Microbial Interaction in Rhizosphere

The rhizosphere is the part of the soil directly influenced by the root system. It is intensively colonized by microorganisms and represents an environmental hot-spot in which the interactions between organisms reach a very complex level. There is a great deal of interest in understanding and manipulating these interactions with a view to the development of new agricultural strategies and plant protection practices.

Microbial Interactions in phyllo-sphere

The phyllosphere supports a tremendous diversity of microbes and other organisms. However, little is known about the colonization and survival of pathogenic and beneficial bacteria alone or together in the phyllosphere across the whole plant life-cycle under herbivore, which hinders our ability to understand the role of phyllosphere bacteria on plant performance.

Interactions with Plants :

A) Legume-Rhizobium Stem, Leaf, Root nodulation

Legume-Rhizobium

  • Several microorganisms have the ability to fix atmospheric nitrogen by symbiotic association with legume plants. Major genera include — Azorhizobium, Allorhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium, and Rhizobium.
  • Root nodule formation by the interaction of Rhizobium and legume plant is an important site of symbiotic nitrogen fixation.
  • The legume plant secrets flavonoids, these flavonoids promote Rhizobium bacteria to synthesize several factors, which enhance the symbiotic association between root hair and Rhizobium.
  • Such factors are referred to as Nod Factors. Following bacterial attachment, the root hair curl and bacteria induce the plant to form infection thread.
  • Rhizobium spreads within infection thread. The nitrogen-fixing form of Rhizobium is called as symbiosome.
  • The symbiosome is are several times greater than the bacteria cell. It consists of bacteroids and peribacteroid space.
  • The leghaemoglobin protects oxygen labile nitrogenase of Rhizobium. Nitrogenasefixes atmospheric nitrogen to form ammonia and alanine then it is supplied to plant.
Rhizobium

Benefits to the plant

Plants are unable to directly uptake the atmospheric elemental nitrogen, this insoluble nitrogen is solubilized by the nitrogen fixation activity of Rhizobium. Thus plants get their soluble nitrogen from Rhizobium

Benefits to Rhizobium

Rhizobiumgets shelter in the nodule synthesized in plant roots. Nitrogenase an enzyme responsible for nitrogen fixation is highly sensitive to the high concentration of oxygen. Nitrogenase of Rhizobium is protected from high oxygen potential inside the nodules.

B) Mycorrhiza: Ecto, Endo, VAM, Orchid

  • Mycorrhizas (Myco = Fungi, Rhizo = Root) – It is the mutualistic association between plants and fungi in which the fungi actually become integrated into the physical structure of roots.
  • The fungus derives nutrition benefits from plant roots, contributes to plant nutrition, and does not cause plant disease. The association involves the fungus mycelia and plant roots.
  • The association exists for prolonged periods with the maintenance of a healthy physiological interaction between the plant and fungus. The association leads to nutrient exchange favorable to both partners. Fungi help in the solubilization of phosphorus and nitrogen for plant nutrition.
  • Types – There are two basic types of mycorrhizae. 1) Ecto-mycorrhizae and 2) Endo-mycorrhizae

1) Ecto-mycorrhizae

In ectomycorrhizae, the fungus such as Ascomycetes or Basidionncetes form an external pseudo parenchymatous sheath more than 40 um thick and constitutes up to 40% of the dry weight of combined root-fungus-structure. The fungal hyphae penetrate the intracellular space of the epidermis and of the cortical region of the root but do not invade the living cells. The morphology of the root is altered, forming shorter, dichotomously branching clusters with reduced meristematic regions. The ectomycorrhizae are predominantly exogenous.

  • Host plant — Ectomycorrhiza are common in gymnosperm and angiosperm including oak, beech, birch, and coniferous trees
  • Fungi — Many fungi can enter into ectomycorrhizal associations, including ascomycetes e.g. truffles and basidiomycetes e.g. Boletus and Amantia.

Benefits to the plants – The plant derives several benefits from its association with the ectomycorrhizal association this includes,

1) The longevity of feeder roots,

2) Increased rates of nutrient absorption from the soil,

3) Selective absorption of certain ions from the soil,

4) Resistance to plant pathogens,

5) Increased tolerance to toxins, temperature, drought, and pH.

Benefits to the fungus – The ectomycorrhizal fungi receive photosynthesis product from the host plant and thus escape intense competition for organic substrates with another soil microorganism.

2) Endo-mycorrhizae

Endo-mycorrhizae invades the living cells of the root which become filled with the mycelial clusters. Mycorrhizal fungi occupy a unique ecological position of being partly inside and partly outside the host. The microscopic appearance of intracellular hyphal clusters causes these to be called Vesicular-

3) Arbuscular (VA) mycorrhizae (VAM).

