Seedling Screening for Lysine‐Plus‐Threonine Resistant Maize1

Abstract

Over 200 maize (Zea mays L.) strains were evaluated for seedling growth on lysine‐plus‐threonine supplemented media in an attempt to find feedback resistant mutants. Five of 92 inbreds, most of which were developed for use in hybrid production, were resistant (i.e., root length on lysine + threonine medium exceeded 50% of control). Resistant inbreds B37 and B76 were from the Iowa Stiff Stalk Synthetic (BSSS) population. Nine of 103 random line isolates from the BSSS population were resistant. From 16 of the 17 original BSSS component lines tested, only Ill. 12‐E was resistant. Seven broad base maize populations did not yield resistant types.Resistance was expressed only when seedlings were derived from germinating whole kernels. Seedlings derived from dissected embryos of resistant strains were inhibited. Studies of kernel aspartokinase and homoserine dehydrogenase activities indicated that alterations in the feedback regulation of these enzymes were not the basis of the observed lysine + threonine resistance.The opaque‐2 version of B37 was inhibited. This observation and amino acid data led to the tentative hypothesis that resistance is a function of the relative amounts of methionine and lysine (M/L ratio) in the kernel with a high M/L ratio leading to resistance and a low M/L ratio leading to inhibition. All three resistant strains analyzed had a high M/L ratio compared with four inhibited strains. Kernels of one strain, BSSS 53, had approximately 21% more total methionine than the other four inbreds analyzed (two resistant, two inhibited) yet retained the typical dent kernel phenotype. Kernels of the resistant strains also tended to have higher percent protein. Specific approaches are suggested for selecting high methionine or high lysine maize.