Deciphering the Genetic Basis of Solar Tracking By Genome-Wide Association Mapping in Sunflower

Plants have evolved to adjust their growth and development in response to various external cues. Growth patterns are often synchronized to daily variations in sunlight, temperature and other factors. Solar tracking in sunflower (Helianthus annuus) is a very intriguing example, wherein during the day the shoot apex orients from east to west following the sun and at night re-orients back to east in anticipation of dawn. This daily movement ceases post-anthesis, and the mature heads maintain an eastward orientation.  To identify genetic loci involved in solar tracking traits, we took a natural variation approach and conducted a genome-wide association study involving a panel of 289 diverse sunflower accessions. Plants were grown in field conditions, solar tracking was recorded with time-lapse videography, the orientation of mature sunflower heads with reference to geographical north was scored at anthesis, and several other growth- and inflorescence-related traits were also measured. Although quantification of many traits is ongoing, we have performed preliminary association analyses for several phenotypes with existing dataset ~5.8k single nucleotide polymorphisms (SNPs) genotyped with an Illumina Infinium SNP array and using a mixed linear model accounting for population structure in TASSEL. Peaks of SNPs significantly associated with variation in mature head orientation and shoot growth rate were identified on multiple linkage groups, indicating that these traits have polygenic genetic architectures and highlighting several noteworthy candidate genes. We anticipate that forthcoming genome resequencing data with more SNPs will improve and refine our efforts to discover functional variants impacting this fascinating, agronomically-relevant growth pattern.