Robert F. Park

Obligate biotrophy features unraveled by the genomic analysis of rust fungi

Rust fungi are some of the most devastating pathogens of crop plants. They are obligate biotrophs, which extract nutrients only from living plant tissues and cannot grow apart from their hosts. Their lifestyle has slowed the dissection of molecular mechanisms underlying host invasion and avoidance or suppression of plant innate immunity. We sequenced the 101-Mb genome of Melampsora larici-populina, the causal agent of poplar leaf rust, and the 89-Mb genome of Puccinia graminis f. sp. tritici, the causal agent of wheat and barley stem rust. We then compared the 16,399 predicted proteins of M. larici-populina with the 17,773 predicted proteins of P. graminis f. sp tritici. Genomic features related to their obligate biotrophic lifestyle include expanded lineage-specific gene families, a large repertoire of effector-like small secreted proteins, impaired nitrogen and sulfur assimilation pathways, and expanded families of amino acid and oligopeptide membrane transporters. The dramatic up-regulation of transcripts coding for small secreted proteins, secreted hydrolytic enzymes, and transporters in planta suggests that they play a role in host infection and nutrient acquisition. Some of these genomic hallmarks are mirrored in the genomes of other microbial eukaryotes that have independently evolved to infect plants, indicating convergent adaptation to a biotrophic existence inside plant cells.

Obligate Biotrophy Features Unraveled by the Genomic Analysis of the Rust Fungi, Melampsora larici-populina and Puccinia graminis f. sp. tritici

Rust fungi are some of the most devastating pathogens of crop plants. They are obligate biotrophs, which extract nutrients only from living plant tissues and cannot grow apart from their hosts. Their lifestyle has slowed the dissection of molecular mechanisms underlying host invasion and avoidance or suppression of plant innate immunity. We sequenced the 101-Mb genome of Melampsora larici-populina, the causal agent of poplar leaf rust, and the 89-Mb genome of Puccinia graminis f. sp. tritici, the causal agent of wheat and barley stem rust. We then compared the 16,399 predicted proteins of M. larici-populina with the 17,773 predicted proteins of P. graminis f. sp tritici. Genomic features related to their obligate biotrophic lifestyle include expanded lineage-specific gene families, a large repertoire of effector-like small secreted proteins, impaired nitrogen and sulfur assimilation pathways, and expanded families of amino acid and oligopeptide membrane transporters. The dramatic up-regulation of transcripts coding for small secreted proteins, secreted hydrolytic enzymes, and transporters in planta suggests that they play a role in host infection and nutrient acquisition. Some of these genomic hallmarks are mirrored in the genomes of other microbial eukaryotes that have independently evolved to infect plants, indicating convergent adaptation to a biotrophic existence inside plant cells.

Global status of wheat leaf rust caused by Puccinia triticina

Leaf rust caused by Puccinia triticina is the most common and widely distributed of the three wheat rusts. Losses from leaf rust are usually less damaging than those from stem rust and stripe rust, but leaf rust causes greater annual losses due to its more frequent and widespread occurrence. Yield losses from leaf rust are mostly due to reductions in kernel weight. Many laboratories worldwide conduct leaf rust surveys and virulence analyses. Most currently important races (pathotypes) have either evolved through mutations in existing populations or migrated from other, often unknown, areas. Several leaf rust resistance genes are cataloged, and high levels of slow rusting adult plant resistance are available in high yielding CIMMYT wheats. This paper summarizes the importance of leaf rust in the main wheat production areas as reflected by yield losses, the complexity of virulence variation in pathogen populations, the role cultivars with race-specific resistance play in pathogen evolution, and the control measures currently practiced in various regions of the world.

