Agriculture Reference
In-Depth Information
The effect of the level of free amino acids on protein
accumulation is quite obvious. Amino acids are the precursors
of storage proteins in the rice grain. Hence, a higher level
of these amino acids will contribute to faster and greater
accumulation of protein in the so called protein bodies (PB),
even if the level of enzymes involved in protein synthesis are
the same in all grains.
The greater capacity for amino acid incorporation in the
developing grain implies a higher concentration of enzymes
involved in protein synthesis.
The low activities of the RNAase and protease and their
lack of relation to the accumulation of protein in the ripening
grain suggests that the equilibrium between synthesis and
degradation is shifted toward the former.
Rice storage proteins (prolamin) that accumulate in PB-1
appear to be synthesized by membrane—bound polysomes
attached to PB-1 or RER and to pass through the membrane into
lumen where they aggregate and are deposited. The proteins
(glutelin and globulin) that accumulate in PB-II, however, seem
to be synthesized by membrane—bound polysomes as a large
precursor and to become sequestered into the cisternal space
of RER, from where they are transferred to vacuolar precursor
of PB-II.
In japonica rice varieties, the activities of glutamine
synthetase gradually increased and then declined as a single
peak curve in the course of grain fi lling. The 15th day after
heading was a turning point, before which the enzymatic
activities in the inferior rice varieties with high protein content
were higher than those in the superior rice varieties with low
protein content and after which it is reversed. It was correlated
with the protein content of rice grain and set back positively
