vided further support by showing that a higher auxin/cytokinin ratio induces root formation and that
changing it converts shoot meristem to root meristem and vice versa [35].
To preserve genetic homogeneity, vegetative propagation (cloning) is important in horticulture, flori-
culture, forestry, and in the conventional breeding and/or biotechnology of higher plants. Therefore, basal
treatment with indolebutyric acid (IBA) and synthetic auxin naphthaleneacetic acid (NAA) is commonly
used to induce adventitious root formation in hard-to-root species.
III. GIBBERELLINS
The unique property of gibberellins (GAs)—that of increasing the growth of plants by greatly elongating
the cells—was discovered by Kurosawa [3]. Studying the symptoms of the rice disease “bakanaebyo”
(“foolish seedling disease”), Takahashi et al. [59] observed that the causal pathogen was a soilborne fun-
gus,G. fujikuroi, the sexual or perfect stage of Fusarium moniliforme, which caused infected seedlings
to grow abnormally taller and to fall over due to their spindly stem structure. They observed further that
when a pure culture filterate was sprayed onto rice seedlings, it produced the same abnormal growth. This
suggested that the abnormal growth of the infected seedlings was caused by a soluble substance(s) pro-
duced by the fungus. Other Japanese biologists showed that the excessive growth was not confined to rice
but that the filterate could induce it in many other species. According to Takahashi et al. [59,60], in 1938
Yabuta and Surniki isolated two crystalline active substances from the culture filterates and called them
gibberellin A and B.
Western scientists became interested in gibberellin research in early 1950 and succeeded in isolating
an active principle from G. fujikuroi. The growth-promoting activity of this compound was similar to that
of the GAs isolated by Japanese investigators, but the chemical nature was clearly different. Therefore, it
was named gibberellic acid (GA 3 ) [59]. The concentration of GAs is usually highest in immature seeds,
reaching up to 18 mg/kg fresh weight in Phaseolusspecies [61]. However, it decreases rapidly as the
seeds mature. In general, roots contain higher amounts of GAs than the shoots, and vegetative tissues con-
tain a comparatively low level of GAs, depending on the types of tissues and their stages of development.
A. Chemical Nature
As of 1993, 84 gibberellins were listed, of which 25 are from fungi, 73 from higher plants, and 14 com-
mon to both [7,59]. Among these, 68 are free and 16 are known to occur in conjugated form [60]. All gib-
berellins are acidic diterpenoids having an ent-gibberellane carbon skeleton and are designated GA 1 ,
GA 2 , GA 3 ,.. ., GA 84. They differ from one another mainly in the numbers and positions of substituent
groups on the ring system and in the degree of saturation in the A ring. Free GAs are divided into two
groups: those possessing an ent-gibberellane skeleton (20 carbons) or ent-20 nongibberellane (19 car-
bons) mono-, di-, or tricarboxylic acids. The terms C-20 and C-19 denote compounds that have retained
and lost, respectively, carbon atom 20, and generally, C-19 GAs are more active than C-20 GAs. They are
grouped in either four- or five-ring systems. The fifth ring is the lactone ring attached to ring A, which is
not present in the ent-gibberellane. The carboxyl group at C-7 seems to be essential for biological activ-
ity. They also seem to be rather stable in plants and are readily interconverted to form glycosides by con-
jugating with sugars.
B. Metabolism
Gibberellins are diterpene, belonging to a large group of naturally occurring compounds in plants known
as terpenoids. All terpenoids are basically built up from isoprene units, which are five-carbon (5C)
compounds.
The linking of two units yields a monoterpene (C-10), of three a sesquiterpene (C-15), of four a diterpene
(C-20).
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