DIFFERENTIALLY AND DEVELOPMENTALLY REGULATED EXPRESSION OF THREE RICE SUCROSE SYNTHASE GENES

¹ Department of Agricultural Chemistry, National Taiwan University No. 1 Roosevelt Road Section 4, Taipei, Taiwan 106, R.O.C.

² Department of Agronomy, National Taiwan University, No. 1 Roosevelt Road Section 4, Taipei, Taiwan 106, R.O.C.

The spatial and temporal distribution of rice sucrose synthase (RSuS) was studied by immunohistochemical and Western analyses using the monospecific antibodies for three RSuS isoforms. In leaf tissues, RSuS1 was localized in the mesophyll while RSuS2 was in the phloem in addition to the mesophyll. In roots, only RSuS1 was found in the phloem. No RSuS3 could be detected in any parts of etiolated seedlings. The expression of each RSus gene is closely linked to the development of grains. Both RSuS1 and RSuS2 were present in aleurone layers of developing grains, but only slightly in endosperm cells. RSuS3 was localized predominantly in the endosperm cells. Co-expression of three genes in the same compartment resulted in the presence of multiple forms of RSuS but predominant expression of one gene in a specific tissue gave the high proportion of homotetrameric forms in the grain. The specific localizations of the three gene products suggested that RSuS1 plays the role of sugar transport into endosperm cells where RSuS3 catalyzed reaction provides the precursors of starch and cell wall synthesis. RSus2, which is ubiquitously expressed, may play a housekeeping role.
This work was supported by grants from the National Science Council, ROC, to AYW, PDL and JCS.