A comprehensive expression analysis of the starch synthase gene family in rice (Oryza sativa L.)

To elucidate the roles of the isogenes encoding starch synthase (EC 2.4.1.21) in rice (Oryza sativa L.), a comprehensive expression analysis of the gene family was conducted. Extensive searches for starch synthase genes were done in the databases of both the whole genome and full-length cDNAs of rice, and ten genes were revealed to comprise the starch synthase gene family. Multi-sequence alignment analysis of the starch synthase proteins from rice and other plant species suggested that they were grouped into five classes, soluble starch synthase I (SSI), SSII, SSIII, SSIV and granule-bound starch synthase (GBSS). In rice, there was one gene for SSI, three for SSII and two each for SSIII, IV and GBSS. The expression pattern of the ten genes in the developing caryopsis was examined by semi-quantitative RT-PCR analysis. Based on the temporal expression patterns, the ten genes could be divided into three groups: (i) early expressers (SSII-2, III-1, GBSSII), which are expressed in the early stage of grain filling; (ii) late expressers (SSII-3, III-2, GBSSI), which are expressed in the mid to later stage of grain filling; and (iii) steady expressers (SSI, II-1, IV-1, IV-2), which are expressed relatively constantly during grain filling. Within a caryopsis, the three gene groups spatially share their expression, i.e. "early expressers" in the pericarp, the "late expressers" in the endosperm" and the "steady expressers" in both tissues. In addition, this grouping was reflected in the expression pattern of various rice tissues: expression in non-endosperm, endosperm or all tissues examined. The implications in this spatio-temporal work sharing of starch synthesis isogenes are discussed.