puccinia: structure,lifecycle

puccinia:

Kingdom                 : Mycetae

Division                   : Amastigomycotina

Sub division            : Basiomycotina

Class                        :  Basidiomycetes

Sub class                 : Teliomycetidae

Order                       : uredinales

Family                     : pucciniaceae

Genus                      : puccinia         

occurrence: 

• the genus Puccina causes rust diseases of many of mans economic plants like barley, wheat, Oats, etc. the name puccina has been given to the genus after  T. Puccini<, an Italian scientist. It is called rust because of the reddish brown color of the spores that are found chiefly upon the surface of the host leaves and stem.
• The most important of all the rusts is P. graminis which causes black stem rust of wheat. It causes the most serious damage to the wheat crop. It is an internal obligate parasite & is found everywhere wheat is grown.
• It is heteroeious, i.e. Requires two host (triticum vulgare) and barbery (Beta vulgaris) to one normal life cycle. Mycelium of one host is unable to grow on the other, and the full life cycle only when  both the hosts are present. The wheat plant is called the primary host and the barbery the secondary or alternate host.
• It is macro cyclic & polymorphic rust, in which life cycle involves five spore forms, each with a different function. The different spore form follows one another in a definite sequence within a life span of one year.

• Vegetative structure: 

• mycelium is well developed and consists of septate and intercellular hyphae, obtthe myaining the nourishment by sending small, round or branced haustoria into the host cells. There is single central pore in each spectrum for the protoplasm communication from one cell to another. The hyphae do not ramify throughout the interior of the host but are restricted to isolated patches on the organ attached.

during the life cycle two types of mycelia are produced, the monokaryotic mycelium found in the alternate host & the dikaryotic mycelium in the primary host.
 

LIFE CYCLE OF FUNGUS:

Uredo stage: 

• the wheat plant becomes infected by aecidiospores which are produced by the fungus on the alternate host (barbery leaf). These aecidiospores are blown off by wind & can infect only the wheat plants. Since the aecidiospores are biuncleate it gives rise to binucleate mycelium (dikaryotic). The hyphae produced ramify through the host tissue, forming haustoria that draw nourishment from surrounding cells of the wheat plant but do not kill them. In the early summer the mycelium leads to the formation of elongated, rust & reddish brown streaks or pustules termed as uredopustules or uredorus on the stem and leaf sheath. This is developed from a mass of dikaryotic hyphae which collect below the epidermis of the host. From the base of the uredosori arise specialized hyphae the sporophores which bear at their apices oval, thick walled brown uredospores. The epidermis of the host plant becomes ruptured by the pressure of the underlying uredospores forming elongated, rusted reddish brown pustules.
• Each uredospore is stalked, broadly ovoid, brown & finely echinulate. It has four germ pores. The uredospores formed are flown by wind currents & carried to other wheat plants where they germinate to produce new crop of uredospores.
• Germination of uredospore within 4-5 days the uredospore germinates producing germ tube, the germ tube does not enter the host tissue directly. It grows over the surface of the epidermis & on reaching stoma, the tip swells up to form an elongated ‘appressorium’. The protoplast of the germ tube migrates into the appressorium & the empty germ tube is cut off from appressorium by cross wall. The appressorium gives rise to a fine branch that passes through the stomata & swells to form a substomatal vesicle from which arise one or more hyphae which ramify through intercellular spaces of the host tissue producing much branched mycelium made up of binucleate cells.
• The uredospores are also called repeating spores because they are produced in successive crops in a season causing widespread infection from plant to plant throughout the field.

• Teleuto stage: 
• letra in summer, after the host plant matures, the same mycelium that gave rise to uredospores now begins to give rise to teliospores or teleutospores. They are produced in the same pustule or develop new pustules termed as teleutosorus or teleutopustules. These telia form elongated, dark brown to black ustules on the stem & leaves of the host plant. Teleutospores are dark brown to black pustules on the surface of the stem & leaves of wheat & it has resulted in the common name black stem rust of wheat for this fungus.
• The teleutospores are stalked, dark brown to black in color, thin walled & two celled, each cell containing a pair of nuclei. The apex of each teleutospore is round or sometimes pointed. Each cell of the spore has a germ pore. The germ pore is situated apically in the upper cell, while in the lower cell; it lies just below the septum.
• When the spore is mature the two nuclei fuse resulting in diploid nuclei each cell. The teleutospores are not capable of immediate germination. It germinates only in the spring.

