Peronospora

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Peronospora is a genus of downy mildews that are obligate plant pathogens.[1] They can cause severe damage to many different cultivated crops, as well as wild and ornamental plants.[2] Peronospora is most closely related to Pseudoperonospora, and together they form the clade of downy mildews with coloured conidia.[3] Peronospora has far more species than any other genus of the downy mildews.[3] However, many species have been moved from this genus to other genera based on phylogenetic evidence.[4] Among these are the species now in Hyaloperonospora, important pathogens of the Brassicaceae.[4] Now, the Peronospora species of most importance is likely Peronospora tabacina.[5]Template:Opinion Peronospora tabacina causes blue mold on tobacco plants and can severely reduce yields of this economically important crop to the point where it has been classified as a bioweapon.[5][3]

History

Peronospora was first described in 1837 by August Carl Joseph Corda, a Czech mycologist and physician, in his first of six volumes of his Icones fungorum hucusque cognitorum.[6] Since then, many of the species originally placed in Peronospora have been allocated to other genera or given rise to new genera based on new techniques such as molecular genetics.[4]

There was an epidemic in 1960 of Peronospora tabacina affecting tobacco plants leading to $25 million in losses across eleven countries, which was about 30 percent of the tobacco plants.[5] Another epidemic that was caused by Peronospora destructor reduced the yield of sweet onions by 25 percent in Georgia, USA in 2012, and led to an estimated $18.2 million in losses.[7]

Habitat and ecology

Most of the Peronospora species are highly specific to their hosts and can generally be found anywhere the host plant grows, or is being cultivated.[3] A large portion of their life cycle is spent inside their host plant. Many species of Peronospora are seedborne pathogens, so the worldwide spread of Peronospora crop-plant pathogens is likely to be a result of unknowingly trading infected seeds to new areas.[3] There are also many Peronospora species that are spread by wind currents, which allows them to disperse over large distances.[3] Peronospora species prefer humid air and cool temperatures.[5]

One clade in the genus is known as the floricolous downy mildews. These species produce conidiophores exclusively on the flowers of their hosts.[8]

General form and structure

The first stage in the Peronospora life history is the sporangia.[5] The sporangia are small spore-like structures about 65 um long that germinate a germ-tube when they are near a leaf stoma.[9][5] A germ tube will come from the sporangium and penetrate the leaf cell where it will form a haustorium.[5] The haustorium absorbs nutrients from the leaf, while hyphae invade the intercellular space, and the leaf will eventually develop a lesion.[5] These lesions often start out yellow and then turn brown as the leaf starts to undergo necrosis.[5] From here, Peronospora can undergo either asexual reproduction or sexual reproduction.[5] Asexual reproduction occurs when the air outside is moist making for favourable conditions.[5] During asexual reproduction, hyphae on the host plant will form sporangiophores, which will produce conidia.[5] The conidia will be dispersed by the wind is able to infect other plants.[5] The asexual cycle only takes five to seven days to complete.[5] Sexual reproduction occurs when the conditions are unfavourable and it needs to withstand harsh environmental conditions.[5] During sexual reproduction, the hyphae will undergo meiosis forming antheridia and oogonia, the only haploid structures in the Peronospora life history.[5] The antheridia will fuse to the oogonia, initiating plasmogamy and then karyogamy, and will result in the production of many oospores.[5] The oospores can then be dispersed by the wind to infect more plants.[5]

Both Peronospora and Pseudoperonospora are characterized by their ability to produce melanized sporangia, but Pseudoperonospora produces zoospores whereas Peronospora cannot.[3]

Practical importance

The model oomycete pathogen, Peronospora parasitica, used to be included in this genus, however it has been reclassified to the genus Hyaloperonospora.[3]

Some species of Peronospora have been considered for their use as a bioweapon or have been classified as potential bioweapons.[3] Peronspora somniferi was considered for its ability to devastate fields of the opium poppy, which could have targeted areas that depend on the crop.[3] The United States has classified Peronospora tabacina as a possible bioweapon, because if it were used to target the US tobacco industry, it would lead to major economic loss.[3]

Genomics and genetics

Only one species in the genus Peronospora has had its genome sequenced and assembled. In 2015, Derevnina et al. performed a de novo sequence assembly of the genome of two Peronospora tabacina isolates using Illumina sequencing.[10] They estimated the genome size to be 68 Mb with a mitochondrial genome of 43 kb.[10] The two assemblies had 61.8x and 128.9x coverage for the nuclear genomes and 6,824x and 43,225x coverage for the mitochondrial genomes.[10] The mitochondrial genome only differed by seven single nucleotide polymorphisms, three small indels, and one copy number variant.[10] Using a program to predict gene models, they found 18,000 potential protein coding genes.[10]

List of species

The following species are placed in genus Peronospora:[11]

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References

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