In some cases, ecto and endo form may be combined and are called ectomycorrhizae.

  • Host plant — endomycorrhiza are common in Ericales which include heath, arbutus azalea, rhododendron, and American Laure, orchis, etc.

VA finds its association with Angiosperm. Gymnosperm, ferns. and Bryophytes. It occurs in wheat. maize, potatoes. beans, soybeans. tomatoes, strawberries, apples, oranges, grapes. cotton, tobacco, tea, coffee, cocoa, sugarcane, rubber trees.

  • Fungi — Many fungi can enter into ectomycorrhizal associations, including Armillariamellea, Rhizoctonialsolani. VA fungi belong to Glomaceae, Acaulosporaceae, and Gigasporaceae. The representative examples are Endogone, Gigaspora margarita, Burkholderia.

Benefits to the plants

1) Nitrogen uptake –Although the fungi do not fix atmospheric nitrogen, the endo-mycorrhizal association may increase plant access to combined nitrogen in soil Selective absorption of certain ions from the soil.

2) Phosphate solubilization — Themycorrhiza fungi can solubilize the insoluble phosphate into soluble and can transfer the phosphate from external sources to the plant. The activity of enzyme required for phosphate solubilization is higher in mycorrhizal roots than nonmycorrhizal roots.

3) Improved growth of host plant- The association of endomycorrhiza improves the growth of the host plant in nutrient-deficient soils. The fungi increase plant growth through increased uptake of nutrients such as phosphate, zinc, sulfate, and ammonium.

4) Phosphate solubilization – Themycorrhiza fungi provides phosphate nutrition to the host plant and thus helps in plant growth.

Benefits to the fungus

1) The ectomycorrhizal fungi receive photosynthesis product from the host plant and thus escape intense competition for organic substrates with another soil microorganism.

2) The plant roots provide a good ecological niche for the fungi

C) Orchid :

  • Orchidmycorrhizae is symbiotic relationship between the roots of plants of the family Orchidaceae and a variety of fungi.
  • All orchids are myco-heterotrophic at some point in their life cycle. Orchid mycorrhizae are critically important during orchid germination, as an orchid seed has virtually no energy reserve and obtains its carbon from the fungal symbiont.
  • The symbiosis starts with a structure called a protocorm. During the symbiosis, the fungus develops structures called pelotons within the root cortex of the orchid.
  • Many adult orchids retain their fungal symbionts throughout their life, although the benefits to the adult photosynthetic orchid and the fungus remain largely unexplained.

D) Lichen

  • The relationships between certain algae/cyanobacteria and fungi result in the formation of lichens. This association is a type of mutualistic inter microbial relationship.
  • Lichens are composed of primary producers i.e. alga (the phycobiont) and consumer (the mycobiont).
  • The algae through the process of photosynthesis produce food material and make it available to the fungal partner whereas a fungal partner provides a sort of protection and mineral nutrient transport to the alga. These two partners form a distinct layer that function as primitive tissue.
  • Lichens grow very slowly but are able to colonize habitats that do not permit the growth of other microorganisms.
  • Most lichens are resistant to extreme temperatures and drying enabling them to grow in hostile habitats such as on rock surfaces where they produce organic acids which help in the solubilization of rock minerals.
  • Some lichens are able to fix atmospheric nitrogen e.g. Lichen Peltigera in tundra soil

Algal partner (phycobiont) – It includes the members of cyanobacteria Chlorophycophyta, and Xanthophycophytathe green algae Trebouxia and Nostoc are very common phycobionts in lichens.

Fungal partner (mycobiont) – It includes the members of Ascomycetes, Basidiomycetes, and Zygomycetes

Types of lichens – Morphologically there are three types of lichens

1) Crustose lichens — Crustose lichens adhere closely to their substrates

2) Foloise lichens- Foliose lichens are leafy in form and are attached to their substrates more loosely

3) Fruticose lichens — Fruticose lichens consist of hollow upright stalks and are the least attached to their substrates.

Environmental significance of lichens

  • Lichens as an indicator of industrial pollution
  • Although lichens are able to occupy some hostile habitats, they are particularly sensitive to industrial pollutants and have been disappearing from industrialized areas.
  • The sulfur dioxide (SO2) is inhibitory to the lichens. probably because it inhibits the algal partner.
  • The reduced efficiency of the photosynthetic activity of an algal partner allows a fungal partner to overgrow and leads to the elimination of the mutualistic relationship.
  • Following the destruction of an algal partner, the fungus is unable to survive alone and is also eliminated from the habitat.