Stomata

Stomata are minute structures on plants found on the outer leaf skin layer, also known as the epidermis. They consist of two specialized cells, called guard cells that surround a tiny pore called a stoma. The word stomata means mouth in Greek because they allow communication between the internal and external environments of the plant. Their main function is to allow gases such as carbon dioxide, water vapor and oxygen to move rapidly into and out of the leaf. Stomata are found on all above-ground parts of plants including the petals of flowers, petioles, soft herbaceous stems and leaves. They are formed during the initial stages of the development of these various plant organs and therefore reflect the environmental conditions under which they grew.

Leaves are the main "food manufacturing" organs of plants. They make food from carbon dioxide and water in the presence of light during a process called photosynthesis. As stomata open in the presence of light, carbon dioxide will diffuse into the leaf as it is converted to sugars through photosynthesis inside the leaf. At the same time, water vapor will exit the leaf along a diffusive gradient through the stomata to the surrounding atmosphere through the process of transpiration. Consequently, plants face the dilemma of taking up carbon dioxide while losing water vapor through their stomata. If this water loss remains unchecked, they can deplete their water reserve. This depletion can become catastrophic to the physiological functioning of the plant given that is the most essential solvent in which biochemical and growth processes occur. Based on Darwinian principles, it is presumed that selective adaptation has driven plants to acquire characteristics which enable them to grow more quickly without diminishing the probability of survival. If plants have not acquired the characteristics to withstand changes in water availability in their growth environment, plants may exacerbate their water shortage by not regulating the size of their stomata apertures in an optimal manner and may fail to survive when water availability declines.

Encyclopedia of earth

Bacillus mucilaginosus

Bacillus mucilaginosus is a common soil bacterium, and usually used as a model bacterium in studying microbe-mineral interactions. Several reaction mechanisms of B. mucilaginosus weathering silicate minerals were proposed. However, the molecule mechanisms and detailed processes were still unclear.

Bacterium-mineral interactions were studied in terms of variations in pH value over the experimental period, variations in mineral composition, weathering rates of silicate minerals and volatile metabolites in the culture medium, etc., to further explore the bacterium-mineral interaction mechanisms. The results showed that B. mucilaginosus could enhance silicate mineral weathering obviously. The weathering rates were quite different for various kinds of silicate minerals, and the weathering rate of weathered adamellite could reach 150 mg/m2/d. Although B. mucilaginosus produced little acidic substance, pH in the micro environment of bacterium-mineral complex might be far lower than that of the circumjacent environment; a large amount of acetic acid was found in the metabolites, and was likely to play an important role as a ligand.

These results appear to suggest that acidolysis and ligand degradation are the main mechanisms of B. mucilaginosus dissolving silicate minerals, the formation of bacterium-mineral complexes is the necessary condition for the bacteria weathering silicate minerals, and extracellular polysaccharides played important roles in bacterium-mineral interaction processes by forming bacterium-mineral complexes and maintaining the special physiochemical properties of micro environment

by MO Binbin  and LIAN Bin

Bacillus megaterium

Bacillus megaterium is a gram positive, spore producing bacteria. It is a Eubacteria and is found in the soil. It has a rod shape and is one of the largest Eubacteria. Colonies form in chains due to sticky polysaccharides on the cell wall. It is very important in the biotech industry due to its size and its' enzyme and cloning abilities. The enzymes help produce synthetic penicillin, modifying corticosteroids and several amino acid dehydrogenases. It also is very efficient in cloning due to no protease degradation of product and good stability of recombinant plasmids.

By Rob Hayden

Azotobacter chroococcum

Azotobacter is a genus of free-living diazotrophic bacteria whose resting stage is a cyst. It is primarily found in neutral to alkaline soils, in aquatic environments, and on some plants. It has several metabolic capabilties, including atmospheric nitrogen fixation by conversion to ammonia. Their unique system of three distinct nitrogenase enzymes makes these bacteria of particular interest to scientists, who may work toward a better understanding of nitrogen fixation and its role in agriculture. Azotobacter spp. have the highest metabolic rate of any organisms.

