Domestication to crop improvement: genetic resources for Sorghum and Saccharum (Andropogoneae)

Abstract

Background: Both sorghum (Sorghum bicolor) and sugarcane (Saccharum officinarum) are members of the Andropogoneae tribe in the Poaceae and are each other's closest relatives amongst cultivated plants. Both are relatively recent domesticates and comparatively little of the genetic potential of these taxa and their wild relatives has been captured by breeding programmes to date. This review assesses the genetic gains made by plant breeders since domestication and the progress in the characterization of genetic resources and their utilization in crop improvement for these two related species.

Genetic resources: The genome of sorghum has recently been sequenced providing a great boost to our knowledge of the evolution of grass genomes and the wealth of diversity within S. bicolor taxa. Molecular analysis of the Sorghum genus has identified close relatives of S. bicolor with novel traits, endosperm structure and composition that may be used to expand the cultivated gene pool. Mutant populations (including TILLING populations) provide a useful addition to genetic resources for this species. Sugarcane is a complex polyploid with a large and variable number of copies of each gene. The wild relatives of sugarcane represent a reservoir of genetic diversity for use in sugarcane improvement. Techniques for quantitative molecular analysis of gene or allele copy number in this genetically complex crop have been developed. SNP discovery and mapping in sugarcane has been advanced by the development of high-throughput techniques for ecoTILLING in sugarcane. Genetic linkage maps of the sugarcane genome are being improved for use in breeding selection. The improvement of both sorghum and sugarcane will be accelerated by the incorporation of more diverse germplasm into the domesticated gene pools using molecular tools and the improved knowledge of these genomes.

Fig. 1.

Time-line displaying the changes in Sorghum nomenclature over time. ¹House et al. (1995); ²Spangler (2003); ³Smith and Frederiksen (2000); ⁴Garber (1950); ⁵Lazarides et al. (1991); ⁶Hodnett et al. (2005), Price et al. (2005a); ⁷Dillon et al. (2007).

Fig. 2.

Growth trial of Sorghum species at seedling stage. Note the broader leaf (far left) of the Eu-sorghum, S. propinquum compared with the Para-Sorghum, Stiposorghum and Heterosorghum species.

Fig. 3.

Variation in Sorghum species seed and caryopsis morphology and size. Letters on the figure denote different species: a–e, S. bicolor caryopsis AusTRCF 322649, 322618, 322620, 322666 and 322611, respectively; f, S. propinquum; g, S. halepense; h, S. macrospermum 322277 seed and caryopsis; i, S. laxiflorum 302503 seed and caryopsis; j, S. grande 302580 seed; k, S. leiocladum 300170 seed and caryopsis; l, S. matarankense 302521 seed and caryopsis; m, S. nitidum 302539 seed; n, S. timorense 302660 seed and caryopsis; o, S. purpureo-sericeum 321134 seed and caryopsis; p, S. versicolor 321126 seed and caryopsis; q, S. amplum 302623 seed and caryopsis; r, S. angustum 302604 seed and caryopsis; s, S. brachypodium 302480 seed and caryopsis; t, S. bulbosum 302646 seed and caryopsis; u, S. ecarinatum 302661 seed; v, S. exstans 302577 seed and caryopsis; w, S. interjectum 302563 seed; x, S. intrans 302390 seed and caryopsis; y, S. plumosum 302489 seed and caryopsis; z, S. stipoideum 302644 seed.

Fig. 4.

The left-hand column shows the variation in the central endosperm and the right-hand column compares the outer layers. (A, B) Representative images of S. bicolor, showing the standard floury and vitreous endosperm, respectively. (C–H) Images from outside the Eu-Sorghums are representative of the variations observed across the species. PB, Protein bodies; M, matrix; S, starch granule; D, indentations left by protein bodies; C, channels; P, pores; CG, small polygonal starch granules forming compound granules.

Fig. 5.

Novel sub-aleurone morphology of the Para-Sorghums and Stiposorghums: (A) the characteristic S. bicolor outer endosperm and pericarp; (B) an increased magnification of the sub-aleurone layer itself; (C) a representative image of the novel morphology found in the undomesticated sorghums; (D) and (E) magnified features of the morphology shown in (C). SA, Sub-aleurone layer; A, aleurone; PB, protein bodies; M, matrix,; S, starch granule; D, indentations left by protein bodies; P, pericarp; CG, small polygonal starch granules forming compound granules.