In this article we will discuss about the life cycle of phyllactinia with the help of suitable diagrams.
Mycelium of Phyllactinia:
The mycelium is partly internal and partly found on the surface of the host. The mycelium spreads on both the surfaces of the host. The hyphae are branched, septate and uninucleate. Although haustoria are not present in epidermal cells, are a small 5-7 celled branch produced by the superficial hypha penetrates the leaf and enters through stromata.
This may develop in substomatal chamber and the intercellular spaces of the adjacent cells. This hypha may give rise to haustoria which are saccate. Since the mycelium is partly found on the surface of the host and is also partly internal, it is known as hemi-endophytic mycelium.
Reproduction in Phyllactinia:
After the fungus has established itself in the host, it starts reproducing asexually during favourable conditions of growth. The asexual reproduction takes place by the formation of conidia produced exogenously in chains at the tip of conidial apparatus.
Development of Conidia:
The conidia are formed on conidial apparatus known as conidiophores. These develop vertically from the superficial mycelium. In the beginning these are formed on both the surfaces of leaves but later they are confined to the lower surface only. The young conidiophore divides transversely to produce two cells-the basal stalk or stipe cell and the upper conidium mother cell.
The latter again divides to form two cells-the upper cell acts as a conidium and the lower cell as subterminal cell. When the first formed conidium is shed, the subterminal cell divides again to form another conidium. Generally the conidia are formed singly but in large numbers forming a white powdery mass on the leaf surface.
Structure and germination of conidium:
The conidium is single celled, clavate, uninucleate, thin walled and hyaline. The conidia are disseminated by wind and upon reaching a suitable substratum germinate to form a new mycelium.
It takes place under unfavourable conditions. The fungus is heterothallic and the gametangia are formed on two closely lying hyphae. The male gametanium is antheridium and the female gametanium is ascogonium.
(a) Antheridium is small clavate, one celled and uninucleate structure.
(b) Ascogonium is thick, ovoid, one celled, uninucleate and closely applied to or coiled around the antheridium.
(c) Plasmogamy- When mature, the wall between the two sex organs dissolves and the nucleus from the antheridium migrates into the ascogonium. Plasmogamy takes place followed with nuclear pairing.
(d) Post Plasmogamy Changes:
After the pairing of the nuclei has taken place, a common binucelate cell is formed. Each nucleus of the pair divides simultaneously. The fertilized ascogonium also divided to form a row of 3-5 cells, the penultimate cell of the row being dikaryotic. This dikaryotic cell produces ascogenous hyphae and subsequently asci.
The young ascus has a dikaryon i.e two nucelei which fuse to form a diploid nucleus. This diploid nucleus divides by meiosis and then by mitosis To form eight nuclei. Of these eight nuclei, six degenerate and only two nuclei survive. These after collecting cytoplasm around them give rise to two ascospores. At maturity each ascus has two ascospores.
Simultaneously, the vegetative hyphae grow upwards from the basal eel ascogonium to form a 2-3 layered sheath around the developing ascocarp (Cleistothecium).
The mature cleistothecium is brown or black in colour, globose with several layered wall and bears appendages.
The appendages are of two types:
(a) An equatorial group of radiating appendages with bulbous swollen bases and
(b) A crown of repeatedly branched mucilage secreting appendages which help in adhesion of cleistothecium to the substratum.
After completion of dormancy, the cleistothecium ruptures irregularly by elongation of asci. The asci come out and the ascospores are liberated from the asci.
The ascospores are ovate to elliptical, haploid, uninucleate and smooth walled. Upon reaching a suitable host, the ascospore germinates to form new mycelium which soon becomes ready to produce comidiophores and conidia.