Genome-wide Association Mapping of Solar Tracking Traits in Sunflower

Plants display subtle and vivid responses to diurnal heterogeneity in the light regime. Solar tracking in sunflower (Helianthus annuus) is a dramatic example, where aerial tissues dynamically change orientation throughout the day, following the position of the sun in the sky from East to West. Recently, studies have revealed that asymmetric stem growth enables the daily pattern in shoot movement. The east side of the stem grows more rapidly during the day, leading to solar tracking, and the west side grows more rapidly during the night, leading to nocturnal reorientation back toward East. The daily cycles of movement slowly ceases with anthesis. We applied a genome-wide association mapping approach to elucidate the genetic basis of solar-tracking traits. The daily shoot movement patterns and the timing of nocturnal reorientation of 288 cultivated sunflower lines grown outdoors were documented using time-lapse photography. At anthesis, the direction of heads with reference to the north was recorded from the plantings at two geographical locations. To associate traits to genetic loci, we fitted ~600k single-nucleotide polymorphisms (SNPs) to traits with mixed linear models accounting for cryptic relationships caused by kinship and PCA as covariates. The top candidate SNPs localized to non-coding sequences upstream of genes involved in light, auxin, and gibberellin signaling. Different SNPs were associated with head orientation at anthesis at different geographical locations, likely suggesting that a complex, environmentally-responsive regulatory network controls this trait. To verify as well as improve the power of genome-wide associations, various additional statistical models were also explored.