Azotobacters, interestingly, contain more DNA than most other bacteria, but their genome size is typical of most prokaryotes. The reason for this above average amount of DNA is not known, but it is possibly because the cells of Azotobacter are larger than those of other bacteria. The DNA of Azotobacter spp. display many similarities, in terms of gene type and recognition factors, to the DNA of Escherichia coli. Genetic information can be transferred between azotobacters or to other bacteria by way of conjugation or transformation. For NCBI's GenBank entry for Azotobacter's unfinished

Diazotrophic organisms such as Azotobacter play a vital role in every ecosystem, working to make nitrogen available to all organisms. Azotobacters and similar bacteria turn nitrogen into ammonia through the process of nitrogen fixation, after which the ammonia is turned into proteins. Nitrogen fixation is used in agriculture in relation to crop rotation and fertilization; soil-dwelling diazotrophs such as Azotobacter are especially useful in gauging the health and virility of the ground. Azotobacters are found worldwide, in climates ranging from extremely northern Siberia to Egypt and India.

From MicrobeWiki

Rhizobium

A classical example of nitrogen fixation is the symbiosis leguminous plants and bacteria of the genus Rhizobium.The seat of symbiosis is within the nodules that appear on the plant roots. In the symbiotic association between Rhizobium and leguminous plant, two organisms interact in such a way as to influence and coordinate the expression of both prokaryotic and eukaryotic gene.

The communications that occur between the plant and the rhizobia during nodule formation and maintenance constitutes a novel opportunity to study signal transduction in a plant system. The expression of nodulation genes in the bacteria is activated by signals from plant roots and as a result the bacteria synthesise signals that induce a nodule meristem and enable the bacteria to enter this meristem via a plant-made infection thread. The chemical signals synthesised by the bacteria are based on a modified amino acid (homoserine lactone) carrying a variable acyl chain substituent, and are called acyl homoserine lactones (AHLs).

By detecting and reacting to these chemicals, individual cells can sense how many cells surround them and whether there are enough bacteria, i.e.a quorum , to initiate the change towards acting in a multi cellular fashion. This is known as 'quorum sensing'.

Beijerinck was the first to isolate and cultivate the bacterium responsible for nodulation and he named it Bacillus radicicola which is now placed in the genus Rhizobium.

Rhizobium is found as a free-living organism in the soil but does not fix atmospheric nitrogen in that state. Only after its association with a leguminous plant and after the formation of root nodules does it fix atmospheric nitrogen in the nodules.

Source : microbiologyprocedure.com



Nitrogen fixation

Nitrogen fixation is the process by which nitrogen is taken from its natural, relatively inert molecular form (N2) in the atmosphere and converted into nitrogen compounds (such as ammonia, nitrate and nitrogen dioxide).

Nitrogen fixation is performed naturally by a number of different prokaryotes, including bacteria, actinobacteria, and certain types of anaerobic bacteria. Microorganisms that fix nitrogen are called diazotrophs. Some higher plants, and some animals (termites), have formed associations with diazotrophs.

Nitrogen fixation also occurs as a result of non-biological processes. These include lightning, industrially through the Haber-Bosch Process, and combustion.

Biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck.

Biological Nitrogen Fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a pair of bacterial enzymes called nitrogenase. The formula for BNF is:

N2 + 8H+ + 8e− + 16 ATP → 2NH3 + H2 + 16 ADP + 16 Pi

Although ammonia (NH3) is the direct product of this reaction, it is quickly protonated into ammonium (NH4+). In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.

In most bacteria, the nitrogenase enzymes are very susceptible to destruction by oxygen (and many bacteria cease production of the enzyme in the presence of oxygen). Low oxygen tension is achieved by different bacteria by: living in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as Leghemoglobin - also spelled leghaemoglobin..

The best-known plants which contribute to nitrogen fixation in nature, are in the legume family - Fabaceae, which includes such  as clover, beans, alfalfa, lupines and peanuts. They contain symbiotic bacteria called rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (family Polygonaceae), which were often referred to as "green manure", since the other natural way of adding nitrogen to the soil is via animal waste products. The entire plant is often ploughed back into the field, thus not only adding more nitrogen, but also improving the soil's organic content and volume.

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