Microbial ecology is known as the intimate association between microorganisms and their environment. Vesicular-arbuscular mycorrhizae, better known as VAM fungi and its host plant are an example of such association. These interactions are stimulated by excretions from the host plant when it experiences unfavorable conditions (8) such as a mineral deficiency. The VAM fungi receives the signals and induces its Myc-factors (8, 11) and triggers a chemical response to this signals received. The VAM fungi interact with the host plant penetrating its root system and having (8) great benefits to the plant increasing its absorption surface area and gaining important growth and functional nutrients and minerals.
Introduction
Microbial ecology is the scientific study of the intimate relationship or interaction of microorganism with its environment either biotic or abiotic (1). The field of microbial ecology is so diverse and unique with multiple disciplines such as animal-microbe interaction, microbe-microbe interactions, e.g.: lichens and symbioses, e.g.: beneficial gut micro flora but the most interesting relationship is plant-microbe interaction (2).
Microbes are omnipresent in our entire world and can also exist in harsh environments such as acidic lakes and tundra's. Studying microbial ecology can help mankind gain more knowledge about the intimate interactions and how they function which can improve our lives in such a way to preserve and restore our environment.
Plants are very important in our environment and it contributes to human existence through providing oxygen (O2). Plants undergo daily stressors; whether it is to fend off pathogens or protect its apparatus against high temperatures therefore they are extremely diverse in its producing metabolites which help protect themselves but this can only occur to some extent. Plants that have a close relationship with microorganisms are very successful in growth and performance and have only positive influences in its ecosystem. Not only does the plant benefit but as well the microbes because the plants provide the microbe with sufficient nutrients as well as a suitable territory (3).
The interaction between plants and microbes can have several beneficial aspects as seen in agricultural and ecological fields. In most countries chemical fertilizers are very commonly use to obtain a higher yield of certain crops especially wheat (Triticum aestivum) and rice (Oryza sativa L.) which sometimes have undesirable side effects on human health especially (5). With amazing research that has been done in the past few years it was found that some microorganisms can be used as "biofertilizers" where beneficial microbes are used to form a relationship with a specific plant e.g.: cyanobacteria for rice (5).
Discussion
Plant-microbe interactions can be either positive or negative depending on the nature of the microbe and what it takes from the plant or secretes into the environment. The positive interaction can be described as a mutualistic or symbiotic relationship between the plant and microorganism for example: Leaf Nodules (6) a typically known where the bacterium occurs, functions and reproduces within a leaf or plant. Sometimes it's also referred to as bacterial endophytes (6), it does not necessarily have any importance to the plant but the major thing these bacteria produce are phytohormones (6) needed for the plants growth. And Mycorrhiza which was first described by A.B. Frank (6). Mycorrhiza mostly occur in plants root system where there are poor soil fertility, the plant provide the fungus with the necessary nutrients for its survival e.g.: carbohydrates whereas the fungi absorb more nutrients from the soil or its surrounding environment.
The most commonly found beneficial fungi is mycorrhizal - there are ecto- and endomycorrhiza which provide the plant with necessary nutrients and water in the already assimilated form which are energy efficient for the plant. The microbe therefore only receives and utilizes photosynthates in return (7). This interaction can also be a form of survival mechanism for the plant when conditions are unfavorable.
For the purpose of this scientific study the in depth study is done on vesicular-arbuscular mycorrhizae, also known as VAM fungi.
Development of Vesicular-arbuscular Mycorrhizae
Presymbiotic stage
Plant-derived signals (8) occur when the plant experiences some sort of stress of deficiency of an important mineral. The AM fungi develop hyphae when these plants signals interact with the fungi. Plants synthesize secondary metabolites (9) such as strigolactones, to protect themselves against harmful pathogens and they use these metabolites to interact with beneficial organisms in order to attract them (9). Strigolactones are a molecule, produced in very small quantities, used for signaling certain responses in the plant.
When the induction process start in the formation of the AM fungi and the root symbioses signaling molecules are present produced by the fungi called Myc-factors (8). These Myc-factors, only a hypothesis (10), are signals in the VAM fungi to produce a chemical signaling response to start with the formation of the symbioses.
The plant actively prepares the cell for the penetration of the fungal hyphae, before the AM development occur plant root morphology and physiology needs to change - this indicates the presence of AM regulated pathways causing the initiation of transcription of AM genes (11). The plant genes that are involved in the development of the AM fungi are recorded to be at least that of seven genes in the formation of the symbiosis in legumes (8). From a genetic point of view the symbiosis initiating factors that are secreted induce transcription of these specific genes resulting in the formation of intracellular changes to successfully accommodate the fungi.
http://www.ces.purdue.edu/extmedia/FNR/images/FNR-104.fig4.gif
Figure 1: A typical arbuscular, endo-, mycorrhizae that penetrate the plant roots together with its spores (7).
Agricultural benefits
An agricultural system that does not use fertilizer or wants to consider organic farming depends solely on VAM fungi which provide more advantages to the host plant (16). These advantages include nutrition (16) through absorption of nutrients and essential minerals; more resistance (16) to pathogens, diseases, drought and these plants can also tolerate heavy metals present in soils.
Many agricultural activities used by farmers such as the use of fertilizers, tillage and monocultures also the planting of non-mycorrhizal plants are unfavorable to VAM fungi (16).
Agricultural management needs to be improved and implemented i.e. the use of VAM associated plants needs to be planted (16) instead of non-mycorrhizal crops - this can be beneficial to more than just the host plant and fungi.
VAM functions
The arbuscular fungi have several functions and play an intimate and important role in plant health and growth. VAM interactions are very important in nutrient and mineral absorption and transporting the nutrient into the root using its external mycelium located on the exterior surface of the roots (8). This relationship can be described as being the "absorptive organs" (12).
Carbon metabolism
Most of the fungus carbon is obtained directly from the host plant (14). The fungus utilizes this carbon sources for its growth and to develop more hyphae when the host plant requires more minerals.
Phosphate
The improving absorption of phosphate is the main benefit of VAM fungi (8, 13). The complex hyphae networks the VAM fungi produced influence the chemical properties of the surrounding soils resulting in the breakdown or formation of important phosphate units (8). Phosphate transporters located in the mycelium transport the phosphate into the plant for further assimilation.
Nitrogen
Nitrogen is essential for the plants growth (16) and normal functioning. It is still unknown if VAM fungi actually play a role in the nitrogen nutrition to plants (15). The interaction between nitrogen-fixing bacteria and VAM can be considered and the bacteria, VAM fungi and plant can play an integrated role.
Conclusion
VAM, vesicular-arbuscular mycorrhizae, has been studies for several years now and it was discovered to have an essential role (8) in plants - for its growth, differentiation, health including defense mechanisms.
It can be concluded that the VAM fungi have an essential role in the plant-microbe interaction and it has been proven to have several advantages to the plant e.g.: better growth and yield because of the absorption of essential nutrients and minerals (8) and protection against pathogens and diseases. This method of interaction can be economically beneficial for a country or farmer as no fertilizers need to be used (16) and the product can be organic.
After studying this important interaction it can be concluded that VAM fungi are very important to the plant for its function - in the near future the unknowns about these interactions, such as the Myc-factors and the possible interlinked interaction between bacteria, fungi and host plant will be discovered and fully understood.
