Biology of two common phyto-pathogenic Oomycetes

Phytophthora infestans

Obligate parasite.

Infect potatoes and tomatoes with late blight disease.

   

Life cycle

Mycelium:  Non septate, coenocytic

Asexual reproduction

In the host tissue inter-cellular somatic hyphae colonizes and club shaped haustorias penetrate the tissues, feeding the parasite.

Sporangiophores are produced from the hyphae and emerge through stomata.

Sporangiophores produce lemon-shaped sporangia laterally and at the terminal end/apex of the sporangiphore. A Sporangium possesses a papilla on its tip.

Outcome of the sporangium depends on the temperature and humidity.

Low humidity or high temperature: Sporangium germinates by a germ tube that later results the somatic hapha.  

High humidity (when raining) or low temperature (12⁰C or below: Cytoplasm of the sporangium divides and gives rise to biflagellate Zoospores. Either by dissolution of papilla or through its opening tip, zoospores are released to outside. Zoospores directly penetrate host tissue and undergo encystment in cells. They develop into germ tubes that later result the hyphae.

Sexual reproduction

Plasmogamy occurs as oogonium punches the antheridium, going through that antheridium, oogonium grows above it. The antheridium therefore appears as a funnel at the base around the oogonium. Karyogamy and meiosis occurs respectively. Oospore is spherical and adapted to adverse environmental conditions. It results out a sporangiophore that again develops into a sporangium.

via www.apsnet.org

Plasmopara viticola

Obligate parasite.

Infect grape vines with Downy mildews.

Life Cycle

Mycelium is non septate and coenocytic.

Life cycle is much similar to Phytophthora infestans as both are oomycites.

Asexual reproduction

Somatic hyphae develops in inter cellular space host tissue penetrating with haustoria.

Hyphae give rise to sporangiophores that emerge through stomata, branching and producing sporangia at each apex of the branches.

Sporangia may either germinate directly into mycelium or produce kidney shaped, biflagellate zoospores, depending on humidity and temperature just like the Phytophthora infestans. Zoospores encyst in cells, germinate and result the somatic hyphae.

Sexual reproduction

Sexual phase is taken place at unfavorable conditions. Plasmogamy occurs by gametangial copulation as oogonium and anthredium come close together. Karyogamy results zygote that later gives the spherical oospore. Oospore divides by meiosis and undergoes germination producing a sporangium.

More about downy mildew http://fruit.cfans.umn.edu/grape/IPM/downy.pdf

Simple facts about Betalain

Betalains are alkaloid pigments which are found in some families of plants belonging to the order Caryophyllales, but in no other plants.

They are named after the Beet family of plants (Beta).

Betalains are not found in plants containing anthocyanin pigments. Structurally they are unrelated. Unlike anthocyanins, they are not pH indicators as their colour is stable over a wide range of pH.  They are oxidised over time going brown in colour. This can be prevented by 0.1% ascorbic acid.

They have also been found in some fungi too e.g. Fly Agaric (mushroom)

Betalains can be divided into two types as betacyanins and betaxanthins; based upon their molecular structure.

Betacyanins

Usually appear red to red violet in colour (absorbance in 535-550nm)

Betaxanthins

Usually appear yellow in colour (absorbance in 475-480nm)

Betalains are found in the vacuole and they are water-soluble.

Also they cause colour in both flowers, fruits and sometimes vegetative organs

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Beetroot contains 2 Betacyanins. Thay are Betanin and a derivative.

Not much is known about the role of betalains. Commercially they are sometimes used as food colourants. As a food dye it’s cheap and no all allergic side effects are figured out yet.

Basic structure of betalain

Controlling metabolism at cellular level: Compartmentation

Metabolism is composed of reaction sequences termed as metabolic pathways. The changes at each step in a metabolic pathway are regulated in order to maintain the stability and survival of the organism. Mainly there are four ways of controlling metabolism. Such one important way of controlling metabolism at cellular level is compartmentation.

