Improvement of Heat Stress Tolerance in Maize and Sorghum by Lipid Alterations of the Plastidic Membrane
Maize and sorghum are two of the most produced cereal crops in the world and are major sources of livestock feed, ethanol fuel, food for human consumption and vegetable oil. Heat stress is a prominent and growing agricultural concern in many areas of the world, negatively impacting production and productivity of crops like maize and sorghum. Heat stress in maize is often observed as leaf firing, tassel blast, increased anthesis-silking interval, and premature senescence and/or plant death while both maize and sorghum are susceptible to decreased agronomic performance and lower yields. Heat stress can also be observed on the biochemical level with changes in lipid composition and saturation. There is no known treatment for heat stress for mature maize plants outside of genetic tolerance. This project seeks to understand the genetic and physiological bases of heat stress tolerance in maize and sorghum. A diversity panel of heat tolerant and susceptible maize lines was developed and phenotyped per se and as hybrids with two testers under heat stress and non-heat stress conditions. We believe this is the first reported association panel using any of the cereal plants comparing lipid composition and plant performance under heat stress conditions. Leaf firing and tassel blast traits only show up in heat stress environments, but were not successfully tied to plant performance in this study as realized through changes in plant height or grain yield. Large decreases in galactolipid unsaturation were observed in response to heat stress, representing a significant increase in overall membrane saturation of the plastidic membranes at high temperatures. Significant correlations were not found between saturation levels and plant performance of individual inbred lines or hybrids. Plant performance of the HTAM panel was not strongly associated to galactolipid saturation, but multiple phospholipid species did have strong associations with plant performance and could be involved in conferring heat stress tolerance. A genome-wide association study (GWAS) is a forward genetics tool that has proven to be very useful to identify genetic loci responsible for changes in a given phenotypic trait. Approximately 700 GWAS runs were completed on lipid and field traits to reveal 78 significant trait to SNP correlations across 40 genetic loci in the Heat Tolerant Association Mapping panel. Many genetic loci had statistically significant associations with multiple lipid traits revealing the correlated responses between specific lipid species. Combinations of MGDG(34:4), DGDG(34:4), PC(34:4), and one principal component had significant associations at the same genetic loci in eleven occurrences. Five of the candidate genes coded for glycosyltransferases, potentially involved in lipid metabolism. Mapping studies also identified multiple candidate genes including PsbQ, malate dehydrogenase, and indole-3-acetic acid amido synthetase. A reverse genetics approach was also used to better understand the biochemical roles of specific genes involved in lipid metabolism as identified in the GWAS results and other mapping studies. Maize and sorghum lines with mutations in or near genes directly involved in acyl chain desaturation and MGDG and DGDG headgroup synthesis were identified and then characterized for lipid and phenotype alterations. Maize mutant fad8b was grown under multiple temperatures regimes and displayed increased saturation that may be due to the mutation of interest, but heat stress tolerance did not seem to be affected. All of the maize mutants showed slight increases in saturation within the segregation analyses, but none of the lipid species were statistically different. Maize is known to have multiple copy number of related genes which may make obtaining obvious phenotypic changes in single mutations difficult, but this is less common in sorghum. Most sorghum mutants had little to no statistically significant changes in lipid concentrations and may not represent functional mutations. Sorghum fad6, which has a stop gained mutation in a FAD6, had statistically significant and biologically relevant changes in lipid composition. Increases in overall saturation were realized through decreases in 18:3 and increases in 18:1 acyl chains on multiple lipid species. We believe fad6 is a novel sorghum mutant useful in further studies. Further studies with fad6 may be conducted to determine how these changes in acyl chain saturation impact thermal tolerance of sorghum.
Tuinstra, Purdue University.
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