- Mobilization of Nutrients
Endosperm is the nutrient storage organ of the seed for the developing embryo. Soon after the axis be-
comes active and root and shoot develop, the nutrient reserve (minerals, fats, starch, and proteins) is mo-
bilized to support the juvenile seedling growth. This supply continues until the root develops the ability
to absorb nutrient ions from the soil and the shoot system begins the photosynthetic process. Large
molecules of proteins, fats, and starch have problems being translocated; therefore, they need to be me-
tabolized to smaller molecules such as amino acids or amides and sugars, which can readily be translo-
cated from source to sink. Gibberellins are known to play a key role in the hydrolysis of starchy en-
dosperm of cereal seeds. In 1960, Yomo in Japan and Paleg in Australia independently observed that GA 3
stimulated the degradation of de-embryoed barley endosperm [60]. A few years later it was demonstrated
that barley embryo axis produces GAs, and it has further been shown by GC/MS as well as by immuno-
chemical methods that grains also contain a large number of GAs [70]. The type of gibberellin is species
specific, but GA 1 and GA 3 are important in barley. The involvement of a living aleurone layer in the
degradation of starchy endosperm in grasses, including barley, was recognized more than a century ago,
but the experimental evidence was not provided until much later.
When isolated aleurone layers are incubated with GA 3 , a large number of enzymes (e.g., -amylase,
protease, ribonuclease, esterase, -1,3-glucanase, acid phosphatase, glucosidase, peroxidase) were se-
creted in the incubation media [70]. Later studies confirmed that GA 3 was required for the de novo syn-
thesis of -amylase,-1,3-glucanase, protease, and ribonuclease. Aleurone layers of barley, wheat, and
wild oat respond to GA 3 by synthesizing hydrolytic enzymes, but most maize cultivars and some culti-
vated oat cultivars do not respond identically. Thus there is considerable genetic variability regarding the
gibberellin response in cereal seeds. However, the role of gibberellins in the mobilization of food is not
as clear in dicots and gymnosperms. The food reserves in these two classes of plants could be starch or
fat, where added GAs may or may not influence degradation.
IV. CYTOKININS
The discovery of cytokinins was an outgrowth of tissue culture research by Skoog and associates. The iso-
lation and identification of kinetin (6-furfurylaminopurine) from aged or autoclaved herring sperm DNA,
and its promotion of cytokinesis (cell division) at concentrations as low as 1 g/L, greatly stimulated re-
search in the field of plant growth and development. Although kinetin does not occur naturally, its dis-
covery greatly supported the concept of the existence of a cell division factor, postulated by Wiesner in
1892.
Haberlandt is generally credited as the pioneer in providing experimental evidence for the hypothet-
ical cell division factor. In 1913 he demonstrated that phloem diffusates could cause cell division in potato
(Solanum tuberosum) parenchyma cells. Later, in 1921, he reported that cell division induced by wound-
ing was prevented if the cut surface was washed and that leaf juice spread over the washed cut surface
would restore it [71]. In the early 1940s, Van Overbeek [71a] observed that coconut milk could sustain
the growth of isolated Daturaembryo. According to Koshizima and Iwamura [72], subsequent work by
Stewart and his associates established that coconut milk markedly stimulated the growth of carrot (Dau-
cus carota) root explants by cell division. They did succeed in isolating the growth-inducing factor, but
it was a mixture rather than a single compound that was effective in their carrot bioassay system. Ac-
cording to Koshizima and Iwamura [72], Skoog and associates, in the late 1940s, using aseptically iso-
lated slabs of nondividing mature stem piths of tobacco plants (var. Wisconsin No. 38), observed cell di-
vision in the presence of vascular tissues. However, the first endogenous cytokinin was isolated from
maize kernels and was named zeatin (Z) [6-(4-hydroxy-3-methyl-2-trans-butanylamino) purine]. Germi-
nating seeds, roots, sap streams, developing fruits, and tumor tissues are rich in cytokinins [73]. There are
25 free cytokinins reported from higher plants [72] and some are active in causing maximum tobacco cal-
lus growth at concentrations as low as 0.004 M.
A. Chemical Nature
All the known endogenous cytokinins are substituted purines attached to the N^6 position of the adenine
ring. These N^6 -substituted adenines can be classified into two groups according to their carbon skeleton
of N^6 substituents: as N^6 -isoprenoid and N^6 -benzyladenine analogues.
PLANT GROWTH HORMONES 511