Pre-harvest Sprouting and Grain Dormancy in Sorghum bicolor: What Have We Learned?

Abstract

The possibility of obtaining sorghum grains with quality to match the standards for a diversity of end-uses is frequently hampered by the susceptibility to pre-harvest sprouting (PHS) displayed by many elite genotypes. For these reasons, obtaining resistance to PHS is considered in sorghum breeding programs, particularly when the crop is expected to approach harvest maturity under rainy or damp conditions prevalence. As in other cereals, the primary cause for sprouting susceptibility is a low dormancy prior to crop harvest; in consequence, most research has focused in understanding the mechanisms through which the duration of dormancy is differentially controlled in genotypes with contrasting sprouting behavior. With this aim two tannin-less, red-grained inbred lines were used as a model system: IS9530 (sprouting resistant) and Redland B2 (sprouting susceptible). Redland B2 grains are able to germinate well before reaching physiological maturity (PM) while IS9530 ones can start to germinate at 40-45 days after pollination, well after PM. Results show that the anticipated dormancy loss displayed by Redland B2 grains is related reduced embryo sensitivity to abscisic acid (ABA) and increased levels of GA upon imbibition. In turn, transcriptional data showed that ABA signal transduction is impaired in Redland B2, which appears to have an impact on GA catabolism, thus affecting the overall GA/ABA balance that regulates germination. QTL analyses were conducted to test whether previous candidate genes were located in a dormancy QTL, but also to identify new genes involved in dormancy. These analyses yielded several dormancy QTL and one of them located in chromosome 9 (qGI-9) was consistently detected even across environments. Fine mapping is already in progress to narrow down the number of candidate genes in qGI-9.

Keywords: Sorghum bicolor; abscisic acid; dormancy QTL; grain sorghum; pre-harvest sprouting; seed dormancy.

FIGURE 1

Germination index for sorghum lines IS9530 and RedlandB2. Caryopses were harvested on different days after pollination (DAP) and incubated for 12 days in water at 25°C (circles with solid lines). Embryos were dissected and incubated at 25°C in distilled water (diamonds) or 50 μM ABA (circles with dashed lines). Each data point is the mean value of two biological replicates (i.e., field plots) each one assessed in triplicate. Bars indicate SE of mean. Time of physiological maturity is indicated with an arrow. Experimental details are described in Rodríguez et al. (2009). Adapted from data first published by Rodríguez et al. (2009) and reproduced by permission of Oxford University Press (doi: 10.1093/aob/mcp184).

FIGURE 2

Genetic linkage map for F2 grain sorghum population, derived from IS9530 and RedlandB2 inbred lines, built with 96 SSRs segregation analysis (Adapted from Cantoro et al. (2016), by permission from Springer, Euphytica, ^®2016). Red bars indicate position of dormancy related QTL as reported by Cantoro et al. (2016). Marker names are shown on the right side of each chromosome and genetic distances between markers (in cM) are indicated on the left side. Positions of Dwarf1, and ABA signaling and GA metabolism candidate genes are shown.