Basidial stage: 

• during the early spring the teleutospores germinate on ground. Under favourable conditions each cell of the teleutospores forms an elongated germ tube like structure called promycelium (epibasidium). The diploid or fusion nucleus of teleutospores migrates into the promycelium where it undergoes meosis & forms. 4 haploid nuclei each of which later forms into the baisdiospore nuclei. Each basidiospore is stalked (sterigmata). Each promycelium gives rise to four basdiospore 2 of + & 2 of –ve strain. Basidiospores infect only barbery.

Pycnidial or spermogonial stage:

•  in the presence of moisture the basidiospore germinate on the surface of the barbery leaf forming a slender germ tube. The germ tube penetrates into the epidermis directly & there it branches freely forming haploid monokaryotic mycelium of either + or –ve strain depending on the Basidiospores strain.
•  Many basidiospores land and infect the same barbery leaf so that both + & -ve mycelia are developed side by side in the barbery tissues.4 days after infection of the host the monokaryotic mycelium branch & grow vigorously between epidermis & the palisade to form dense mat. There are primordial of pycnia (spermogonia).These are seen as yellow or red patches on the leaf surface.
• In addition to the formation or pycnia primordial few monokaryotic mycelium grow towards the lower epidermis & form structures called ‘aecia primordial’. These mycelium do not develop further unless & until it is dikaryotized.
• The spermagonia may be+ or  -ve depending on the mycelium from  which it arises. A mature spermogonium is flask shaped with an opening called ostiole.
• At the base of the spermoginal cavity pseudo parenchymatous mass of uninucleate cells are present from which arise number of elongated uninucleate spermatiophores. Spermatiphores gives rise to pycnidiospores(spermatia) in chains.they ooze out on the leaf surface in small droplets of nectar through the ostioles.other than spermatiophores thin hyphae arise which are long & protrude out from the  ostiole which is the receptive hyphae(female gametangia).spermatia act as male cells.
• Insects like flies are attracted to the leaf by sugary necter. Some spermatia stick to the leg & proboscis of the visiting insects and thus are carried from one pycnium to the receptive hyphae of other pycnia. If a pycniospore of one strain comes into contact with a receptive hyphae projecting from a pycnium of opposite strain or vice versa, they fuse and the nucleus passes into the receptive hyphae moves downward establishing dikaryon condition. The nucleus repeatedly divides  & the resulting nuclei pass through pores in the hyphae cross walls.

Aecidial stage: 

• Each aecium primordium, which has been previously formed on the lower surface of the host leaf by the same mycelium that gave rise to spermogonium on the upper surface of the leaf, consist only of an upper closely packed weft of uninucleate hyphae the basal cells & a lower group of parenchyma like cells.
• Fusion of the spermatium with receptive hyphae is followed by migration of spermatial nuclei into the receptive hyphae & through septal perforation of the mycelium to the basal cells of aecial primoridia which thus became dikaryotized. Each binucleate cell has a + & a –  ve  strain nuclei. If due to any reason dikaryotisation fails, the protoaecidium aborts & the aecidial cup is not formed.
• The diakryotized (binucleate) basal cells are called aecidiospores mother cells or sporophores & from them are cut off terminally the chains of biuncleate spores the aecidiospores, alternating with sterile cell or disjunction cells. The neighboring marginal  at the base of the aecidium divide & form the pridium which surrounds cells in aecidium.
• The aecidial cup remains partly embedded in the leaf tissue & partly projects above it. The edge of peridium bends upward & a bell shaped structure is formed. They appear on the lower surface structure of the infected leaf as white cups, filled with golden yellow powder of spores. The developing aecidium exerts pressure on the leaf tissue & rulptures the peridium as well as the lower epidermis & as a result, the exposed binucleate aecidiospores fall from the aecidium.
• These aecidiospores are unicellular, thin walled. Polyhedral in shape & areunable to reinfect barbery but it infects only the wheat plants repeating the life cycle.