Plant Growth Substances - American Chemical Society

3 Gibberellin Biosynthesis in the Fungus Gibberella fujikuroi and in Higher Plants

BERNARD O. PHINNEY

Downloaded by UNIV LAVAL on June 21, 2014 | http://pubs.acs.org Publication Date: September 27, 1979 | doi: 10.1021/bk-1979-0111.ch003

Department of Biology, University of California, Los Angeles, CA 90024

The gibberellins (GAs)1 were o r i g i n a l l y identified as secondary metabolites of the fungus Gibberella fujikuroi Saw Wr. (Fusarium moniliforme Sheld.). While these compounds have no apparent r o l e i n the fungus, they have been found to elicit a variety of responses i n higher plants (seed plants) including shoot elongation, sex expression, f r u i t growth, and seed germination (1,2). The early descriptions of GA-induced elongation, especially those associated with genetic dwarfism ( 3 , 4 ) , led to the idea that GAs might be naturally-occurring i n normal, nondwarf strains of higher plants. The f i r s t evidence for the presence of gibberellin-like substances i n higher plants came from the fact that semipurified extracts from such material would mimic a GA-induced growth response when applied to genetic dwarfs ( 5 , 6 , 7 ) . This evidence was soon followed by the isolation and chemical identification of GAs from higher plants (8,9,10). Since then 53 GAs have been identified as naturally occurring, 22 of them being found i n the fungus G. fujikuroi, and40of them i n higher plants including members of the Gymnospermae (e.g. pines) and the Angiospermae (flowering plants) (1,11). Although gibberellin-like substances have also been obtained from other groups of plants such as algae, other fungi, bacteria, mosses and ferns, these substances have yet to be i d e n t i f i e d chemically as GAs. It seems l i k e l y that GAs w i l l be found to occur universally i n the plant kingdom. A l l GAs are tetracyclic diterpene acids; they can be d i v i ded into two types, the C20~GAs and the C ] q - G A s (Figure l ) . The unraveling of the details of the biosynthetic origin of the GAs has proven to be less d i f f i c u l t than the diversity of structures would suggest. Some of the simplifying factors are (l) that the origin of the GAs i n both the fungus and higher plants i s through a single terpenoid pathway leading to the common GA-precursor, GAi ~ aldehyde ; (2) that 20 of the 22 fungal GAs have now been biosynthetically related to each other v i a two pathways; and (3) that, although kO GAs have been identified from higher plants, less than 15 have been found i n any one 2

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0-8412-0518-3/79/47-lll-057$05.50/0 © 1979 American Chemical Society In Plant Growth Substances; Mandava, N.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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PLANT GROWTH

Figure 1.

SUBSTANCES

Numbering system for ent-kaurene and the GA's (GA ) and structures of ent-kaurene, GA , GA , and GA 12

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In Plant Growth Substances; Mandava, N.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

the


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PHINNEY

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Gibberellin Biosynthesis

species of plant. This review w i l l present an overall picture of GA biosyn thesis i n plants. In addition the role of GA biosynthesis i n relation to GA-induced shoot elongation w i l l be b r i e f l y dis cussed, as w i l l the correlation of levels of endogenous GA-like substances with elongation growth i n higher plants. A l l biosynthetic steps relating GAs to each other and to GA-precursors presented here are based on feeds of radiolabeled GAs and their precursors to plant preparations, including intact plants, tissue sections, and c e l l free homogenates The evidence for the specific steps described here has recently been reviewed i n de t a i l by Hedden et a l . (11) and by Graebe and Ropers (1). The latter also includes a c r i t i c a l evaluation of information on the physiology of the GAs. The details of the biochemistry of polyisoprenoid biosynthesis has recently been reviewed by Beytia and Porter (12) and Goodwin (13). From MVA to GA^-aldehyde i n the Fungus and i n Higher Plants (Figure 2) The earliest steps (MVA to GGPP) for polyisoprenoid biosyn thesis are identical for a l l plants and animals (12,13). They involve the well-known diterpene pathway, MVA — M V A P — M V A P P — I P P + DMAPP — G P P —»> FPP — G G P P . The enzymes catalyz ing these steps have been studied extensively, especially from animals (liver) and yeast, and to a more limited extent from higher plants. In some cases the enzymes have been purified to homogeneity; most have been only p a r t i a l l y purified. In both plants and animals a major branch at FPP leads to the production of squalene and the steroids. In plants, three major branches occur at GGPP, of which one leads to the carotenoids v i a phytoene, a second to the phytyl group of chlorophyll, and a t h i r d to the GAs. The steps i n the pathway from GGPP to GAi2~aldehyde are unique to plants and involve the reactions GGPP — e n t - k a u r e n e — ^ ent-kaurenol — e n t - k a u r e n a l — ^ ent-kaurenoic acid — ^ ent-7cy-hydroxy kaurenoic acid—GA]_2~aldehyde. The two step reaction, GGPP — ^ CPP — ^ ent-kaurene, i s catalyzed by entkaurene synthetase (15). This enzyme or enzyme complex i s responsible for (1) the proton-initiated cyclization to form the A and Β rings of the b i c y c l i c intermediate CPP (A a c t i v i t y ) , and (2) the loss of pyrophosphate, cyclization and rearrangement of the resulting carbonium ion, and loss of H from carbon-17 to produce ent-kaurene (B a c t i v i t y ) . ent-Kaurene i s the f i r s t com mitted intermediate i n the biosynthetic pathway leading to the GAs, and i t has been suggested that the A a c t i v i t y may be a limiting step i n GA biosynthesis ( l 6 ) . The enzymes catalyzing the steps from MVA to ent-kaurene are soluble. After production of ent-kaurene, carbon-19 i s sequentially oxidized to give ent-kaurenol, ent-kaurenal, and ent-kaurenoic +


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P L A N T G R O W T H SUBSTANCES

CPP

é?/7/-KAURENE

Figure 2. The GA hiosynthetic pathway from MVA to GA -aldehyde. This pathway is found in the fungus Gibberella fujikuroi and higher plants 12


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acid. This i s followed by a hydroxylation on carbon-7 to give ent-7a-hydroxykaurenoic acid. There i s some evidence that the enzymes for the pathway from MVA to ent-7a-hydroxykaurenoic acid are present i n subcellular organelles - proplastids and chloroplasts (ώ,Ιδ,ΐΧίΙ^) . characteristics of the enzymes cataly zing the steps between ent-kaurene and ent-7

Plant Growth Substances - American Chemical Society

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