Investigating the Genetics of Lateral Branching in Barley

Lateral branches, called tillers, in barley directly contribute to grain yield. Precisely modifying the number and quality of tillers may improve plant architecture and yield. However, a better understanding of the genetics underlying tillering is necessary to achieve this optimization. Therefore, the natural genetic diversity of the USDA-ARS National Small Grains Core Collection (NSGCC), which contains cultivars, landraces, and breeding lines from all over the world, was exploited to understand how tillering traits relate to other traits and to identify associated quantitative trait loci (QTL) through genome-wide association mapping. A subset of the most genetically diverse spring two-row and six-row lines from the NSGCC was grown in the field in Saint Paul, MN in 2014 and 2015. Data for numerous traits were obtained including: weekly average tillers, average productive tillers, days to heading, height, and other yield-related traits. In 2014, height and days to heading were both weakly, positively correlated with average productive tillers. Tiller number was strongly influenced by environment as indicated by drastically different tillering capacity between 2014 and 2015 and low heritability estimates.  As expected, on average two-row lines tillered more than six-row lines. However, one quarter of the six-row lines tillered more than the two-row average, and some six-row lines were able to reach the highest tillering capacity observed in the entire population. Three tillering QTL were detected in 2014: one on chromosome 2H that overlaps the Photoperiod-H1 (Ppd-H1) locus, a major regulator of photoperiod sensitivity; one on 5H; and one on 7H. Four tillering QTL were detected in 2015: three on 3H, one of which overlaps the circadian gene Lux1, and the same one on 5H as in 2014. The most significant heading date QTL detected overlapped Ppd-H1 in 2014 and Lux1 in 2015, indicating that tillering is influenced by genes controlling heading date.