The pciniaresentation describe the life cycle of Pythium, Albugo, Erysiphae, Claviceps, Ustilago, Puccinia. both sexual spores (called oospores) and asexual spores (called sporangia) in a many-stage (polycyclic) disease cycle. infection stag heads oospore productt0n zoo spores. \. SEXUAL c germ1not1~. Oospores -overwintering spores. 8. Figure 8. Life cycle of Albugo candlda.
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In this article we will discuss about the life cycle of albugo with the help of suitable diagrams. It is well developed and consists of branched, aseptate, coenocytic hyphae. The hyphae live and ramify in the intercellular spaces of the susceptible lie tissue. The hyphal wall contains cellulose and not chitin.
The hyphal protoplasm is granular and vacuolate in the older parts. It contains numerous nuclei, oil globules and glycogen. Electron micrographs reveal or presence of mitochondria, endoplasmic reticulum and ribosomes. The cytoplasmic membrane which is closely appressed to the hyphal wall forms lomasomes. Septa remain suppressed in the actively growing hyphae but appear to separate reproductive structures and to seal chcle injured parts.
The fungus mycelium grows vigorously. The hyphae branch and ramify within the host attacking the tissues adjoining the point of infection. Sometimes both Albugo and Peronosporaoccur on the same host particularly Capsella bursapastoris.
Albugo can, however, be distinguished from Peronospora by the smaller diameter of its hyphae and more numerous, vesicular haustoria Fig. The intercellular hyphae of this obligate parasite produce intracellular haustoria in the mesophyll cells of the host Alubgo. The haustorium arises as a lateral outgrowth at cyce site where the hyphal wall is tightly pressed against the mesophyll cell wall. An electron- dense amorphous material known as the penetration matrix is usually deposited at the site of contact between the host and the hypha cell walls.
Albugo – Wikipedia
It is described as the penetration site Fig. The slightly crescent-shaped bulge of the haustorial mother cell known as haustorial initial perforates the host cell wall at the penetration site and protrudes into the lumen of the mesophyll cell to develop into a haustorium. It is albuto by the invaginated host plasma membrane. With light microscope cyclr haustorium is seen as a small, spherical structure consisting of two parts namely: The haustorial stalk passes through the penetration site to connect the haustorial body to the hyphal wall in the intercellular space between the mesophyll cell.
Usually one or two, sometimes more, haustona are seen in the thin peripheral layer of the host cell cytoplasm albuo to the chloroplasts. When the mycelium has reached a certain stage of maturity it epidermis produces pads of hyphae at certain areas just below the epidermis. The tips of hyphae constituting the mat grow verticlly into short, upright, thick-walled, unbranched club-shaped hyphae.
They are arranged in a closely packed palisade like layer forming a sorus between the epidermis and the mesophyll of the host leaf. Each sporangiophore cycld constricted at its junction with the subtending hypha. The lower two-third portion of sporangiophore is narrow, thick-walled, with a undulating surface whereas the upper one-third is broader, thin-walled with a smoother surface.
According to Khanthe sporangiophore wall towards its proximal end consists of two layers, the outer more electron-dense and thicker than the inner layer. In cydle lower fungi Phycomycetes Albugo is unique in that its lemon- shaped sporangia are produced in basipetal chains at the tips of clavate sporangiophores.
Two different views have been put forth to explain their mode of development. According to one view, the sporangial chains in Albugo are abstricted by percurrent proliferation. The second view advocates the blastic mode of development. The sporangia in Albugo which are cut off in succession are arranged in a basipetal chain albguo the sporangiophore.
According to Hughesthey are produced by successive proliferations of the sporangiophore subtending a sporangium. This mode of development of sporangia is termed per-current proliferation. Albjgo the sporangiophore increases in length as each successive sporangium is cut off from each successive proliferation at a higher level than the previous one.
The first formed sporangium is cyclf aleuriosporangium. Reaching a certain size it is delimited from the sporangiophore by a basal septum. The latter eventually splits into two halves so that the subsequent proliferation of sporangiophore involves the exposed half septum.
In Albugo each successive sporangium is capable of seceding from the sporangiophore or from the young sporangium. The second sporangium is thus formed by proliferation of the sporangiophore with total involvement of the half of the fractured transverse septum exposed by the seceding first sporangium above it. Apart from this, septum is seen at the apex of the young sporangium.
Od second sporangium is delimited in the same manner as the first.
