Handbook of Plant and Crop Physiology

(Steven Felgate) #1

Our knowledge regarding the biosynthesis of GA in plants and crops stems from feeding^14 C-labeled
acetic acid and mevalonate to G. fujikuroithrough the culture medium. It was observed that^14 C from
these two compounds was incorporated into gibberellic acid (GA 3 ) [59]. Graebe et al. [62], using a cell-
free system [the endosperm nucellus of wild cucumber (Marah macrocarpus; formerly Echinocystis
macrocarpaGreene)], reported incorporation of [^14 C]mevalonic acid into kaurene, kaurenol, and ger-
anylgeraniol. The pathway commonly accepted is 3-acetyl-CoA →mevalonic acid →isopentenyl py-
rophosphate (a five-carbon terpenoid) →geranylgeranyl pyrophosphate (a 20-carbon compound) →co-
palyl pyrophosphate →kaurene→kaurenol→kaurenal→kaurenoic acid →GA 12 -aldehyde. The
GA 12 -aldehyde is a branch point to the formation of various GAs. Pathways to various GAs differ mainly
in the position and sequence of hydroxylation, and more than one pathway can operate in the same plant.
The details of the pathways are covered comprehensively in several excellent works [59,60].
The metabolism of GAs in plant tissue is not well understood, and very meager information exists
regarding its eventual fate. There is evidence that considerable interconversion of gibberellins (i.e., one
GA can be converted to another GA) takes place in the plant. Immature seeds from “summer”-grown
Pisum sativumwere fed with GA 9 , which was metabolized to GA 51 and dihydro-GA 31 and its conjugate.
But in “winter”-grown seeds, the metabolites were GA 20 and GA 51. Another metabolite, gibberellethione,
was isolated from immature seeds of Pharbitis nil[60]. Degradation of commonly used GA 3 appears to
be slow. However, during the active growth phase, most of the gibberellins are metabolized to inactive
forms by hydroxylation or by conjugation with glucose to form glucosides.


C. Transport


Gibberellins are known to be synthesized in all young, actively growing organs, vegetative or reproduc-
tive, including immature and mature seeds. Understanding their transport within the plant pertains pri-
marily to work with excised coleoptile, stem, or petiole segments in a donor-tissue-receiver system.
Transport has generally been observed to be nonpolar, but occasionally, basipolar movement has been re-
ported [28] with a velocity up to 1 mm/hr. However, information regarding endogenous movement is
rather indirect. It has been noted to occur in the phloem by the same mechanism and in a pattern similar
to that with which other assimilates move. Gibberellins have been isolated from phloem sieve tube saps
as well as from the xylem stream. Experimental evidence using^14 C-labeled GA shows an interchange be-
tween phloem and xylem [17]. This suggests that GA is transported both symplastically and apoplasti-
cally. Its phloem transport rate was similar to that of other assimilates. In analogy with the source-to-sink
movement of assimilates in phloem, perhaps the polar movement observed was to a growth center rather
than to the morphological base.


D. Biological Activity


The physiological properties of these highly active compounds are wide ranging, but extensive growth
and de novo enzyme synthesis are the most significant. The GAs act synergistically with other hormones
in what might be called a system approach. The best known response is the stimulation of internode
growth of dwarf maize, pea, and bush bean, which after treatment with GA attains the normal height. In
some cases, but not all, dwarfism does in fact seem to be correlated with endogenous GA deficiency. The
most detailed analysis, at the molecular level, has been done with dwarf mutant of maize, known as dwarf-
5 (d 5 ) [59,60]. The height of the dwarf-5 mutant is about one fifth that of its parent, due to a single gene
mutation causing a deficiency of GA. When treated with GA, the mutant attains the height of normal
maize. The action of many GAs is similar to that of IAA, including cell elongation, promotion of cambial
activity, induction of parthenocarpy, and stimulation of nucleic acid and protein synthesis. The GAs vary
greatly in their biological activity, and GA 3 and GA 7 are considered to have the widest range. In ferns, al-
gae, and fungi, GAs have also been shown to influence growth and development.
The content of GAs varies depending on the types of tissues and their stages of growth. Tissues other
than seeds usually contain very low amounts (e.g., 0.3 g/kg in young bamboo shoots) [60]. Roots are con-
sidered to be the richest source of GAs, and most GAs, as such or in bound form, are supplied to the shoot.



  1. Bioassay


A number of bioassays, such as elongation of dwarf maize, pea, and rice seedlings and of lettuce (Lac-
tuca sativa) hypocotyl, Avenaleaf segments, and chlorophyll retention in Rumexleaf disks [63,64], have


PLANT GROWTH HORMONES 509

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