Bridging the Gulf: Phytophthora and Downy Mildews Are Connected by Rare Grass Parasites

Citation: Thines M ( Bridging the Gulf: Phytophthora and Downy Mildews Are Connected by Rare Grass Parasites Marco Thines 0 Frederick M. Ausubel, Massachusetts General Hospital, United States of America 0 Institute of Botany, University of Hohenheim , Stuttgart , Germany Downy mildews and root and foliar rots caused by Phytophthora are among the most destructive plant pathogens and therefore have attracted considerable attention during the past two decades. Although it has been realized that a close phylogenetic relationship exists, so far sharp distinction has been made between the obligate biotrophic downy mildews and the hemibiotrophic Phytophthora. In the study presented here, it is shown that a continuum of character states from hemibiotrophic Phytophthora species to obligate biotrophic downy mildews is present. Intermediate character states between downy mildews and Phytophthora species exist in several rare parasites of grasses, which are not embedded within the major clades of the downy mildews but are placed sister to these, with unresolved affinities to both these clades and to Phytophthora. They still have retained traits hitherto thought to be exclusive for Phytophthora. A careful review of previous research is presented and it is highlighted that uniquely for downy mildews, Poakatesthia may form an intracellular mycelium, growing through several host cells. In addition, scanning electron microscopy reveals that sporangiophore growth is not determinate in Viennotia and that outgrowth from sporangiophores is very similar to Phytophthora infestans. It is concluded that the sharp morphological distinction between downy mildews and Phytophthora species (that are often placed in separate families and even different orders), is rather artificial, since all features thought to be exclusive to Phytophthora or the downy mildews are united in the rare grass-parasitizing down mildew genera Viennotia and Poakatesthia and the enigmatic genus Sclerophthora. Therefore, several paradigms regarding the distinction between Phytophthora and the downy mildews need to be reconsidered. - Phytophthora species are among the most destructive rot-causing pathogens of plants, responsible for several catastrophic events, like the sudden oak death in North America, caused by Ph. ramorum [1] and the Irish Potato Famine caused by Ph. infestans [2,3]. These pathogens can be grown on synthetic media and are hemibiotrophic in nature. The downy mildews can cause severe damage in several important crops, such grape, cucurbits, sunflower, spinach, lettuce, sorghum, millet, tobacco and spinach. Downy mildews are generally believed to be obligate biotrophic and fully dependent on living host cells although the reasons why are still obscure so they can not be grown on media. Downy mildews are mainly host genus or even host species specific, while Phytophthora species often have a much wider host range. In addition to these characteristics, there are three main morphological characteristics that are thought to distinguish Phytophthora species from downy mildews. First, downy mildews do not form intracellular mycelium, but invade host cells only by haustoria [4], while Phytophthora is able to grow through both living and dead cells [5]. Second and equally important, sporangiophore growth is terminated in downy mildews, i.e. once sporangia are formed no further growth takes place, while in Phytophthora the sporangiophore may grow further after sporangia have formed. Third, it is generally believed that in downy mildews all sporangia ripen simultaneously, while in Phytophthora sequential maturation takes place. These differences were thought to be of major importance and have been used as an argument to postulate a deep divide between downy mildews and Phytophthora, placing the downy mildews in a family of its own, the Peronosporaceae, while Phytophthora on the basis of its thallus growth and general morphological characteristics has usually been placed in the family Pythiaceae of the order Pythiales [6,7]. Recent molecular phylogenetic investigations [812], however reveal that downy mildews are very closely related to a paraphyletic Phytophthora. Only in one study [10], was there significant support for a monophyly of the downy mildews, but this result could be attributed to an artefact due to a too narrow search radius, especially in Maximum Likelihood [13] phylogenetic reconstruction [14]. However, in all molecular phylogenies computed so far, the clade containing both downy mildews and Phytophthora was found to be monophyletic, often with maximum support [15,10]. This appears at odds with the developmental and morphological differences between these two groups and the fact that Phytophthora infected plants usually rot and die, whilst those infected with downy mildews may remain almost or completely asymptomatic for long periods and may even recover, particularly in secondary infections of natural populations. However, the effectors secreted by these pathogens are similar [1619]. The complex interactions involved in pathogenesis [2022] differ mostly in respect of the final necrotic phases of infection in Phytophthora which is initiated by necrosis inducing proteins [23]. Although necrosis can also be observed in later stages of infection in some downy mildews, such as Bremia, only seldom do plants die from infections with this pathogen. Bremia is not culturable on artificial media and therefore it is unlikely that necrotic parts of infected plants can serve as a source of nutrition for the hyphae. In general, the downy mildews have evolved into well adapted obligate biotrophic pathogens, which may be transmitted with the seeds of their host plants and many hardly cause observable symptoms. This is especially the case in Basidiophora and some species of Hyaloperonospora, which systemically infect their hosts and in effect develop endosymbiotically [24]. Because of the close phylogenetic relationship revealed by molecular studies and the similarity in the effector genes between the hemibiotrophic Phytophthora and the obligate biotrophic downy mildews, the question then whether some missing links between these two groups still exist. Recent molecular phylogenetic studies [1012,25], revealed that the graminicolous downy mildews appear to be sister clades to the monophyletic major clades of the downy mildews, which contain around 95 % of all species so far described (i.e. brassicolous downy mildews Hyaloperonospora, Perofascia; downy mildews with coloured conidia Peronospora, Pseudoperonospora; downy mildews with pyriform haustoria Basidiophora, Benua, Bremia, Novotelnova, Paraperonospora, Plasmopara, Plasmoverna, Protobremia). However, in none of the analyses to date did these apparent sister relationships receive high bootstrap support and therefore this molecular evidence must be treated cautiously. If the before mentioned artefact in phylogenetic reconstruction [10,14] is taken int (...truncated)