Recent Fungal Diseases of Crop Plants: Is Lateral Gene Transfer a Common Theme?

A cursory glance at old textbooks of plant pathology reveals that the diseases which are the current scourge of agriculture in many parts of the world are a different set from those that were prominent 50 or 100 years ago. Why have these new diseases arisen? The traditional explanations subscribe to the “nature abhors a vacuum” principle—that control of one disease creates the condition for the emergence of a replacement—but does little to explain why the new pathogen succeeds. The emergence of a new disease requires a series of conditions and steps, including the enhanced fecundity of the new pathogen, enhanced survival from season to season, and spread around the world. Recently, evidence was obtained that wheat tan spot emerged through a lateral gene transfer event some time prior to 1941. Although there have been sporadic and persistent reports of lateral gene transfer between and into fungal plant pathogens, most examples have been dismissed through incomplete evidence. The completion of whole genome sequences of an increasing number of fungal pathogens no longer allows such proposed cases of lateral gene transfer to be dismissed so easily. How frequent are lateral gene transfers involving fungal plant pathogens, and can this process explain the emergence of many of the new diseases of the recent past? Many of the apparently new diseases are dependant on the expression of host-specific toxins. These are enigmatic molecules whose action requires the presence of plant genes with products that specifically encode sensitivity to the toxin and susceptibility to the disease. It is also notable that many new diseases belong to the fungal taxon dothideomycetes. This review explores the coincidence of new diseases, interspecific gene transfer, host-specific toxins, and the dothideomycete class.

Broadening the Genetic Basis of Verticillium longisporum Resistance in Brassica napus by Interspecific Hybridization

Verticillium wilt caused by the vascular fungal pathogen Verticillium longisporum is one of the most important pathogens of oilseed rape (Brassica napus sp. oleifera) in northern Europe. Because production of this major oilseed crop is expanding rapidly and no approved fungicides are available for V. longisporum, long-term control of the disease can only be achieved with cultivars carrying effective quantitative resistance. However, very little resistance to V. longisporum is available within the gene pool of oilseed rape, meaning that interspecific gene transfer from related species is the only possibility for broadening levels of resistance in current varieties. The amphidiploid species B. napus can be resynthesized by crossing the two progenitor species Brassica oleracea and Brassica rapa, hence resistant accessions of these two diploid species can be used as resistance donors. In this study a total of 43 potential B. rapa and B. oleracea resistance donors were tested with regard to their reaction to a mixture of two aggressive V. longisporum isolates, and resistances from diverse lines were combined by embryo rescue-assisted interspecific hybridization in resynthesized rapeseed lines. Progenies from crosses of the two B. rapa gene bank accessions 13444 and 56515 to the B. oleracea gene bank accessions BRA1008, CGN14044, 8207, BRA1398, and 7518 showed a broad spectrum of resistance in pathogenicity tests. Of 45 tested resynthesized lines, 41 lines exhibited a significantly higher level of resistance than the moderately V. longisporum-tolerant oilseed rape cultivar Express. These lines represent a promising basis for the combination of different resistance resources in new varieties.

Gamma irradiation induced deletions in an alien chromosome segment of the wheat 'Indis' and their use in gene mapping

Deletion mutants were produced in a translocated chromosome segment derived from Thinopyrum distichum (Thunb.) Löve. Spikes of the translocation line 'Indis' were irradiated with gamma rays at dosages of 15, 20, and 25 Gy. The irradiated spikes were pollinated with 'Inia 66' pollen and the F2 and F3 generations screened for translocation mutants, using the genes for leaf rust resistance and yellow endosperm pigmentation as markers. Finally, endopeptidase polymorphisms were utilized to select mutant translocation homozygotes within each of 29 families. An investigation of polymorphisms at the α-Amy-D2 and Wsp-D1 loci of chromosome arm 7DL revealed that 'Indis' did not produce an α-AMY-D2 product, but it did produce a novel WSP-D1 protein. The mutants were characterized for their leaf and stem rust resistances and the presence of WSP-D1 and yellow flour pigments. The stem rust resistance gene could not be accurately mapped. The linear order of the remaining loci on 7DL was centromere - leaf rust resistance - Wsp-D1 and yellow pigment. The data obtained suggested that the 'Indis' translocation has homo(eo)logy to the Lr19 translocation and homoeology to 7DL of common wheat.Key words: wheat, interspecific gene transfer, translocation, deletion mapping.

Monosomic alien addition lines of rice: production, morphology, cytology, and breeding behavior

Interspecific hybrids between three breeding lines of Oryza sativa (2n = 24 = AA) and 18 accessions of O. officinalis (2n = 24 = CC) were obtained through an embryo-rescue technique. The crossability ranged from 1.0 to 2.3%. The AC hybrids showed a low amount of chromosome pairing (0–4 bivalents) and were completely male sterile. The BC1 progeny, obtained upon backcrossing to O. sativa were allotriploid (AAC) and were also male sterile. Further backcrosses to O. sativa produced 94 BC2 plants with chromosome numbers of 2n to 2n + 6. Monosomic alien addition lines (AA + 1C) corresponding to the 12 chromosomes of the haploid complement of O. officinalis were isolated from among forty 2n + 1 BC2 plants. These addition lines differ from their diploid sibs by an array of morphological features and resemble the 12 primary trisomics of O. sativa. The female transmission rates of alien chromosomes varied from 6.6 to 26.8%. Four of the 12 monosomic alien addition lines transmitted the alien chromosome through the male gametes.Key words: chromosome pairing, introgression, embryo rescue, allotriploid, genes for insect resistance, interspecific gene transfer.

Chromosomal structure of homozygous common wheat lines obtained from (wheat × Agropyron) × Aegilops speltoides derivatives. II. A screening of paracentric inversions

The presence of inversions is important in the evolution and cultivar differentiation of wheats, Even though we can't have conclusive cytological proof of the presence of inversions in wheat, we can use the indirect way of screening paracentric inversions through chromosome configuration at anaphase I and II. This work presents the comparison of 'Chinese Spring' structure (with respect to the presence of inverted segments) to (i) 52 homozygous wheat lines obtained in the progenies of the cross between a wheat × Agropyron intermedium substitution line and Aegilops speltoides homoeologous pairing promotor and (ii) three wheat cultivars ('Vilmorin 27', 'Ducat', and 'Tormes') that were used in developing the above mentioned lines. The three cultivars present inverted segments relative to 'Chinese Spring' structure. Some of these inverted segments are possibly maintained in some of the lines studied. Four of them show the primitive structure of 'Chinese Spring' even though none of the wheat parents had it. Finally, other lines show new structure that could have been due to the Aegilops speltoides effect, but the method used did not allow us to reach a definite conclusion on this point. It is interesting to emphasize that the use of this homoeologous pairing promotor led to a chromosomal structure that was not so different from that existing in wheats.Key words: pairing (homoeologous), hybridization (interspecific), gene transfer, Triticum, Agropyron, Aegilops.