In this article we will discuss about:- 1. Introduction to Phytophthora Infestans 2. Vegetative Structure of Phytophthora Infestans 3. Reproduction.
Introduction to Phytophthora Infestans:
The most important species is Phytophthora infestans, which causes late blight disease of potato by which the Ireland famine took place in 1845.
The pathogen is most destructive in low temperature and high humidity. Some other species of Phytophthora are also pathogenic to different hosts. A list of pathogens along with diseases caused by them is given in Table 4.2.
Vegetative Structure of Phytophthora Infestans:
Vegetative body of Phytophthora infestans is of eucarpic thallus, consisting of n. celium. The mycelium is profusely branched and consists of aseptate hyaline, irregularly branched hyphae, where septa may develop in the older parts and at the base of sex organs. The branches arise at right angles.
The mycelium grows both intra- and intercellularly. During intercellular growth, it develops fingerlike haustoria inside the neighbouring host cells. The hyphae are about 4-8 µm in diameter with about 0.01 µm thick wall.
The cell wall lacks chitin and is mainly composed of glucan (about 90%). The protoplasm contains numerous nuclei, mitochondria, endoplasmic reticulum, ribosome, dictyosome and vacuoles.
Reproduction in Phytophthora Infestans:
Phytophthora infestans reproduces asexually and sexually.
1. Asexual Reproduction:
Asexual reproduction takes place by indirect and direct germination of sporangium and also by chlamydospore formation.
Sporangium Formation:
The sporangia develop on sporangiophore, which come out from the infected leaf through stomata or through piercing’ the epidermal cell towards lower side (Fig. 4.22A). The sporangiophore is branched (P. infestans).
The sporangium develops terminally (Fig. 2.22B-D), but it becomes shifted to lateral position by further growth of the sporangiophore (Fig. 4.22E, F) which develops again a new sporangium at its tip. Thus the sporangiophore behaves as sympodia and the branching of conidiophore of P. infestans is called sympodial branching.
The sporangium is thin-walled, hyaline to light yellow coloured, spherical, oval or pear- shaped with a small stalk-below and thin papilla above (Fig. 4.22G). It measures 22-25 pm x 16-25 µm.
The mature sporangium is detached from the sporangiophore and is dispersed by rain splash.
Depends on the environmental condition, the germination of sporangium may be indirect or direct. Low temperature (below 1 5°C) and wet condition i.e., high humidity (about 100%) favour indirect germination, while high temperature (above 20°C) and dry condition i.e., low humidity (below 90%) favour direct germination of sporangium.
During indirect germination, the content of sporangium breaks into 3-13 or more small uninucleate segments (Fig. 4.22H). Each segment becomes kidney-shaped zoospore and it develops two laterally placed flagella of which one being whiplash and other tinsel type (Fig. 4.22I). These zoospores can swim in water-film of rain or dewdrop.
On coming in contact with host surface, they first lose their flagella, become encysted (Fig. 4.22K) and then germinate by producing germ tube (Fig. 4.22L). The germ tube then penetrates directly through stoma and causes infection (Fig. 4.22M) or the germ tube at its tip may produce an appresorium. The appresorium develops infection peg towards its lower side and causes further infection piercing through epidermal cell.
During direct germination, the sporangium behaves as conidium and germinates directly by producing multinucleate germ tube which causes infection like that of zoospores.
2. Sexual Reproduction:
The sexual reproduction is of oogamous type which takes place with the help of the definite male and female reproductive organs, known as antheridium and oogonium, respectively. The sex organs arise at the tip of short lateral hyphae. The species of Phytophthora may be homothallic (P. himalayensis) or heterothallic (P. infestans).
Depending on the development of antheridium and oogonium, Phytophthora is of two types:
Paragynous and Amphigynous:
1. In Paragynous species (Fig. 4.23A) like P. cactorum, the antheridia are attached laterally on the oogonial wall, developed from same or different hyphae.
2. In Amphigynous species (Fig. 4.23B) like P. infestans, P. erythroseptica, and P. cactorum, the antheridium develops first and a nearby hypha penetrates through the developing antheridium and emerges out, its tip inflates to form a spherical oogonium. The antheridium remains as collar at the base of the oogonium.
The detailed process of sexual reproduction of Phytophthora infestans is given (Fig. 4.24):
Development of Antheridium in Phytophthora Infestans:
Antheridium is formed at the tip of the lateral hypha. The tip swells up and then a partition wall develops at the base which cuts off the upper antheridium containing one or two nuclei. The nuclei undergo repeated mitosis by stimulus of developing oogonium and form 8-12 nuclei. At maturity only one nucleus remains functional and the others degenerate.
Development of Oogonium in Phytophthora Infestans:
Oogonium also develops from the tip of neighbouring hypha. The hypha initially pierces the developing antheridium and its tip swells up to form a globose oogonium above the antheridium. Initially the oogonium is multinucleate and those nuclei again undergo repeated mitosis to form about 24 nuclei.
The protoplast of the oogonium differentiates into outer periplasm and central ooplasm. Initially both the regions are multinucleate, later all the nuclei except one pass from ooplasm to the periplasm. The nuclei in the periplasm later degenerate and the ooplasm with its one nucleus functions as an egg.
Fertilisation in Phytophthora Infestans:
After maturation of antheridium and oogonium, a single nucleus from the antheridium passes to the oogonium through fertilisation tube and fertilises the egg resulting in the formation of oospore. Karyogamy is delayed till the oospore wall matures.
The oospore is double-walled, consisting of comparatively thin exospore and thick endo- spore. Parthenogenic development of oospore also takes place in P. infestans.
Germination of Oospore:
The oospore undergoes maturation for several weeks to months. Before germination, the oospore nucleus (2n) undergoes repeated mitotic division and forms a number of diploid nuclei. The oospore then produces germ tube which cuts off terminal sporangium at its tip by transverse septa. The nuclei of the sporangium first undergo meiotic division followed by several mitotic divisions.
Each nucleus along with some cytoplasm metamorphoses into a biflagellate kidney-shaped zoospore. The zoospores after release from the zoosporangium remain motile for some time. On contact with suitable host, it becomes encysted and then germinates to form new mycelium again.
Some Views Regarding Meiotic Stage:
The place of meiosis in Phytophthora infestans is still not clear. According to Sansome (1963, 65, 67) and others, vegetative thallus is diploid and reduction division (meiosis) takes place in gametangium to form haploid gametes.
According to Day (1974) however, the place of occurrence of meiosis in the life cycle is still a vexing question. On the other hand, Webster (1980) viewed that reduction division occurs at the time of oospore germination and the zoosporangium bears haploid zoospores.