Molecular mechanism of methionine differentiation in high and low methionine maize lines
Since maize is a primary food stuff for humans and livestock, its amino acid balance is important for proper nutrition. Methionine, an essential amino acid and a primary source of sulfur, is lacking in maize endosperm. Several maize populations were developed through breeding with enhanced methionine content in comparison with normal maize populations. BS31HM (high methionine) and BS31LM (low methionine) maize were among such populations created by the selection from the highest or lowest methionine content population from original BS31 maize. Candidate gene approach was adopted to determine the difference between the two populations at transcript level of the selected genes in the endosperm. The genes selected were mostly expressed in the endosperm and could be involved in enhanced methionine biosynthesis. The selected genes, that is, 15-kDa β-zein, 16-kDa γ-zein, 19-kDa α- zeinB1, 27-kDa γ-zein, 22-kDa α-zein and 18-kDa δ-zein were responsible for coding of endosperm storage proteins when analyzed through RT-PCR. Similarly, expression level relative to the high population (2-ΔΔct) values were also calculated for BS31HM and BS31LM, respectively. These values were found as 1 and 0.25, 1 and 0.07, 1 and 0.10, 1 and 0.15, 1 and 0.33, 1 and 0.43 for 27-kDa γ-zein, 22- kDa α-zein, 18-kDa δ-zein, 15-kDa β-zein, 16-kDa γ-zein and 19-kDa α-zeinB1, respectively, in both populations. The p-values were determined by student’s t-test at confidence level of 95%. The expression of 18-kDa δ-gene, 15-kDa β-gene and 16-kDa γ-gene were found to be significant (p < 0.05) in high methionine maize population when compared with low methionine maize population. Non significant (p > 0.05) differences in the expression level of 27-kDa γ-gene, 22-kDa α-gene and 19-kDa α- gene were observed in both HM and LM maize populations. From these results it can be concluded that all zein genes did not show expression equally in high and low methionine maize populations.
Key words: Maize, methionine, zein, storage protein, amino acid, real time PCR.