Contribution of granule bound starch synthase in kernel modification of quality protein maize
The role of gbssI and gbssII genes, encoding granule bound starch synthase enzyme I and II, respectively, in quality protein maize (QPM) were studied at different days after pollination (DAP). Total RNA was used for first strand cDNA synthesis using the ImpromIISriptTM reverse transcriptase. No detectable levels of gbssI were observed. The results showed a gradual increase in expression of gbssII from 6 to 30 DAPS and gbss expression was higher in non-QPM than QPM material. Modification level 1 and 2 had lower expression levels than level 4. In general, grain endosperm opaqueness modification is rated on a scale of 1(completely hard / vitreous) to 5 (soft opaque). Overall, this study showed that gbss activity, based on its isoform gbssII was high at 30 DAP when maximum accumulation of amylose occurs. It was generally compromised in QPM than in non-QPM materials and appeared to be influenced by opaque donor parent. Protein analysis revealed a mar ked increase of GBSS in both QPM and non QPM. There was a higher proportion of GBSSII than I extracted from developing endosperm in both QPM and non QPM. At 18 DAP, the amounts of extractable GBSSII and I were lower in modified maize line 4 comparable to amounts in level 1 modified germplasm. In contrast, at 30 DAP; the GBSSII and I proteinquantities at level 2 were lower than level 4, but comparable to level 3. The major finding was that transcript use was unreliable because the genes under study (gbssI and II) were influenced by other non-allelic interactions. The stability of both GBSS and zein proteins coupled with the use of SDS-PAGE implied that only one was more reliable and required further validation. It was clear from this study, that kernel modification was regulated by complex genetic interactions. Fairly distinct systems such as the starch synthases, zein proteins and perhaps other non-allelic genetic systems all appeared to act in a complementary manner to regulate kernel texture.
Key words: Amylose, protein maize, zein proteins