New sources of rust resistance from alien species : meliorating linked defects and discovery

This paper presents a review of projects undertaken over the past 2 decades aimed at improving the yield and/or quality attributes of translocation lines carrying rust resistance genes from species related to wheat, so as to make these lines more suitable for use in breeding programs. Homeologous recombination between the alien chromosome segments and normal wheat chromosomes was induced in a ph1bph1b background. Lines with shortened alien chromatin were selected using dissociation patterns of molecular-based markers. A new line of bread wheat was developed containing a shortened chromosome 1RS segment carrying rust resistance gene SrR (Secale cereale L.), in which a defect affecting dough-quality appears to have been deleted. In addition, several advanced lines were produced with modified 6Ae#1L chromosome segments with Sr26 (Thinopyrum ponticum), 2S#1 chromosome segments with Sr32, and a previously unnamed gene, a 2S#2 chromosome segment with Sr39 (Triticum speltoides), 4G#1 chromosome segments with Sr37, and 2G#2 chromosome segments with Sr40 (T. timopheevii).

Mycosphaerella nubilosa a synonym of M. molleriana

Mycosphaerella leaf blotch is a serious disease of Eucalyptus in South Africa. Both M. nubilosa and M. molleriana have been reported to occur in this country. An examination of numerous collections has failed to show reliable differences between collections identified as either M. nubilosa or M. molleriana. Further studies have shown the type of M. molleriana to be incorrectly measured, and that the two species are conspecific, with M. nubilosa being a synonym of the earlier described M. molleriana.

Stem rust of wheat in Australia

Annual pathogenicity surveys of Puccinina graminis f. sp. tritici (Pgt), initiated at the University of Sydney in 1919, have continued without interruption to the present day. The population structure of Pgt over the past 85 years has been strongly influenced by exotic introductions in 1925 (race 126), 1954 (race 21), and 1969 (races 194 and 326), subsequent random mutations to virulence, and selection of genotypes with virulence matching resistance genes in cultivars. Pathotypes detected in Australia over the past 10 years trace back to either races 21, 194, or 326. Based on varietal resistance and pathogenic variability, previous workers identified 3 periods between 1919 and 1970: from 1919 to 1938, cultivars lacked effective resistance genes; from 1938 to 1964, cultivars released with single genes for resistance (Sr6, Sr11, Sr9b, Sr36, Sr17), and new pathotypes with corresponding virulences were detected; from 1965 to 1970, and beyond, cultivars with multiple resistance genes were deployed in many regions, significantly reducing yield losses. During this third phase, and until now, cultivars were protected by resistance genes Sr2, Sr9g, Sr12, Sr13, Sr17, Sr22, Sr24, Sr26, Sr30, Sr36, and Sr38, singly or more commonly in combinations. Overall inoculum levels and pathotype diversity in Pgt have declined in all wheat-growing regions since the mid 1970s, likely as a consequence of the release of cultivars with gene combinations. Despite the low levels of stem rust in Australia over the past 30 years, resistance is still a top priority in many breeding programs. The development of virulence for Sr38 in WA in 2001 was a timely reminder of the need for continued vigilance if the sustained genetic control of the past 30 years is to continue.

Distribution of Pathotypes with Regard to Host Cultivars in French Wheat Leaf Rust Populations

ABSTRACT Isolates of wheat leaf rust collected from durum and bread wheat cultivars in France during 1999-2002 were analyzed for virulence on 18 Thatcher lines with single genes for leaf rust resistance (Lr genes). Sampling focused on the five most widely grown bread wheat cultivars (two susceptible and three resistant) to allow statistical comparison of diversity indexes between the cultivars. Leaf rust populations from durum and bread wheats were different. The diversity of the bread wheat leaf rust pathotypes, as measured by the Shannon index, ranged from 2.43 to 2.76 over the 4 years. Diversity for wheat leaf rust resistance was limited in the host since we postulated only seven seedling resistance genes in the 35 cultivars most widely grown during 1999-2002. Leaf rust populations were strongly differentiated for virulence within bread wheat cultivars, and diversity was higher on those that were resistant, mainly due to a more even distribution of virulence phenotypes than on susceptible cultivars. The pathogen population on the susceptible cv. Soissons was largely dominated by a single pathotype (073100), whereas all other pathotypes virulent on cv. Soissons either decreased in frequency or remained at a low frequency during the period studied. Several pathotypes including the most complex one were found only on resistant cultivars, even though most of them were virulent on the susceptible cv. Soissons. Specific interactions were necessary, but not always sufficient, to account for pathotype distribution and frequencies on the cultivars, suggesting that selection for virulence to host resistance genes is balanced by other selective forces including selection for aggressiveness.

