Hyaloperonospora

Hyaloperonospora is a genus of oomycete, obligate, plant pathogens that was originally considered to be part of Peronospora.^[1] Species in this group produce a disease called downy mildew and can infect many important crops.^[1] From the 19 downy mildew producing genera, Hyaloperonospora has been grouped with Perofascia in the brassicolous downy mildews.^[1] In the group of downy mildews, Hyaloperonospora is the third biggest genus.^[1] The most famous species in the genus is the Hyaloperonospora parasitica, or also known as Hyaloperonospora arabidopsis.^[2] This species has become a model organism from its ability to infect the model plant Arabidopsis thaliana.^[2] It is used to study plant-pathogen interactions, and is currently the only Hyaloperonospora species that has an assembled genome.^[2]

Hyaloperonospora
Hyaloperonospora brassicae on cabbage
Scientific classification
Domain:	Eukaryota
Clade:	Sar
Clade:	Stramenopiles
Clade:	Pseudofungi
Class:	Oomycetes
Order:	Peronosporales
Family:	Peronosporaceae
Genus:	Hyaloperonospora Constant.

History

In 2002, Hyaloperonospora was discovered and described by Constantinescu, O. and Fatehi, J. using morphological and molecular characteristics.^[3] Later, Göker et al., also used molecular phylogenetic techniques showing that the group was different enough from the other Peronospora species to be its own taxon.^[4] Hyaloperonospora along with Perofascia were the first downy mildews described using their molecular phylogenies.^[1]

Habitat and ecology

Hyaloperonospora can be found on plants from about 20 different tribes of Brassicaceae.^[1] They can generally be found anywhere their host plant grows, due to human transport from seed trade.^[1] Hyaloperonospora parasitica is unlike most other species in the family in that it has a very wide host range, infecting a variety of crops^{[citation needed]}. Another important interaction is with Hyaloperonospora brassicae, which also has a wider host range infecting many Brassica species^{[citation needed]}.

General form and structure

Hyaloperonospora differs from Perofascia in that its sporangiophores are tree-like, its haustoria are lobate to globose, and the walls of its oospores are relatively thinner.^[1]

The life history does not differ from that of Peronospora, the genus that Hyaloperonospora species used to be classified under^{[citation needed]}. It begins as sporangia, which are small spore-like structure, and when it lands next to a leaf stoma, it germinates a germ-tube.^[5] The germ tube enters the leaf cell creating a haustorium, which allows the mould the uptake nutrients from the leaf.^[5] The mould will continue to grow, with hyphae extending into the leaf's intercellular space.^[5] This invasion kills some of the leaf cells and the leaf will develop a lesion followed by necrosis.^[5] If the conditions are favourable, the mould will undergo asexual reproduction and produce a tree of sporangiophores out of the leaf.^[5] The sporangiophores will produce conidia that can be dispersed by the wind to another plant.^[5] If the conditions in the leaf were unfavourable, the mould can undergo sexual reproduction and produce haploid antheridia and haploid oogonia through meiosis.^[5] These two structures are the only non-diploid stages of the Hyaloperonospora.^[5] The antheridia will fuse to the oogonia inducing plasmogamy followed by karyogamy to form diploid oospores.^[5] The oospores will then be dispersed through the wind to infect more plants.^[5]

Practical importance

Hyaloperonospora arabidopsis infects the model plant Arabidopsis thaliana, and by association has become a model pathogen for studying plant-pathogen interactions.^[2] Studying these interactions should give us insight into how we can more effectively protect our crops from deadly eukaryotic pathogens. It is also used as a model in the Arabidopsis eFP Browser as one of the nine biotic stresses.^[6]

Genomics and genetics

The Hyaloperonospora arabidopsis genome was first sequenced and assembled in 2008 using Sanger and Illumina sequencing, by Baxter et al.^[7] They reported a genome size of 78 Mb with 9.5x coverage of the nuclear genome and did not assemble the mitochondrial genome.^[7] They also found that 42% of the genome consisted of repetitive elements.^[7] 14,543 protein coding genes were predicted using a program to detect gene models.^[7]

In 2015, two more isolates of Hyaloperonospora arabidopsis were sequenced using Illumina HiSeq with 90x coverage, and reported genome sizes of 70 Mb and 74 Mb^{[citation needed]}.

Species include

• Hyaloperonospora arabidopsidis (Gäum.) Göker, Voglmayr, Riethm., Weiss & Oberw., 2003
• Hyaloperonospora brassicae (Gäum.) Göker, Voglmayr, Riethm., Weiss & Oberw., 2003
• Hyaloperonospora parasitica (Pers.) Constant., 2002
• Hyaloperonospora drabae
• Hyaloperonospora floerkeae (Kellerm.) Constant. 2002
• Hyaloperonospora camelinae
• Hyaloperonospora dentariae (Rabenh.) Voglmayr 2013
• Hyaloperonospora isatidis
• Hyaloperonospora erophilae
• Hyaloperonospora cardamines-enneaphyllos Voglmayr 2013
• Hyaloperonospora cheiranthi
• Hyaloperonospora arabidis-alpinae
• Hyaloperonospora sisymbrii-loeselii
• Hyaloperonospora sisymbrii-sophiae
• Hyaloperonospora malyi (Lindtner) Voglmayr 2013
• Hyaloperonospora tribulina (Pass.) Constant. 2002
• Hyaloperonospora barbareae
• Hyaloperonospora lobulariae
• Hyaloperonospora berteroae
• Hyaloperonospora galligena
• Hyaloperonospora hesperidis
• Hyaloperonospora dentariae-macrophyllae
• Hyaloperonospora cardamines-laciniatae
• Hyaloperonospora rorippae-islandicae
• Hyaloperonospora lunariae
• Hyaloperonospora cochleariae
• Hyaloperonospora lepidii-perfoliati (Savul. & Rayss) Constant. 2002
• Hyaloperonospora thlaspeos-perfoliati
• Hyaloperonospora nesliae
• Hyaloperonospora thlaspeos-arvensis
• Hyaloperonospora cardaminopsidis
• Hyaloperonospora nasturtii-aquatici
• Hyaloperonospora praecox
• Hyaloperonospora norvegica
• Hyaloperonospora niessliana (Berl.) Constant. 2002