When considering a eukaryotic cell; it’s composed of many different compartments known as sub cellular organelles in contrast to a prokaryotic cell. Each organelle contributes a specific metabolic function with different enzymatic activities under different conditions. These metabolic pathways needed to be interconnected to maintain the metabolism. Therefore compartmentation allows the distribution of metabolites and enzymes between the membranes bounded organelles. It’s the method that enables the cell to obtain different metabolic activities at the same time for more energy efficiency. For an example, glycolysis occurs in the cytoplasm and provides the mitochondria substrates such as pyruvates from carbohydrate oxidation. TCA cycle occurs within the mitochondrion matrix while oxidative phosphorylation occurs in mitochondria inner membrane. However, altogether all these three processes are included in one metabolism called ‘cell respiration’. Enzymes of each process are found where the processes take place.

Fat metabolism is another one good example for compartmentation. Fatty acids synthesis takes place in cytoplasm. Cytoplasm contains the required enzymes for the anabolism of fatty acid synthesis. But these fatty acids are degregaded in mitochondria supplying Acetyl Co A for TCA cycle. Mitochondria also contain the enzymes require for the catabolism of fat.

RNA polymerase allows RNA synthesis by using a template of DNA strand. It catalyzes the synthesis of RNA. RNA synthesis takes place in nucleus. DNA transcription also occurs in nucleus. Therefore both the RNA polymerase and DNA templates are available, so that the RNA synthesis is favorable. When considering protein synthesis, all the organelles used for it are close to one another. The nucleolus makes ribosome which synthesizes proteins. Endoplasmic reticulum is near the nucleus to receive Ribosomes and synthesized proteins. The Golgi apparatus is also near the endoplasmic reticulum for storing, packaging and distribution. Therefore all the activities within a cell are cooperated with one another.

Hence, compartmentation helps metabolic controlling by maintaining substrates, regulators and enzymes in separate locations with only favoured access between them.

Plant senescence in brief

Senescence

Senescence is an important program in which the function of a cell/tissue/whole plant naturally leads to its death.

Senescence involves in plant aging.

When tissues senesce they produce enzymes which can recycle the “expensive” materials and reroute the sub-units to somewhere else that growth takes place.

Overall senescence

·         Occurs in the whole plant.

·         Death takes place right after flower and setting.

·         When the flower dies the whole plant dies too.

·         Ex: Asteraceae ( Sun flower)

image via Wikipedia

Top senescence

·         After the growth season the part above the ground dies remaining the underground part that can grow for several years.

·         Therefore a rhizome is present.

·         Ex: Weed, Grass

       

Deciduous senescence

·         In dry seasons like winter and summer, the leaves falls and trunk remains.

·         Leaves fall due to avoid the heavy transpiration occurs in the dry season.

·         Ex: Deciduous plants

image via ehow.co.uk

Progressive senescence

·         Older parts die due to the senescence and the new organs and tissues continue their development.

·         Ex: Green trees

image via clasohm.com

As above mentioned, senescence can occur in cells as well as tissues.

Cell senescence

·         Can be described according to the senescence in cell membrane and organelle level.

Senescence in cell membrane

Lipid phase change

·         Plasma membrane has the fluid mosaic structure.

·         This structure changes as the liquid crystalline state changes into the solid-gel state.

·         Therefore the membrane gets inflexible and hard.

Degradation and preoxidation of lipids

·         Lipid content gets decreased.

·         It occurs as the lipid synthesis decreases and lipase content increases.

·         Phospholipase, lipoxygenase, MDA and active O₂ content increase.

Increase in Phospholipase activity

·         Mainly the activity of Phospholipase D takes place in plants.

Biomembrane degradation and leakage

·         The equilibrium of ions breaks down.

·         Metabolisms get disordered.

Senescence in organelle level

·         Ribosomes and RER break down.

·         Chloroplasts collapse and mitochondrion cristae are swollen.

·           As the vacuoles break down, all the toxics inside them is released to the cell plasma.

·         Therefore the autophagy takes place and the senescence of organelles activates.