Inheritance and Genetic Mapping of the Reduced Height (Rht18) Gene in Wheat

Short-statured plants revolutionized agriculture during the 1960s due to their ability to resist lodging, increased their response to fertilizers, and improved partitioning of assimilates which led to yield gains. Of more than 21 reduced-height (Rht) genes reported in wheat, only three—Rht-B1b, Rht-D1b, and Rht8—were extensively used in wheat breeding programs. The remaining reduced height mutants have not been utilized in breeding programs due to the lack of characterization. In the present study, we determined the inheritance of Rht18 and developed a genetic linkage map of the region containing Rht18. The height distribution of the F2 population was skewed towards the mutant parent, indicating that the dwarf allele (Rht18) is semi-dominant over the tall allele (rht18). Rht18 was mapped on chromosome 6A between markers barc146 and cfd190 with a genetic distance of 26.2 and 17.3 cM, respectively. In addition to plant height, agronomically important traits, like awns and tiller numbers, were also studied in the bi-parental population. Although the average tiller number was very similar in both parents, the F2 population displayed a normal distribution for tiller number with the majority of plants having phenotype similar to the parents. Transgressive segregation was observed for plant height and tiller number in F2 population. This study enabled us to select a semi-dwarf line with superior agronomic characteristics that could be utilized in a breeding program. The identification of SSRs associated with Rht18 may improve breeders’ effectiveness in selecting desired semi-dwarf lines for developing new wheat cultivars.

Ppd1, Vrn1, ALMT1 and Rht genes and their effects on grain yield in lower rainfall environments in southern Australia

Allele-specific markers for important genes can improve the efficiency of plant breeding. Their value can be enhanced if effects of the alleles for important traits can be estimated in identifiable types of environment. Provided potential bias can be minimised, large, unbalanced, datasets from previous plant-breeding and agronomic research can be used. Reliable, allele-specific markers are now available for the phenology genes Ppd-D1, Vrn-A1, Vrn-B1 and Vrn-D1, the aluminium-tolerance gene TaALMT1, and the plant-stature genes Rht-B1 and Rht-D1. We used a set of 208 experiments with growing-season rainfall of <347 mm from southern Australia to estimate the effects of seven frequent combinations of the phenology genes, an intolerant and a tolerant allele of TaALMT1, and two semi-dwarf combinations Rht-B1b + Rht-D1a (Rht-ba) and Rht-B1a + Rht-D1b (Rht-ab) on grain yield in lower rainfall, Mediterranean-type environments in southern Australia. There were 775 lines in our analyses and a relationship matrix was used to minimise bias. Differences among the phenology genes were small, but the spring allele Vrn-B1a might be desirable. The tolerant allele, TaALMT1-V, was advantageous in locations with alkaline soils, possibly because of toxic levels of aluminium ions in subsoils. The advantage of TaALMT1-V is likely to be highest when mean maximum temperatures in spring are high. Rht-ab (Rht2 semi-dwarf) was also advantageous in environments with high mean maximum temperatures in spring, suggesting that for these stress environments, the combination of Vrn-B1a plus TaALMT1-V plus Rht-ab should be desirable. Many successful cultivars carry this combination.

Seedling growth under osmotic stress and agronomic traits in Bulgarian semi-dwarf wheat: comparison of genotypes with Rht8 and/or Rht-B1 genes

Early-season drought is an adverse environmental factor affecting seedling emergence and crop establishment in winter wheat grown in continental climates. The different semi-dwarfing (Rht, reduced height) genes exert various effects on seedling growth, plant height and productivity. The occurrence of the major Rht genes in 69 Bulgarian bread wheat cultivars was postulated by molecular markers. Cultivars carrying Rht-D1b+Rht8 (1), Rht-B1b/d (6), Rht-B1b/d+Rht8 (19), or Rht8 (43) were identified. The three latter groups were compared in respect to germination and seedling growth in response to polyethylene glycol-induced osmotic stress, plant height and main yield components. Cultivars carrying only Rht8 produced longer roots, coleoptiles and shoots, and had smaller root-to-shoot length ratio in non-stress and in osmotic stress conditions compared with those carrying Rht-B1b/d or Rht-B1b/d+Rht8. The same cultivars produced shorter culms, shorter spikes with fewer spikelets, reduced number and mass of grain in the main spike, and reduced 50-grain mass. The Rht genic effects were confirmed on Rht near-isogenic lines. The preferential semi-dwarfing allele selection aiming to combine the advantages of Rht8 and Rht-B1b/d alleles in specific eco-climatic conditions and to improve breeding efficiency in wheat is discussed.

Plant Height Affects Fusarium Crown Rot Severity in Wheat

Effects of plant height on Fusarium crown rot (FCR) disease severity were investigated using 12 pairs of near-isogenic lines (NILs) for six different reduced height (Rht) genes in wheat. The dwarf isolines all gave better FCR resistance when compared with their respective tall counterparts, although the Rht genes involved in these NILs are located on several different chromosomes. Treating plants with exogenous gibberellin increased FCR severity as well as seedling lengths in all of the isolines tested. Analysis of the expression of several defense genes with known correlation with resistance to FCR pathogens between the Rht isolines following FCR inoculation indicated that the better resistance of the dwarf isolines was not due to enhanced defense gene induction. These results suggested that the difference in FCR severity between the tall and dwarf isolines is likely due to their height difference per se or to some physiological and structural consequences of reduced height. Thus, caution should be taken when considering to exploit any FCR locus located near a height gene.