Postulation of leaf (brown) rust resistance genes in 70 wheat cultivars grown in the United Kingdom

Multi-pathotype tests on 70 U.K. wheat cultivars permitted postulation of eight known seedling genes for resistance to Puccinia recondita f. sp.tritici either singly or in combinations. The most commonly detected gene was Lr13 (present in approximately 57% of cultivars), followed by Lr26 (22%), Lr37 (20%), Lr10 (17%), Lr17b (LrH) (10%), Lr1 (7%), Lr3a (6%) and Lr20(4%). This information permitted assessments of adult plant resistance (APR) in some cultivars, in field nurseries inoculated with pathotypes of P. recondita f. sp. tritici of known pathogenicities for characterized seedling resistance genes. APR was identified in eleven cultivars, including Avalon and Maris Ranger, which lacked detectable seedling resistance genes. The results provided a better understanding of specific resistances in the cultivars tested than was available from previous reports.

Molecular genetic variability of Australian isolates of five cereal rust pathogens

Rust fungi cause economically important diseases of cereals, and their ability to rapidly evolve new virulent races has hindered attempts to control them by genetic resistance. PCR-based molecular tools may assist in understanding the genetic structure of pathogen populations. The high multiplex DNA fingerprinting techniques, amplified fragment length polymorphisms (AFLP), selectively amplified microsatellites (SAM) and sequence-specific amplification polymorphisms (S-SAP) were assessed for their potential in investigations of the genetic relationships among isolates of the wheat rust pathogens, Puccinia graminis f. sp. tritici (Pgt), Puccinia triticina (Pt), and P. striiformis f. sp. tritici (Pst), the oat stem rust pathogen P. graminis f. sp. avenae (Pga), and a putative new P. striiformis special form tentatively designated Barley grass yellow rust (Bgyr). Marker information content, as indicated by the number of species-specific fragments, polymorphic fragments among pathotypes, percentage of polymorphic loci, and the marker index, was highest for the SAM assay, followed by the AFLP and S-SAP assays. UPGMA analysis revealed that all marker types efficiently discriminated the five different taxa and Mantel tests revealed significant correlations between the marker types. Within pathogen groups, the marker types differed in the amount of variation detected among isolates; however, the major differences were consistent and polymorphism was generally low. This was reflected by the AMOVA analysis that significantly partitioned 90% of the genetic variation between taxa. Of the three marker types, SAMS were the most informative, and have the potential for the development of locus-specific microsatellites.

Studies on the origin, spread, and evolution of an important group ofPuccinia recondita f. sp.tritici pathotypes in Australasia

Wheat brown rust pathotype (pt) 104-2,3,(6),(7), 11 was first detected in Australasia in Victoria during 1984. Although it appeared similar to a pre-existing pathotype, 104-2,3,6,(7), detailed greenhouse test revealed nine pathogenic differences between the two rusts. Six differences involved contrasting virulence/avirulence for the resistance genes/specificitiesLr12, Lr27+Lr31 andLr16, and three uncharacterised genes, present in the wheat cultivars Gaza and Harrier, and in triticale cultivar Lasko. Differences in partial virulence between the pathotypes were found for the genesLr2a, Lr13 andLr26. A comparison of the phenotypes for 13 isozyme systems in the two pathotypes revealed two differences, including aPgm2 allele in pt 104-2,3,(6),(7),11 not found in other contemporary AustralasianPuccinia recondita f. sp.tritici pathotypes. On the basis of these differences, it was concluded that pt 104-2,3,(6),(7),11 was introduced into the Australasian region before or during 1984.Seven variants of pt 104-2,3,(6),(7),11, that differed by single virulences, were detected during 1984–1992. Pt 104-2,3,(6),(7),11 and a derivative pathotype with virulence forLr20 underwent rapid increases in frequency, largely displacing pathotypes which predominated before 1984. Although first detected in eastern Australia, both pathotypes spread to New Zealand, and the derivative pathotype appeared in Western Australia. The rapid spread and increase of these pathotypes could not be explained by host selection. Pt 104-2,3,(6),(7),11 and derivatives may therefore be more aggressive than other contemporary Australasian pathotypes.