**1. Introduction**

Numerous studies have associated the consumption of high-fiber foods with decreased risk of type 2 diabetes, obesity, and heart disease [1,2]. Consumer awareness of the health benefits of dietary fiber-rich foods has driven product development of high-fiber foods in the baking industry. As a good source of fiber, whole wheat flour has become increasingly popular globally with the number of whole grain products increasing from 164 in 2000 to 3378 in 2011 [3].

Unlike refined white flour, whole wheat flour contains the all the anatomical portions of the grain are together in the same proportions as they occur in the intact kernel [4]. The bran component (including the outer portions of the kernel: pericarp, germ, and aleurone) has a significant impact on whole wheat flour functionality [5–7]. Due to its high fiber concentration, wheat bran binds considerable amounts of water, which interferes with proper gluten hydration and leads to detrimental effects on bread quality [8]. Bran water retention capacity (WRC), which is a measure of how much water the bran will imbibe, varies among genotypes and growing conditions and is correlated with whole grain flour functionality [5,9]. Therefore, WRC is an important trait in the evaluation of bran quality.

As wheat is milled, the bran may fracture into many small particles or remain as larger pieces. The tendency for bran to fracture into smaller pieces is known as friability. Initial research on wheat milling was focused on a reduction in bran friability (ability to retain large particles) to facilitate e fficient separation of white flour from the bran by sifting [10]. However, as whole wheat flour has become more popular, there have been numerous studies showing that bran particle size a ffects whole wheat flour functionality [6,11]. Reported results are somewhat contradictory, but they do show that very large particles are undesirable due to interference with gluten network formation [6]. Furthermore, bran WRC also increases with increased bran particle size [12]. Therefore, for whole wheat flour, it is desirable to have high bran friability.

In recent years, genome-wide association studies (GWAS) have been used extensively to determine the genetic basis of agronomic, physiological, and disease resistance traits in bread wheats [13–33]. Relatively fewer studies have used GWAS to identify loci that control end-use quality traits, including grain protein concentration, flour yield and color, gluten strength, and mixing properties, among others [16,34–41]. However, these studies have focused on end-use quality for refined wheat flour applications. No studies have focused on quality traits related to whole grain end-use quality.

Our previous study showed that among eight traits measured on wheat bran, including phenolic compounds, antioxidant activity, protein, ash, lipoxygenase activity, and free thiol groups, bran particle size and WRC showed the most significant impact on whole wheat bread volume and texture among the other chemical and physical parameters tested [9]. Therefore, the objectives of this study were to: (1) investigate the variation in bran friability and WRC in the Hard Winter Wheat Association Mapping Panel (HWWAMP), a diverse set of 299 winter wheat lines representing germplasm from the hard winter wheat growing regions of the US [42,43]; (2) conduct a genome-wide association study (GWAS) to identify genomic regions associated with bran friability and WRC; and (3) identify underlying genes associated with bran friability and WRC and investigate their annotations to determine potential genes that may be important for these two traits.
