Genetic diversity in Zea mays primary root growth rate, global gene transcript profile and proline content in response to osmotic stress and osmotic stress recovery
This thesis is a study of genetic variation in Zea mays primary root traits in response to osmotic stress and stress recovery. Using a set of up to 25 maize inbred genotypes, chosen as representatives of modern, locally adapted inbred lines, genetic variation was tested in the three traits of root growth rate, global transcriptome profile, and proline content. A large-scale hydroponic growth system, developed in this investigation, allowed for concurrent, ongoing, and non-destructive root trait observation. The system yielded repeatable trait information where genotype explained up to 60% of the total variation in observed root growth rates. In response to stress, growth rate was reduced from 80 to 94%; in response to stress removal, growth rate recovered from 34 to 86% of pre-stress growth. Genotypes differed in relative root growth rate performance when compared before, during, and after the osmotic stress, indicative of genotype by growth environment interaction. Genotypes showed from 7.4 to 17.4% of genes that were stress-responsive, or from 6.0 to 17.6% of genes that were recovery-responsive. Genotypes also differed in the percentage of stress-responsive genes whose transcript abundance change was reversed upon recovery, ranging from 49.8 to 73.3%. In addition, the percentage of stress-responsive genes of opposite abundance change due to recovery showed positive correlation with percentage of growth rate restoration upon stress removal. Genetic variation in proline content was observed before, during, and after osmotic stress. Under osmotic stress, proline content was 1.6 to 3.5 times as high as before the stress; proline content observed during osmotic stress underwent a 60 to 90% reduction upon stress removal. Proline content under osmotic stress was positively correlated with root growth rate under osmotic stress. This study is the first to report genetic variation in each of the three traits in a set of elite maize inbreds. Availability of this genetically variable material, along with the large-scale hydroponic system, provides a powerful tool to further investigate the molecular basis of genotype differences in root growth and growth responses, and to further investigate the role of these traits and variation in seedling and plant stress tolerance.