Genetic and environmental factors affecting grain texture in common wheat

Abstract

Thirteen wheat cultivars grown in six locations were compared for kernel weight, protein content and grain texture, as determined by the Single Kernel Characterization System (SKCS). Moreover, puroindolines a (Pin-A) and b (Pin-B) bound to starch were quantified by densitometric scanning of A-PAGE fractionations. All cultivars shared allele Pina-D1a coding for wild-type Pin-A, and differed from each other in allele composition at Pinb-D1 coding for Pin-B. Cultivars with Pinb-D1a exhibited soft grain and high amounts of Pin-A and Pin-B compared to cultivars with Pinb-D1b or Pinb-D1d. Significant genetic variation for grain hardness and Pin-A level was detected in soft cultivars. The ratio between Pin-A and Pin-B levels in soft cultivars was approximately 6:5, whereas it varied between 9:5 and 10:1 in hard cultivars. Protein content was significantly correlated with Pin-B content (r=0.34) and SKCS value (r=0.36) in soft wheats. Significant correlations (0.68 and 0.73 for soft and hard wheats, respectively) were observed between Pin-A and Pin-B levels. Grain hardness was not correlated with puroindoline levels and Pin-A/Pin-B ratio in both textural classes. By contrast, kernel weight was found to act as a major environmental factor affecting grain texture in both soft and hard wheats.

Introduction

In common wheat (Triticum aestivum L.) the Pina-D1 and Pinb-D1 loci on the short arm of chromosome 5D encode for puroindolines a (Pin-A) and b (Pin-B), respectively (Giroux and Morris, 1997). Cultivars with soft grain possess wild-type a alleles at those loci (Gazza et al., 2005; Morris, 2002), and accumulate Pin-A and Pin-B on the surface of starch granules in the developing endosperm (Blochet et al., 1993; Gautier et al., 1994; Greenwell and Schofield, 1989; Morris et al., 1994). On the other hand, medium-hard and hard wheat cultivars contain mutant alleles at Pinb-D1 (Corona et al., 2001a, Corona et al., 2001b; Giroux and Morris, 1997, Giroux and Morris, 1998; Lillemo and Morris, 2000), and deposit reduced or null amounts of Pin-B on starch granules (Corona et al., 2001a, Corona et al., 2001b), whereas most wheats with extra-hard grain texture lack Pin-A due to the presence of a null allele at Pina-D1 (Gazza et al., 2005; Morris, 2002). Interestingly, the presence of a null Pin-A allele was found to drastically reduce the amount of wild-type Pin-B bound to starch granules as well (Corona et al., 2001a; Pogna et al., 2002), suggesting that deposition of Pin-B on starch granules in developing kernels requires the presence of Pin-A (Corona et al., 2001a; Giroux and Morris, 1998). More recently, the synergic effects of wild-type Pin-A and Pin-B on kernel hardness have been demonstrated in wheat lines transformed with both Pina-D1a and Pinb-D1a sequences, the softening effects of these alleles being correlated with accumulation of puroindolines on starch granules (Hogg et al., 2004).

The lipid-binding properties of puroindolines are likely involved in their interaction with the amyloplast membranes, and are also assumed to account for the permeabilizing properties of puroindolines on bacterial and fungal membranes (Blochet et al., 1993; Jing et al., 2003; Krishnamurthy et al., 2001). In addition, the affinity of puroindolines for polar lipid is claimed to be responsible for their effects on crumb structure of bread and rheological properties of wheat dough (Dubreil et al., 1997, Dubreil et al., 1998; Gautier et al., 1994; Igrejas et al., 2001).

Genes other than Pina-D1 and Pinb-D1 were shown to contribute to variation in kernel texture, their effects being very small relative to the puroindoline genes. In particular, quantitative trait loci (QTL) with single-factor effects on kernel texture occur on chromosomes 1A, 1B, 1D, 2A, 2B, 2D, 3A, 3B, 3D, 4B, 5AL, 5B, 5DL, 6B and 6D (Breseghello et al., 2005; Campbell et al., 1999; Galande et al., 2001; Igrejas et al., 2002; Perretant et al., 2000; Sourdille et al., 1996; Turner et al., 2004), whereas three other QTLs with interaction effects are located on chromosomes 5A, 6D and 7A (Sourdille et al., 1996). Additional factors correlated with hardness are protein content (Bushuk, 1998; Giroux et al., 2000; Igrejas et al., 2002; Martin et al., 2001; Slaughter et al., 1992), hectolitre weight (Pomeranz and Williams, 1990) and kernel weight (Giroux et al., 2000; Martin et al., 2001; Pomeranz and Williams, 1990).

Kernel hardness is a main determinant in end product quality because of its strong effects on milling conditions, granularity of flour and starch granule integrity. Here this important characteristic of wheat grain is analysed in 13 common wheat cultivars grown at seven locations in a field trial with three replications. Analysis of variance (ANOVA) was used to assess the effects that Pina-D1 and Pinb-D1 loci, puroindoline deposition on starch granules, protein content and kernel weight have on kernel texture, as determined by the Single Kernel Characterization System (SKCS).