Potato Late Blight Pathogen Research Articles

Origins, lineage-specific expansions, and multiple losses of tyrosine kinases in eukaryotes.

Tyrosine kinases are important components of metazoan signaling pathways, and their mutant forms are implicated in various malignancies. Searching the sequences from the genomes of 28 eukaryotes and the GenBank, we found tyrosine kinases not only in metazoans but also in the green algae Chlamydomonas reinhardtii, the potato late blight pathogen Phytophthora infestans, and the protozoan pathogen Entamoeba histolytica, contrary to the current view that tyrosine kinases are animal-specific. Based on a phylogenetic analysis, we divided this gene family into 43 subfamilies and found that at least 19 tyrosine kinases were likely present in the common ancestor of chordates, arthropods, and nematodes. Interestingly, most of the subfamilies have conserved domain organizations among subfamily members but have undergone different degrees of expansion during the evolution of metazoans. In particular, a large number of duplications occurred in the lineage leading to the common ancestor of Tagifugu and mammals after its split from the Ciona lineage about 450 to 550 MYA. The timing of expansion coincides with proposed large-scale duplication event in the chordate lineage. Furthermore, gene losses have occurred in most subfamilies. Interestingly, different subfamilies have similar net gain rates in the chordates studied. However, the tyrosine kinases in mouse and human or in fruit fly and mosquito mostly have a one-to-one relationship between species, indicating that static periods of 90 Myr or longer in tyrosine kinase evolution have followed large expansion events.

Multiple pathways regulate the induction of genes during zoosporogenesis in Phytophthora infestans.

Zoospores are a critical component of the disease cycles of most oomycete pathogens. To better understand this stage, genes induced during zoosporogenesis were identified from Phytophthora infestans, the potato late blight pathogen. Using cDNA arrays representing 2,600 genes expressed during zoosporogenesis, 69 genes showing >fourfold increases in mRNA levels were identified, of which 22 exhibited >100-fold induction. Included were putative protein kinases, transcription factors, ion channels, and other regulators. The expression of 15 genes was characterized in detail using zoosporogenesis time courses, other developmental stages, different temperature regimes, and tissue treated with signaling inhibitors. The latter were of interest because zoosporogenesis is known to be cold induced and inhibited by calcium channel blockers such as verapamil; moreover, in this study, inhibitors of phospholipase C (U-73122) and inositol trisphosphate receptor-gated calcium channels (2-aminoethoxydiphenyl borate) also were shown to block zoosporogenesis. The results indicated that the cytoplasmic and transcriptional changes occurring during zoosporogenesis are regulated by several pathways. For example, verapamil inhibited zoosporogenesis but not the up-regulation of most genes; the induction of some genes required while others were independent of calcium or phospholipid signaling; and, although most genes were induced in sporangia at 10 degrees C but not 24 degrees C, one was induced at both temperatures.

A Galpha subunit controls zoospore motility and virulence in the potato late blight pathogen Phytophthora infestans.

The heterotrimeric G-protein pathway is a ubiquitous eukaryotic signalling module that is known to regulate growth and differentiation in many plant pathogens. We previously identified Pigpa1, a gene encoding a G-protein alpha subunit from the potato late blight pathogen Phytophthora infestans. P. infestans belongs to the class oomycetes, a group of organisms in which signal transduction processes have not yet been studied at the molecular level. To elucidate the function of Pigpa1, PiGPA1-deficient mutants were obtained by homology-dependent gene silencing. The Pigpa1-silenced mutants produced zoospores that turned six to eight times more frequently, causing them to swim only short distances compared with wild type. Attraction to the surface, a phenomenon known as negative geotaxis, was impaired in the mutant zoospores, as well as autoaggregation and chemotaxis towards glutamic and aspartic acid. Zoospore production was reduced by 20-45% in different Pigpa1-silenced mutants. Transformants expressing constitutively active forms of PiGPA1, containing amino acid substitutions (R177H and Q203L), showed no obvious phenotypic differences from the wild-type strain. Infection efficiencies on potato leaves ranged from 3% to 14% in the Pigpa1-silenced mutants, compared with 77% in wild type, showing that virulence is severely impaired. The results prove that PiGPA1 is crucial for zoospore motility and for pathogenicity in an important oomycete plant pathogen.

