Lecture - 7 Diplomastigomycotina: Peronospora
Sub-division: Diplomastigomycotina:- Flagellated cells bear two different types of flagella whiplash type as well as tinsel type. It includes 1 class -
Oomycetes:-
> Most of the members are aquatic but some grow on the soil and the others attack the shoots of terrestrial plants.
> The primitive forms are unicellular, whereas the advanced forms have a well-developed branched, filamentous, and coenocytic mycelium.
> The cell wall is mainly composed of cellulose-β-glucan and chitin is altogether absent.
> A sexual reproduction takes place by means of pyriform or reniform biflagellate zoospores, which are devoid of cell wall. In pyriform zoospores, the flagella are attached anteriorly and in reniform laterally. One flagellum is of whiplash type and the other of tinsel type.
> In primitive aquatic form, zoosporangia are not well differentiated from somatic hyphae, whereas in advanced forms distinct zoosprangia are found.
> Some forms also produce non-motile asexual spores, known as conidia.
> Most of the Oomycetes are eucarpic, but the members of the order Lagenidiales are holocarpic.
> The sexual reproduction is of oogamous type. The male and female sex organs are known as antheridia and oogonia, respectively. The sex organs are either terminal or intercalary.
> It is now established that meiosis in Oomycetes takes place within the diploid nuclei of gametangia (antheridia and oogonia) hence is called gametangial-meiosis. That is, zygotic meiosis is absent. As a result, the vegetative thallus is diploid.
> The gametes are non-flagellate. The fusion of gametes results in the formation of thick-walled oospores.
> Oomycets show interesting parallel changes in their biology and morphology, which are closely connected with their phylogeny.
> Classification:-
4 Orders:-
a. Laginidiales:-
Family:- Lagenidiaceae
Eg.- Lagenidium, Lagenisma, Myzocytium
b. Saprolegnials:-
Family:- Saprolegniaceae
Eg.- Saprolegnia, Leptolegnia, Geolegnia, Brevilegnia, Aphanomyces, Achlya, Isoachlya, Dictyuchus, Pythiopsis, Thraustotheca
c. Leptomitales:-
2 Families:-
i. Leptomitaceae:- Eg.- Leptomitus, Apodachlya
ii. Rhipidiaceae:- Eg.- Aqualinderella, Rhipidium, Apodachlyella
d. Peronosporales:-
4 Families:-
i. Pythiaceae:- Eg.- Pythium, Phytophthora
ii. Peronosporaceae:- Eg.- Peronospora, Sclerospora, Plasmopara, Basidiophora, Bremia
iii. Peronophythoraceae:- Eg.- Peronophythora
iv. Albuginaceae:- Eg.- Albugo
Peronospora:-
1. Classification:-
Kingdom:- Myceteae
Division:- Mastigomycota
Sub division:- Diplomastigomycotina
Class:- Oomycetes
Order:- Peronosporales
Family:- Peronosporaceae
Genus:- Peronospora
2. Habit and Habitat:-
> 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.
> 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.
> There are also many Peronospora species that are spread by wind currents, which allows them to disperse over large distances.
> Peronospora species prefer humid air and cool temperatures.
> It causes downy mildew disease in crops.
i. P. brassicae:- Downy mildew of Mustard.
ii. P. destructor:- Downy mildew of onion.
3. Thallus structure:-
> Well developed and profusely branched mycelium.
Hyphae are aseptate, coenocytic and branched.
4. Reproduction:-
> The first stage in the Peronospora life history is the sporangia.
> The sporangia are small spore-like structures about 65 um long that germinate a germ-tube when they are near a leaf stoma.
> A germ tube will come from the sporangium and penetrate the leaf cell where it will form a haustorium.
> The haustorium absorbs nutrients from the leaf, while hyphae invade the intercellular space, and the leaf will eventually develop a lesion.
> These lesions often start out yellow and then turn brown as the leaf starts to undergo necrosis.
> From here, Peronospora can undergo either asexual reproduction or sexual reproduction.
a. Asexual reproduction:-
- Asexual reproduction occurs when the air outside is moist making for favourable conditions.
- During asexual reproduction, hyphae on the host plant will form sporangiophores, which will produce conidia.
- The conidia will be dispersed by the wind is able to infect other plants.
- The asexual cycle only takes five to 7 days to complete.
b. Sexual reproduction:-
- Sexual reproduction occurs when the conditions are unfavourable and it needs to withstand harsh environmental conditions.
- During sexual reproduction, the hyphae will undergo meiosis forming antheridia and oogonia, the only haploid structures in the Peronospora life history.
- The antheridia will fuse to the oogonia, initiating plasmogamy and then karyogamy, and will result in the production of many oospores.
- The oospores can then be dispersed by the wind to infect more plants.
- Peronospora is characterized by their ability to produce melanized sporangia, but do not produce zoospores.
