Question

What physical and chemical traits are characteristics of the microbial environment associated the rhizome for plants?

What are the primary ways that carbon and energy flow through that system? Use the four classes of metabolism in your answer and state which class is most dominant and also if all four classes of metabolism are present. Please also include how primary and secondary production, predation, and decomposition/heterotrophy may factor into the rhizome ecosystem.

Answer 1

Rhizome Ecosysytem

Rhizome, also called creeping rootstalk, horizontal underground plant stem capable of producing the shoot and root systems of a new plant. Rhizomes are used to store starches and proteins and enable plants to perennate underground.

Generally, the concentration of the endophytic bacteria is more at the root than at shoot tissueThis makes endophytes to be considered as untapped source of natural products .Endophytes also provide plant growth regulators, by providing resistance to diseases and also by assisting in phytoremediation

. A large number of plant species are shown to be associated with bacteria like Pseudomonas, Bacillus, Azospirillum etc. (Chanway 1996). various species of bacteria like Pantoea, Azospirillum, Methylobacterium, Rhizhobium, Herbaspirillum, Burkholderia etc. are found to be endophytically associated. These bacterial species contribution to the yield and growth of the rice plants P. putida, S. maltophilia, and S. pasteuri are present as endophytes in wide variety of plants

production of auxin-like molecules Indole 3 acetic acid (IAA) being an auxin can stimulate both rapid responses like cell elongation and long term responses like cell division and differentiation in plants Indole 3 acetic acid (IAA) is shown to be produced by many root associated bacteria including Enterobacter sp., Pseudomonas sp., and Azospirillium sp.

plant growth promoting bacteria (PGPB) are also shown to exhibit other properties like ACC deaminase, phosphate solubilization, siderophore production,

endophytic bacteria were isolated from ginger rhizome and one among the isolates was found to have the ability to produce IAA, ACC deaminase and siderophore.

Q2 ANS>

A)

1.Energy Flow in Ecosystem:

Energy Flow in Ecosystem Begins with Sun. Green plants use water, carbon dioxide and sun light to make glucose, through the process of ‘ Photosynthesis’.

Movement of Energy:

Movement of Energy The flow of energy is the most important factor that controls what kind of organisms live in an ecosystem and how many organisms the ecosystem can support

2.Carbon flow in ecosystem

Most of the carbon dioxide enters the living world through photosynthesis. The organic compounds synthesised are passed from the producers (green plants) to the consumers (herbivores and carnivores). During respiration, plants and animals release carbon back to the surrounding medium as carbon-dioxide. The dead bodies of plants and animals as well as the body wastes, which accumulate carbon compounds, are decomposed by micro-organisms to release carbon dioxide.

B)the major biogeochemical cycles are as follows:

(1) Water Cycle or Hydrologic Cycle

(2) Carbon-Cycle

(3) Nitrogen Cycle

(4) Oxygen Cycle

Dominating metabolism is Nitrogen Metabolism

The soil has limited amounts of nitrogen. Plants and microbes compete for this nitrogen.

Nitrogen Cycle.The main part of nitrogen metabolism is the Nitrogen Cycle.There are three main pools of nitrogen – atmosphere, soil and biomass.

Atmospheric Pool

The process of converting atmospheric nitrogen (N₂) to ammonia (NH₃) is ‘Nitrogen fixation‘. Atmospheric nitrogen is fixed in three ways – biological, industrial and electrical.

• ‘Biological nitrogen fixation’ involves living organisms that reduce nitrogen to ammonia/The above processes fix atmospheric nitrogen into the soil. This nitrogen is then taken up by plants and animals, consequently.

Biomass Pool

When plants and animals die, the organic nitrogen within them decomposes to ammonia. This process is ‘Ammonification‘ and it returns nitrogen back to the soil. Some of this ammonia evaporates and re-enters the atmosphere while a major part of it is converted by soil bacteria into nitrate as follows:

(i) First, ammonia is oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus.

(ii) Then, nitrite is further oxidised to nitrate by Nitrobacter.These reactions are called ‘Nitrification‘ and the nitrifying bacteria are ‘Chemoautotrophs‘. Plants absorb the nitrate thus formed and transport it to the leaves where it is reduced to ammonia. This ammonia forms the amine group of amino acids. Nitrates in the soil are also reduced to nitrogen during ‘Denitrification‘ by Pseudomonas and Thiobacillus. In this way, nitrogen keeps cycling in the ecosystem.

Biological Nitrogen Fixation

Only a few prokaryotes can use the atmospheric nitrogen as N₂ and reduce it to ammonia. This reduction of nitrogen to ammonia by living organisms is ‘biological nitrogen fixation’. The enzyme it needs for this reaction – nitrogenase is present exclusively in prokaryotes and these microbes are called N₂ – fixers. These N₂ – fixers can be symbiotic or free-living. Some examples of free-living N₂ – fixers are Azotobacter, Bacillus, Anabaena, Nostoc etc.

Symbiotic Biological Nitrogen Fixation

The most popular example in this category is the symbiotic relationship between Rhizobium and the roots of legumes such as sweet pea, garden pea, lentils. The association is visible as nodules (small outgrowths) on the roots. Another example is the microbe Frankia that also produces nitrogen-fixing nodules on the roots of non-leguminous plants.

Did you know that if you cut through a nodule, its central portion is red or pink? What gives it this colour? It is the presence of leguminous haemoglobin or leg-haemoglobin. Let’s learn a little more about these nodules.

Nodule Formation

Another form of nitrogen metabolism is nodule formation. Nodule formation involves several interactions between the roots of the host plant and Rhizobium. They are:

• The Rhizobia multiply, colonise and attach themselves to the epidermal and root-hair cells of legumes.
• The root-hair curls allowing the bacteria to invade, create an infection thread and reach the cortex of the root. Here the bacteria initiate nodule formation.
• The infection thread then releases the bacteria into the cells. This leads to the differentiation of special nitrogen-fixing cells. In this way, the nodule establishes a direct connection for exchange of nutrients with the host.
• The nodule contains the enzyme nitrogenase which converts atmospheric nitrogen to the first stable product of nitrogen fixation – ammonia. The reaction is:

N₂ + 8e^– + 8H⁺ + 16ATP —→ 2NH₃ + H₂ + 16ADP + 16P_i

The energy required for the above reaction (8ATP for each NH₃ produced) comes from the respiration of host cells. Also, the enzyme nitrogenase is very sensitive to molecular oxygen and therefore needs anaerobic conditions. The oxygen-scavenger called leg-haemoglobin protects nitrogenase from oxygen in the nodules.

Interestingly, this microbe lives as aerobic, free-living organism where nitrogenase is not functional. But during nitrogen-fixation, they become anaerobic, thus protecting the enzyme from oxygen.