Nitrate reductase, a nitric oxide-producing enzyme: induction by pathogen signals

Nitric oxide (NO) is believed to act as an effector for defense signaling in plant cells. Nitrate reductase (NR) is one of the NO-producing enzymes in plants. Here, we report that infection of Phytophthora infestans, the fungal pathogen of potato late blight, into potato tubers caused a transient increase in the NR transcript in an incompatible, but not a compatible, interaction. Treatment of potato tubers with the fungal elicitor hyphal wall components (HWC) from the fungus induced NR gene at the transcriptional and protein level. Soluble proteins from HWC-treated tubers exhibited enhanced nitrite-dependent NO production. The inhibitor experiments for protein kinase and extracellular Ca2+ on the HWC-induced accumulation of the NR transcript suggested involvement of a calcium-independent protein kinase in regulation of the NR gene. Additionally, we isolated cDNA clones encoding NR from a potato tuber cDNA library; two isogenes were designated StNR5 and StNR6, respectively. Reverse transcription-polymerase chain reaction analyses using gene-specific primers showed that transcripts for both StNR5 and StNR6 were induced by HWC in potato tubers. This is the first report that pathogen signals induce expression of the NR gene.

Phytophthora infestans: populations, pathogenicity and phenylamides.

Isolates of Phytophthora infestans (Mont) de Bary (the potato and tomato late blight pathogen) resistant to phenylamides appeared in Europe and North America in the late 1970s and early 1990s respectively. Concurrent, but coincidentally, with both these events there were radical structural shifts in the pathogen populations as immigrant genotypes from Mexico displaced the indigenous populations. Both A1 and A2 mating type isolates are now present in blighted crops, permitting alternative inoculum via germinating sexually produced oospores to influence dynamics of late blight populations. Studies of inheritance of ploidy, host-specific pathogenicity, mating type and resistance to antibiotics and phenylamide fungicides have provided insight into mechanisms of variation in this potent pathogen.

The pipg1 gene of the oomycete Phytophthora infestans encodes a fungal-like endopolygalacturonase.

Endopolygalacturonases (endoPGs) are plant cell wall-degrading enzymes that have been implicated in the invasion of plant tissue by pathogenic microbes. EndoPGs have been described from bacteria, plants, insects and numerous species of phytopathogenic fungi. In this study, we describe the first endoPG sequence from oomycetes, a unique group of eukaryotic plant pathogens that exhibit fungal-like filamentous growth but share little taxonomic affinity to fungi. The characterized gene, pipg1, was identified from the potato late-blight pathogen, Phytophthora infestans, and was predicted to encode a secreted glycoprotein with all the signature sequences of endoPGs. Pipg1 was expressed during preinfection and infection stages. Phylogenetic analysis of endoPGs indicated that pipg1 forms a unique class that is significantly more similar to fungal endoPGs than to plant or bacterial ones. This unexpected affinity between PIPG1 and fungal endoPGs contrasts with phylogenies obtained using ribosomal sequences or compiled protein sequences from mitochondrial and chromosomal genes, raising interesting questions about the evolution of these enzymes in oomycetes.

Potato late blight threatens Russian crop

New virulent types of the potato late blight pathogen have emerged in Russia and are far more aggressive than the pathogen that triggered the Irish potato famine of the 1840s. Unlike the old strains, the new pathogen can survive harsh winters in the soil, further endangering crops. The fungus originates from a 25 000 tonne shipment of potatoes from Mexico in the 1980s. Research emphasis for resistance is on the survival of the fittest by crossing domestic potatoes with wild relatives from Latin America, which have developed resistance naturally by co-existing with the disease for millennia. In February, Robert Plaisted (Cornell University, USA) announced a blight-resistant variety – the New York 121 – a white potato derived from Peruvian and Argentinean descent.

The Importance of Archival and Herbarium Materials in Understanding the Role of Oospores in Late Blight Epidemics of the Past

ABSTRACT Nineteenth and early twentieth century botanists and mycologists collected healthy and infected plant materials from many regions of the world. Some of these plant collections preserved in herbaria around the world contain samples that are of considerable significance to epidemiologists, population biologists, and botanists. The advent of the polymerase chain reaction (PCR) and the development of molecular marker technology has made DNA amplification from herbarium material a reality. In this mini-review, archival letters and herbarium samples are used to track the historical role of oospores in the biology of the potato late blight pathogen. DNA was successfully amplified by PCR with the Phytophthora infestans-specific PCR primer, PINF, and the universal primer, ITS 5, from oospores observed in a field sample of potato collected by G. P. Clinton in 1902. This experiment demonstrates the potential to utilize molecular methods to amplify DNA from historical samples of the late blight pathogen and represents the earliest definitive record of oospores of the pathogen in field samples in the United States. Studies based upon such materials and techniques, although high risk and laborious, have the potential to open a new window to epidemics of the past.

A Global Marker Database for Phytophthora infestans.

