The starch stored temporarily in the leaf sheath of rice plants is
translocated to the grain, contributing to the grain yield. In this paper, the
relationship between starch accumulation and the activities of enzymes
involved in the starch biosynthetic pathway in the leaf sheaths of rice plants
was examined to elucidate the regulation mechanism of starch accumulation in
the temporary sink organ. When the starch content was compared between
different leaf sheath positions on the main stem, the 14th leaf sheath counted
from the bottom, which elongated just before anthesis, showed about a
four-fold higher value than the 10th leaf sheath. Among the enzymes involved
in carbohydrate metabolism, the activity of starch branching enzyme (BE,
EC2.4.1.18) was greatly higher in the 14th leaf sheath than in the 10th leaf
sheath, while that of ADPglucose pyrophosphorylase (EC2.7.7.27) was similar
between the two leaf sheaths. The starch content increased rapidly in the
period around anthesis in the 12th and 14th leaf sheaths, but did not in the
10th and 11th leaf sheath. In the higher leaf sheath position, the activity of
BE changed with noticeably similar trend to the starch content. Soluble starch
synthase (SSS, EC2.4.1.21), granule-bound starch synthase (GBSS, EC2.4.1.21)
and plastidial fructose- 1,6-bisphosphatase (FBPase, EC3.1.3.11) were also
significantly correlated with the starch content. These results suggest that
BE is involved in regulation of starch metabolism, possibly in collaboration
with other enzymes such as SSS, GBSS and plastidial FBPase in temporary sink
organs like the leaf sheath.
On the mechanism of starch accumulation in tissues surrounding spots in leaves of rice plants due to the attack of Cochliobolus miyabeanus
