Richard R.-C. Wang

Structure and dynamics of retrotransposons at wheat centromeres and pericentromeres

Little is known of the dynamics of centromeric DNA in polyploid plants. We report the sequences of two centromere-associated bacterial artificial chromosome clones from a Triticum boeoticum library. Both autonomous and non-autonomous wheat centromeric retrotransposons (CRWs) were identified, both being closely associated with the centromeres of wheat. Fiber-fluorescence in situ hybridization and chromatin immunoprecipitation analysis showed that wheat centromeric retrotransposons (CRWs) represent a dominant component of the wheat centromere and are associated with centromere function. CRW copy number showed variation among different genomes: the D genome chromosomes contained fewer copies than either the A or B genome chromosomes. The frequency of lengthy continuous CRW arrays was higher than that in either rice or maize. The dynamics of CRWs and other retrotransposons at centromeric and pericentromeric regions during diploid speciation and polyploidization of wheat and its related species are discussed.

Agropyron and Psathyrostachys

Wheatgrass encompasses five genera, and wildrye species belong to three genera of the tribe Triticeae. These grasses have wide distribution in the temperate regions of the world. Thus, they are highly variable in morphological traits, ecological adaptation, geographic distribution, and genomic constitutions. Consequently, wheatgrass and wildrye grasses possess valuable traits, such as resistance to biotic stresses (diseases and insect pests), tolerance to abiotic stresses (cold, heat, drought, salinity, and toxic minerals, etc.), and nutritional contents (grain proteins). Species of various known genome constitutions (P, St, Ns, E, ESt, StH, StY, StHY, StWY, NsXm, and StHNsXm) in the perennial Triticeae have been successfully crossed with wheat. Some perennial Triticeae grasses were intentionally being used as tertiary genetic resources for wheat improvement via chromosome engineering techniques since 1950s. However, from this review, it is evident that, due to their closer genome relationships with the ABD genomes of wheat, the genomes E and St in the genus Thinopyrum are more likely involved in gene transfers from perennial Triticeae to wheat. Furthermore, linkage drags are limiting the usefulness of many early-day alien gene transfers. With the advent of new molecular techniques to clone targeted desirable functional genes and precision chromosomal manipulation to enhance gene recombination, we may overcome this major problem in utilizing alien genes in wheat breeding. But before that time comes, many tasks need to be done. The preservation and evaluation of the biodiversity in wheatgrass and wildrye species should be an ongoing effort. Molecular research on perennial Triticeae grasses needs to be accelerated to catch up with that on annual cereal crops. New uses of wheatgrasses and wildryes, such as perennial wheat and biofuel crops, should be exploited. The scientific knowledge and tools developed from the fundamental research will benefit both cereal and forage breeding.

Characterization of fructan biosynthesis in big bluegrass (Poa secunda)

Summary Most cool-season grasses contain multiple types of fructans. One exception is big bluegrass (Poa secunda Presl.). When grown under specific controlled environmental conditions it synthesizes only β-2,6-linked fructans. This study analyzed fructan accumulations, enzyme activities and gene expression in big bluegrass. Detachment/illumination and cool treatments effectively induced the accumulation of fructans in leaf tissues. Enzyme assays indicated that 6-SST (sucrose : sucrose 6-fruc tosyltransferase)- and 6-SFT (sucrose : fructan 6-fructosyltransferase)-like activities were the major enzyme activities involved in fructan biosynthesis in big bluegrass leaves. A full-length cDNA of the putative 6-SFT gene was cloned using RT-PCR and RACE techniques. The deduced amino acid sequence showed 69 percnt; identity with barley 6-SFT. Homology was also high with other fructosyltransferases and some invertases. The abundance of putative 6-SFT mRNA showed a coincidence with fructan accumulation and 6-SFT activity. We suggest that 6-SFT is the major enzyme involved in fructan biosynthesis in big bluegrass but it may also exhibit limited 6-SST activity.