A marker database was compiled for isolates of the potato and tomato late blight pathogen, Phytophthora infestans, originating from 41 locations which include 31 countries plus 10 regions within Mexico. Presently, the database contains information on 1,776 isolates for one or more of the following markers: restriction fragment length polymorphism (RFLP) "fingerprint" consisting of 23 bands; mating type; dilocus allozyme genotype; mitochondrial DNA haplotype; sensitivity to the fungicide metalaxyl; and virulence. In the database, 305 entries have unique RFLP fingerprints and 258 entries have unique multilocus genotypes based on RFLP fingerprint, dilocus allozyme genotype, and mating type. A nomenclature is described for naming multilocus genotypes based on the International Organization for Standardization (ISO) two-letter country code and a unique number. Forty-two previously published multilocus genotypes are represented in the database with references to publications. As a result of compilation of the database, seven new genotypes were identified and named. Cluster analysis of genotypes from clonally propagated populations worldwide generally confirmed a previously published classification of "old" and "new" genotypes. Genotypes from geographically distant countries were frequently clustered, and several old and new genotypes were found in two or more distant countries. The cluster analysis also demonstrated that A2 genotypes from Argentina differed from all others. The database is available via the Internet, and thus can serve as a resource for Phytophthora workers worldwide.

The origin of Phytophthora infestans populations present in Europe in the 1840s: a critical review of historical and scientific evidence

A critical review of available historical and scientific evidence related to the question of the origin of the first European populations of the potato late blight pathogen Phytophthora infestans is presented. It shows that the bases for the current theories of a direct introduction of the fungus into Europe and North America from either a Mexican or an Andean centre of origin and of diversity are questionable. An alternative theory, involving a three‐step process: (i) migration from central Mexico to South America several centuries ago; (ii) migration from South America to the US in 1841–1842, and (iii) migration to Europe from either South America, US or both in 1843–1844, is in good agreement with both the historical records and with the genetics and structure of current populations.

Arachidonic Acid Induces Local but not Systemic Synthesis of Salicylic Acid and Confers Systemic Resistance in Potato Plants toPhytophthora infestansandAlternaria solani

When the lower leaves of young potato plants (cv. Bintje) were treated with an aqueous suspension of the polyunsaturated fatty acid arachidonic acid (AA), a threefold increase in the amount of free salicylic acid (SA) in the treated leaves was observed 3 h after treatment. The highest level of free SA was measured 6 h after treatment, reaching 3.21 g/g fresh weight compared to 0.15 g/g fresh weight in controls. The levels of conjugated SA in the treated leaves increased up to 51 g/g fresh weight 3 days after treatment compared to 0.83 g/g fresh weight in controls. Four days after treatment with AA the upper leaves became resistant to the potato late blight pathogen, Phytophthora infestans, but no accumulation of SA was observed. This systemic protection was observed in plants treated with AA at concentrations as low as 10 ppm and is unlikely to have arisen from AA translocated in the plant. In addition, treatment with AA protected potato plants against the early blight pathogen, Alternaria solani, indicating a broader spectrum of protection. Local but not systemic accumulation of a pathogenesis-related (PR)1-like protein was observed 24 h after treatment with AA. Application of SA on potato leaves did not confer resistance against P. infestans, but a local accumulation of a PR1-like protein was observed 24 h after treatment. Younger leaves of healthy susceptible and resistant potato plants contained higher levels of SA than older leaves, and this gradient correlated with their natural resistance to P. infestans. In general, cultivars showing field resistance contained higher amounts of conjugated SA than did susceptible cultivars.

Race structure and dynamics in populations of Phytophthora infestans

The structure and dynamics of races of the potato late blight pathogen Phytophthora infestans in several populations collected worldwide during 1966–1993 were investigated using mathematical estimates of phenotypic diversity (Shannon and Rogers indices) and of virulence complexity. As expected, the highest diversity of races was found in central Mexico, whereas European and other American populations had consistently lower numbers of races and were usually dominated by one or a few phenotypes. Rogers indices were generally close to 0.5 when calculated for populations collected over successive years from the same area and for populations from neighbouring regions collected at the same period of time, indicating that some geographical and temporal structuring was the rule. This is consistent with a metapopulation structure for P. infestans and provides evidence for evolutionary patterns driven by founder effects. The mean number of virulence genes per race was highly variable among populations. Unnecessary virulences were frequent in most if not all populations. In all populations investigated, the rarest virulence genes were found in highly complex races, indicating a predominant influence of mutation events on race evolution. Key words: potato, late blight, virulence, diversity, complexity, population structure.