As the second sporangium increases in size it pushes the first upward without disjunction. The process is repeated resulting in a chain of sporangia. Probably the septum seen at the apex of each younger sporangium thickens on both sides to form a connective between the successive sporangia in the chain.
According to Hughes, besides increase in length of sporangiophores, this method of sporangium development is accompanied by marked lamination and thickening of the walls of the sporangiophores.
The mature sporangiophores are thus longer, more thick-walled and show annellations. Thakur corroborated findings of Hughes on formation of sporangia by percurrent proliferation in Albugo. According to Khan the sporangiophore has a fixed sporogenous locus at its apex.
The sporangial initial arises as a bud from it A. It contains about nuclei and dense cytoplasm. The two wall layers of the sporangium initial are continuous with those of the sporangiophore wall. Reaching a certain size, the initial is delimited by a basal septum near the sporogenous locus.
It becomes the first sporangium and the oldest in the chain B. The septum is formed by the centripetal growth of the inner layer of the sporangiophore wall C.
A nearly complete septum has a narrow central canal and consists of three layers, upper and lower electron dense and the thick middle one of less electron density D. After the completion of the basal septum and conversion of the initial into a full-fledged sporangium, a new sporangium initial grows as a bud from the sporogenous locus B.
It pushes the newly-formed sporangium upward. Thus only one sporangium is formed at a time. As the second sporangium initial grows to the normal size, it is also delimited by the formation of a basal septum as the first. The repetition of the process results in the formation of a basipetal chain of sporangia.
Soon after the formation of the first sporangium, the breakdown of its basal septum begains. It is the middle layer which starts disintegrating from its periphery inwards whereas its upper layer fonns the wall of the upper sporangium and the lower layer completes that of the lower sporangium E-F.
The fibrous product of dissolution of the middle layer is held in position by the pellicle which covers both the sporangia and the sporangiophore.
It is seen as a connective or disjunctor between the successive sporangia in the chain. Khan did not notice any increase in length of the sporangiophore during sporangia formation nor did he observe any annellations on the sporangiophore surface. They are small, hyaline, nearly spherical or lemon- shaped structures with a smooth or somewhat punctate surface.
The basipetal arrangement of sporangia in the chain with the oldest at the top and the youngest at the base of the chain serves two useful purposes: It bears remnants of the connectives or disjunctor pads at both the ends.
The sporangium wall is differentiated into two distinct layers. The outer is more electron dense than the inner. Within the sporangium wall is the highly convoluted plasma membrane enclosing the dense cytoplasm containing up to 4 nuclei. Besides, the cytoplasm contains endoplasmic reticulum, mitochondria, perinuclear, dictyosomes, ribosomes both free lifr attached to endoplasmic reticulum, vesicles of various kinds and lipid droplets.
Towards maturity the sporangial wall especially, its inner layer increases in thickness and alubgo number of lipid droplets decrease as the sporangia matured. The oldest lfe have none. The endoplasmic reticulum becomes accumulated in the peripheral cytoplasm. Towards the end of sporangial maturation, the dictyosomes become quiescent, mitochondria decreased in number and also the amount of endoplasmic reticulum.
The sporangial wall increased 3-fold the thickness. The chains of sporangia lengthen and press on the epidermis above. This causes the leaf surface to bulge. The overlying epidermis eventually bursts over the growing sporangial sorus and exposes the white shining pustules consisting of masses of sporangia. The pustules look like white blisters.
The exposed sporangia are white. The distal ones by this time have matured. As the sporangia mature the connectives or gelatinous pads between them dry, shrink and finally disintegrate in moist air. The sporangia in the chain thus separate. They are then blown away in the air by wind or washed away by rain water.
Landing on a suitable host the sporangia begin to germinate within two or three hours under suitable conditions. At the time of germination they behave in either of the following two ways depending on temperature conditions: In the presence of moisture and low temperature, the sporangium xycle as a zoosporangium B.
It absorbs water and swells. A few vacuoles appear in its granular cytoplasm. Later the vacuoles disappear and the multinucleate protoplast undergo division. It divides to cyclle five or eight polyhedral uninucleate daugher protoplasts. Meanwhile an obtuse papilla forms on one side of the sporangium. Each daughter protoplast shapes into a slightly concave-convex zoospore E.
It has a disc-like contractile vacuole on one side and is furnished with two flagella, one short and one long. The former is of tinsel type and the later whiplash.