Mobilization of StoredFood Reserves
• The food reserves are commonly found as storage forms of
starch, proteins, and fats in cotyledons of dicots and in the
endosperm of monocots.
• Stored reserves are complex, water-insoluble compounds,
their digestion into simple molecules is the first essential
step in their utilization during germination.
• The digestion is facilitated by hydrolytic enzymes that
include proteases, lipases, and amylases.
• The utilizationof starchy endosperm during the germination
of cereal grains are among the most specialized of all
endosperm types in angiosperms, conserving most of their
starchy reserves in the bulky inner cells. One to four layers
of aleurone cells are found surrounding the starchy
endosperm.
• The involvement of the plant hormone gibberellic acid (GA)
in the hydrolysis of endosperm reserves, the highlights of
this work are of particular interest as perspectives on the
mechanism of GA action.
5.
Regulation of α-AmylaseGenes by Gibberellic Acid
• In this process, barley grains imbibe water and begin to
germinate when the starch in the endosperm is hydrolyzed to
sugars. It was noted that the cleavage of the starch molecule
involves the action of hydrolytic enzymes, primarily α-
amylase, which are secreted into the starchy endosperm.
• The source of the α-amylase was traced to the cells of the
aleurone layers. However, if the embryo is removed from the
grain, which is then imbibed, the aleurone cells fail to make
significant amounts of the enzymes, including alpha-amylase.
6.
• By theearly 1960s, it became known that GA produced by the
embryo during imbibition of the grain is the decisive factor
that diffuses into the aleurone cells to stimulate the formation
of α-amylase and the breakdown of starch.
• The products of starch hydrolysis are translocated to the
embryo to promote its germinative growth.
• Experiments done during this period to clarify the role of GA
in the barley system was the demonstration that embryoless
half grains respond to low concentrations of the hormone and
cause hydrolysis of the starchy endosperm just as rapidly as
when the embryo is present.
8.
• Pilner andVarner, (1967) observed that the induction of α-amylase by
GA in the aleurone cells is the result of de novo synthesis and not due
to the release of an active form of an inactive precursor.
• This result was obtained by labeling the enzyme newly synthesized by
the aleurone cells in the presence of GA with heavy oxygen, 18O, and
separating it by isopycnic equilibrium centrifugation from the 16O-
labeled enzyme already synthesized by the cells before exposure to
the hormone.
• Another observation that has added to our understanding of the
complexity of the system is that several isozymes of α-amylase, which
separate neatly into a high isoelectric point (type B or high-pI) group
and a low isoelectric point (type A or low-pI) group are produced by
the aleurone cells in response to the hormone.
9.
• The actionof GA on the aleurone cells of germinating barley
grain also includes the de novo synthesis of a host of other
hydrolytic enzymes including proteases, glucanases,
phosphatases, deoxyribonuclease (DNase), and ribonuclease
(RNase).
• The ability of GA to trigger the production of hydrolytic
enzymes during the germination of barley grain raises the
question as to whether the enzymes are encoded by newly
synthesized mRNAs.
• A variety of experiments have pointed to the conclusion that
GA stimulation of α-amylase activity is due to the production
of new mRNA.
10.
• Varner andChandra (1964) showed that the synthesis and
release of the enzyme in the aleurone cells is inhibited if
actinomycin D is present during the first few hours of GA
action.
• Incorporation of the inhibitor into the medium about 7
hours after GA addition has little effect on α-amylase
formation, whereas a protein synthesis inhibitor is still
highly effective in inhibiting enzyme synthesis.
• Studies on rice aleurone cells have shown that protein
factors from GA-treated tissues bind to a specific region of
the 5'-upstream promoter of a rice α-amylase gene
11.
• The responseof plant cells to hormonal and environmental stimuli
was framed in the context of signal transduction pathways
involving a second messenger.
• The ubiquitous divalent cation Ca2 + has held the center stage as a
second messenger in the signal transduction pathway involving
hormones.
• There is some evidence implicating Ca2 + and the calcium binding
regulatory protein calmodulin in the GA induced secretion of α-
amylase by barley aleurone cells.
• Gilroy and Jones (1992) showed that not only does GA increase the
level of cytosolic Ca2 + in aleurone cell protoplasts, but that this
increase precedes the effects of the hormone on enzyme synthesis.
12.
• Other effectsof GA on the aleurone cells are the elevation of
uptake of Ca2+ into the endoplasmic reticulum (ER) and an
increase in calmodulin levels.
• The expression of genes encoding a-amylase sets in motion a
series of events leading to the mobilization of the enzyme and
the organization of the protein export machinery of the
aleurone cells. As is the case for many secreted proteins, a role
for the ER as the site of synthesis of α-amylase and for the
vesicles of the Golgi apparatus in the transport of the protein to
the plasma membrane has been accepted.
