Abstract
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.
Highlights
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]
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, in secondary infections of natural populations
In light microscopy (LM), the apophyses, which are typical for Phytophthora, can be observed in Viennotia (Fig. 1)
Summary
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]. 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 whether some missing links between these two groups still exist. The basic question arising from this molecular work, addressed in this study, is whether there are any characteristics in the graminicolous downy mildews that are reflecting the close relationship of downy mildews and Phytophthora and the rather basal position of the graminicolous downy mildews that was revealed by recent molecular phylogenetic investigations
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