Differentially and developmentally regulated expression of three rice sucrose synthase genes

The spatial and temporal distribution of sucrose synthase (RSuS) in rice (O ryza sativa L.) was studied by Western and immnnohistochemical analyses usi ng the monospecific antibodies for three RSuS isoforms, In leaf tissues, RS uS1 was localized in the mesophyll: while RSuS2 was in the phloem in additi on to the mesophyll. In the roots, only RSuS1 was found in the phloem, No R SuS3 could be detected in any parts of etiolated seedlings. The expression of each RSus gene is closely Linked to the seed development. RSuS1 was pres ent in the aleurone layers of developing seeds, and at a low level in endos perm cells. RSuS2 was evenly distributed in seed tissues other than the end osperm. RSuS3 was localized predominantly in the endosperm cells. The tissu e specific localizations of the three gene products suggest that RSuS1 play s a role in sugar transport into endosperm cells where the reaction catalyz ed by RSuS3 provides the precursor of starch synthesis. RSus2, which is ubi quitously expressed, may play a housekeeping role.