Suppressive mechanism of seed coat pigmentation in yellow soybean

Abstract

In soybean seeds, numerous variations in colors and pigmentation patterns exist, most of which are observed in the seed coat. Patterns of seed coat pigmentation are determined by four alleles (I, i(i), i(k) and i) of the classically defined I locus, which controls the spatial distribution of anthocyanins and proanthocyanidins in the seed coat. Most commercial soybean cultivars produce yellow seeds with yellow cotyledons and nonpigmented seed coats, which are important traits of high-quality seeds. Plants carrying the I or i(i) allele show complete inhibition of pigmentation in the seed coat or pigmentation only in the hilum, respectively, resulting in a yellow seed phenotype. Classical genetic analyses of the I locus were performed in the 1920s and 1930s but, until recently, the molecular mechanism by which the I locus regulated seed coat pigmentation remained unclear. In this review, we provide an overview of the molecular suppressive mechanism of seed coat pigmentation in yellow soybean, with the main focus on the effect of the I allele. In addition, we discuss seed coat pigmentation phenomena in yellow soybean and their relationship to inhibition of I allele action.

Keywords: CHS genes; RNA silencing; dsRNA; pigmentation; seed coat; siRNA; soybean.

Fig. 1

Soybean seed pigmentation patterns determined by the four alleles at the I locus (from left to right): I, light (non-pigmented) hilum; iⁱ, dark (pigmented) hilum; i^k, saddle-shaped pigmented region; i, full pigmentation.

Fig. 2

Schematic representation of the structure of GmIRCHS and GmASCHS. A 5′-flanking region, exon1 and intron of GmJ1 are represented by a gray line, gray box and stippled box, respectively. A pseudoCHS3 and its 3′-downstream region are indicated by an open box and a broad black line, respectively. The position and relative orientations of a pseudoCHS3 inverted repeat are indicated by black horizontal arrows. The 78 bp distance between a pseudoCHS3 (1087 bp) and its complementary sequence (1087 bp), which comprise the inverted repeat of pseudoCHS3, is indicated. The 16-bp unique sequence is denoted by a thin black line.

Fig. 3

Model for induction and maintenance of RNA silencing of CHS genes in the seed coat of yellow soybean. The 5′-monophosphorylated ends and modified 3′-ends of CHS siRNAs are indicated by a circled ‘P’ and star, respectively. A: production of primary CHS siRNAs from GmIRCHS transcripts. B: guidance of RISC to CHS mRNAs and cleavage of CHS mRNAs. C: CHS dsRNA synthesis and production of secondary CHS siRNAs. D: degradation of cleaved CHS mRNAs and reduction of CHS mRNA level.

Fig. 4

Pigmented seeds produced by the yellow soybean (cv. Toyohomare). Left to right: yellow seeds with light hilum, mottled seeds induced by Soybean mosaic virus infection, fully pigmented seeds produced by recessive mutation of allele I to allele i, and discoloration induced by low-temperature treatment.