Biosystematics and evolutionary relationships of perennial Triticeae species revealed by genomic analyses

Understanding the classification and biosystematics of species in Triticeae Dumort., an economically important tribe in the grass family (Poaceae), is not an easy task, particularly for some perennial species. Does genomic analysis facilitate the understanding of evolutionary relationships of these Triticeae species? We reviewed literature published after 1984 to address questions concerning: (1) genome relationships among the monogenomic diploid species; (2) progenitors of the unknown Y genome in Elymus polyploids, X genome in Thinopyrum intermedium, and Xm genome in Leymus; and (3) genome constitutions of some perennial Triticeae species that were unknown or misidentified. A majority of publications have substantiated the close affinity of the Eb and Ee genomes in Th. bessarabicum and Th. elongatum, supporting the use of a common basic genome symbol. The E genome is close to the St genome of Pseudoroegneria and ABD genomes of Triticum/Aegilops complex, providing an explanation for transferring genes from the E to ABD genomes with relative ease. Although the solid proof is still lacking, the W, P, and especially Xp genomes are possible origins for the Y genome of polyploid Elymus. The absence of the E genome and the allopolyploidy nature of tetraploid Leymus species have been unequivocally confirmed by both cytogenetic and molecular studies. However, the donor of the Xm genomes of Leymus was only speculated to be related to the P genome of Agropyron and F genome of Eremopyrum. Intermediate wheatgrass (Th. intermedium) has been extensively studied. The presence of the St (as the previously designated X) genome in Th. intermedium is now unequivocal. Its two more closely related E1 and E2 genomes are shown to be older versions of the E genome rather than the current Eb and Ee genomes. Speciation of Th. intermedium was similar to that of Triticum aestivum, in which the Js/Es (like B) genomes had the greatest differentiation from the current J (Eb) genome owning to repetitive sequences of the V genome, whereas its St (like D) had the least differentiation from the current St genome. Species with unknown or misidentified genomes have been correctly designated, including those with the ESt, StP, StPY, StWY, EStP, HW, StYHW, and NsXm genomes. Some of those species have been transferred to and renamed in appropriate genera.

Homoeology of Thinopyrum junceum and Elymus rectisetus chromosomes to wheat and disease resistance conferred by the Thinopyrum and Elymus chromosomes in wheat

Thirteen common wheat “Chinese Spring” (CS)-Thinopyrum junceum addition lines and three common wheat “Fukuhokomuji”(Fuku)-Elymus rectisetus addition lines were characterized and verified as disomic additions of a Th. junceum or E. rectisetus chromosome in the wheat backgrounds by fluorescent genomic in situ hybridization. Another Fuku-E. rectisetus addition line, A1048, was found to contain multiple segregating E. rectisetus chromosomes. Seven partial CS-Th. junceum amphiploids were identified to combine 12–16 Th. junceum chromosomes with CS wheat chromosomes. The disomic addition lines AJDAj5, 7, 8, 9, and HD3508 were identified to contain a Th. junceum chromosome in homoeologous group 1. Two of them, AJDAj7 and AJDAj9, had the same Th. junceum chromosome. AJDAj2, 3, and 4 contained a Th. junceum chromosome in group 2, HD3505 in group 4, AJDAj6 and AJDAj11 in group 5, and AJDAj1 probably in group 6. The disomic addition lines A1026 and A1057 were identified to carry an E. rectisetus chromosome in group 1 and A1034 in group 5. E. rectisetus chromosomes in groups 1–6 were detected in A1048. The homoeologous group of the Th. junceum chromosome in HD3515 could not be determined in this study. Several Th. junceum and E. rectisetus chromosomes in the addition lines were found to contain genes for resistance to Fusarium head blight, tan spot, Stagonospora nodorum blotch, and stem rust (Ug99 races). Understanding of the homoeology of the Th. junceum and E. rectisetus chromosomes with wheat will facilitate utilization of the favorable genes on these alien chromosomes in wheat improvement.

A proposed mechanism for loss of heterozygosity in rice hybrids

A phenomenon, loss of heterozygosity (LOH), was discovered in hybrid plants involving a selected plant (named AMR) of the Chinese rice cultivar ‘ZhongxinNo. 1’ as one parent. In these hybrids and some of their progenies, somatic variations were manifested by molecular genotypes and/or morphological phenotypes in vegetative parts of the same plant. Random amplified polymorphic DNA (RAPD) markers for the parents have been followed through the F3 generation. RAPD markers were uniformly present or absent in all plants within some or all F2 panicle rows derived from F1 hybrids involving AMR. In contrast, RAPD markers segregated in the Mendelian manner for dominant markers in panicle rows derived from control hybrids. Certain F2 panicle rows from F1hybrids involving the special rice became fixed for all assayed RAPD markers. Genotype fixation was confirmed by molecular assays and field observations of the F3 progenies. We propose a new biological mechanism, called ‘assortment mitosis,’ as being responsible for the observed phenomenon. The use of this mechanism in plant hybrids allows the development of uniform progenies as early as the F2 generation. Therefore, the time required to obtain fixed non-parental type progenies for subsequent performance trials can be drastically shortened. Utilizing this mechanism in plant breeding represents a new approach and requires the modification of conventional plant breeding procedures.