwarf-1 block has been defined chemically. The dwarf-2 and dwarf-3 blocks are based on bloassay data only. The initial steps subsequent to GA] 2 l< hyde are not yet known, hence the alternate routes via 6A 2 6A53-aldehyde.

Probably the most significant conclusion that comes from our genetic and biochemical studies is that must be the only gibberellln in the early-13-hydroxylation pathway that is active per se in the control of shoot elongation in maize. Other members of the pathway are active through their metabolism to GAj. This conclusion is supported by data from three kinds of approaches - bioactivity studies I radiolabeled feeds, and analysis of endogenous GAs in mutant and normal seedlings. 

Phinney, B. O. and C. R. Spray, Plant hormones and the biosynthesis of gibberellins The early-13-hydroxylation pathway leading to GAi, in Biochemistry of the Mevalonic Acid Pathway to Terpenoids (G. H. N. Towers and H. A. Stafford, eds.). Recent Advances in Phytochemistry Vol. 24, 203-218, Plenum Press, New York, 1990. 

Fig. 6. The gibberellin biosynthetic pathway in Zea mays shoots. The position in the pathway controlled by each of the dwarf mutants, dh d2, d2, and d5. is shown by a cross line on the appropriate arrow. This early-13-hydroxylation pathway leads to GA,. the bioactive GA controlling shoot elongation in maize. The absence of G A,. G A-,. and G A.. in the dl mutant, together with the accumulation of GA., [6]. suggest that the dl gene controls a common step leading to GA, and GA-,...

This report describes unpublished work, including collaborative studies with Prof B.O. Phinney and his colleagues at UCLA and with Dr. J.B. Reid and colleagues at the University of Tasmania. These studies, which will be documented elsewhere in full, are concerned with the early 13-hydroxylation pathway — the metabolism of GA20 and the related enzymology and the non-13-hydroxylation pathway — the metabolism of GAg. An attempt is made to draw together the results from which the following conclusions are derived. 

A composite pathway, summarizing the results presented in this report, is shown for the early 13-hydroxylation pathway from GA20 in Fig. 1 and for the non-13-hydroxylation pathway from GAg in Fig. 2. 

In the non-13-hydroxylation pathway, GA9 is analogous to GA20 in the early 13-hydroxylation pathway (Fig. 1). The pathway from GAg to GA7 in the Marah and Malus systems, and the pathway from GA20 to GA3 in maize is different from that in the fungus. This difference has phyllogenetic significance. 

EARLY-13-HYDROXYLATION PATHWAY HIGHER PLANTS ONLY... 

Tt is the purpose of this report to analyze the early-13-hydroxylation pathway in maize. (It is apparently the only pathway in vegetative shoots of maize). In addition, comments will be made on new approaches to the study of the enzymology of this most interesting class of plant hormones, the gibberellins. 

GA36 and GA 3, have been shown to be metabolites from radio-labeled feeds of GA]J+ to the fungus (2 ). Thus the mechanism for the loss of carbon-20 is also unresolved in the early-3-hydroxylation pathway. 

GA] 3 (named GA3g), and the C g gibberellins, GA (named 6A5g), and GA3 (named GA g). Thus they are potential members of an early-12o-hydroxylation pathway analogous in oxidation and hydroxylation pattern to the early-3-hydroxylation pathway alternatively, they may be a series of single step metabolites that branch from an early-3-hydroxylation pathway. Recent and extensive radiolabeled feeds to cell-free systems from pumpkin suggest both possibilities (, ). In this system the conversions are highly... 

Although it may be too early to feel absolutely secure about the functional structural correlations, these studies establish that steroids, depending upon their structure, can guide the NADPH hydrogen generated by the hexose monophosphate shunt into either the hydroxylation pathway through the cytochrome P450 electron transport chain or into biosynthetic pathways. 

EARLY-3-HYDROXYLATION PATHWAY FUNGUS HIGHER PLANTS... 

Figure 8.3 Brassinosteroid biosynthetic pathway. DWF4 and CPD catalyze the 22a- and 23a-hydroxylation reaction, respectively, in both the early and late C-6 oxidation pathways. CYP85 catalyzes the C-6 oxidation of 6-deoxytyphasterol and 6-deoxocastasterone and tentatively the C-6 oxidation of 6-deoxocathasterone and 6-deoxoteasterone (arrows not shown). DDWF1 dark-induced DIP/7-like protein 1 DWF4 DWARF4 CPD CONSTITUTIVE PHOTOMORPHOGENESIS AND DWARF.

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