Detection of amino acid variants

--Deborah A. Muenchrath and Ronald L. Phillips

Lysine, methionine, and threonine are synthesized in a common branched pathway, the aspartate pathway. The enzymes in the pathway are regulated by allosteric feedback inhibition by the end product amino acids. Exogenously added lysine and threonine (LT) synergistically inhibit root growth of germinating kernels. Phillips et al. (Crop Sci. 21:601, 1981) screened over 200 maize lines for root growth on medium supplemented with equimolar amounts of lysine plus threonine. Relative inhibition was determined by the ratio of root lengths of seedlings grown on LT medium (Treatment) to root lengths of seedlings grown on basal medium (Control), the T/C ratio. In the 1981 study, the T/C of B37 opaque-2 was less than the inbred B37 T/C. Whole kernel amino acid analyses indicated the o2 version had a lower ratio of methionine to lysine (M/L), and to lysine-plus-threonine, than the wildtype. A relationship between the response to LT medium (T/C) and the M/L ratio was hypothesized. Based on this relationship, a screening system was proposed to detect high lysine amino acid variants by simply screening lines on LT-supplemented medium and searching for lines with low T/C ratios.

The objective of the current study is to test the hypothesis that the LT response is a function of the M/L ratio; i.e., high M/L results in resistance and low M/L leads to inhibition of root growth on LT medium. Nine B37 endosperm mutants, provided by F. Salamini, were evaluated for LT response and amino acid composition. The nine genotypes were tested in two groups to permit timely handling of the materials. The T/C and M/L data for each group are presented in Table 1.

A highly significant correlation between M/L and T/C (r=0.98) exists for group 1. A similar trend is observed for group 2 (Figure 1); however, the T/Cs of sugary-2 and the o2:su2 double mutant are reduced. Without the su2 lines, the correlation coefficient between M/L and T/C in group 2 is increased from 0.63 to 0.97, consistent with the correlation for group 1. The reduced T/C of the su2 mutants may be due to linkage of su2 with a locus affecting seedling vigor, or related to endosperm starch or protein composition. Other endosperm mutants will be evaluated. Other inbred lines and their o2 versions have been tested; in each case, the T/C and M/L are reduced in the o2 version. Based on the correlation between M/L and T/C, seedling screening on LT-supplemented medium may effectively detect amino acid variants.

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