Mycorrhizal fungi have one of the most important relationships with plants in terms of survival and nutrient acquisition.
Mycorrhizal fungi have a symbiotic relationship with plant roots, meaning they work together in their survival. Plant roots contain root hairs that are smaller extensions of the main tap root, these hairs attempt to reach further distances and access smaller pore within the soil; however, they are not as efficient as they hope to be. Mycorrhizal fungi have filamentous hyphae which have a greater surface area to volume ratio compared to root hairs and are extremely efficient in nutrient acquisition. Plants that have a symbiotic relationship with mycorrhizal fungi have a huge advantage and a competitive edge in nutrient uptake. The mycorrhizal fungi are able to expand the reach of the plants root system resulting in greater nutrient accessibility. The thin hyphae are able to access smaller aggregate pore sizes and acquire a greater amount of elemental nutrients. The hyphae not only are small enough to reach smaller soil pore, but they also have a large surface area to volume ratio, enabling more nutrients to be absorbed. They are able to reach a greater volume of soil and, within that volume, they are able to absorb a greater amount of nutrients. Both the plant and mycorrhizal benefit from this interaction. The fungi have a constant and reliable carbon source and the plant is supplied an abundant amount of nutrients such as phosphorus and water that they could not obtain without the mycorrhizae’s help.
Mycorrhizal fungi can be categories into two groups: Endomycorrhizae and Ectomycorrhizae. With the mycorrhizal penetrating root cells or occupying the extracellular space between plant root cells, respectively. For this blog, I will be focusing on the endomycorrhizae structures. Endomycorrhizal have a characteristic “dwarf tree” structure labeled arbuscular, that penetrate the cells of the roots of a plant. These structures are finely branched hyphae that allow for rapid nutrient exchange from the hyphae straight into the root cell.
This structure allows for direct nutrient absorption into the plant's cells. The arbuscular hyphae also have vesicles that are storage structures for nutrients that the mycorrhizal captured out in the soil.
There are many papers that support the notion that arbuscular mycorrhizal fungi (AM) improve plants overall health in water-stressed conditions but few they attempt to combine all aspects of AM benefits. The papers that I examined, focused on wheat and the fruiting plant, Chile Ancho.
Ghazi Al-Karaki is a very prominent scientist in the field of arbuscular mycorrhizal effects on wheat. Both papers I reviewed of theirs have very similar results. One of his research projects was conducted to determine the effects of AM fungi on growth, and grain yield of wheat. From this study, they were able to conclude that AM fungal colonization of roots had been shown to increase drought resistance of wheat (Al-Karaki 2004). AM fungi increased biomass and grain yield regardless of water content, and wheat plants that contained these AM had a great advantage in a water-stressed environment (Al-Karaki 2004). Within this study, they also tested two different species of mycorrhizal: G. mossae and G. etunicatum. From this portion of the experiment, it was determined that plants that contained G. etunicatum fungi also had higher biomass, grain yield, and colonization of AM fungi in wheat compared to G. mossae fungi (Al-Karaki 2004). Giving reason that not all mycorrhizal fungi are the same.
Another research project was conducted to determine the effects of AM fungi on growth, phosphorus, and mineral uptake of wheat (Al-Karaki 1997). Within this experiment, they conducted tests on two different genotypes of wheat crops, CR057 (drought-resistant) and CR006 (drought-sensitive). Using these two genotypes to measured differences in mineral content and dry matter weight. The plants that were inoculated with mycorrhizal fungi had enhanced concentrations of phosphorus, iron, zinc, copper, and manganese in the plant body and increased the weight of total dry matter in all the soil moisture contents (Al-Karaki 1997). This increasing plant growth was probably due to mycorrhizal enhancement of phosphorus uptake. Phosphorus is associated with photosynthetic reactions and mycorrhizal plants have a greater capability of phosphorus uptake. The more phosphorus, the greater activity of photosynthesis and thus greater sucrose production which will increase the biomass of the plant. However, within the study CR057 had lower total root dry matter compared to CR006. Plants that are drought-sensitive are phosphorus deficient in drought conditions and when mycorrhizal are added to the system it increases the efficiency of phosphorus uptake. Showing that certain plants can have greater responses to the mycorrhizal symbiotic relationship. Drought-sensitive plants benefitted the most out of the mycorrhizal infections (Al-Karaki 1997). Mycorrhizal association with plant roots not only enhanced growth and mineral elemental uptake but may also confer greater resistance to drought.
Mycorrhizal fungi also help with fruit production. Hortencia G. Mena-Violante and colleagues studied the effect of AM and drought conditions on fruit quality in Chile Ancho. They used Chile ancho because it is a very important source of vitamin C in the Mexican diet and it is prominently grown in a semiarid (desert) valley where water is the limiting factor. Water availability is an important factor that influences vegetable and fruit production because droughts result in stressed plant growth. Under drought conditions, plants that contained AM fungus produced fruit that was larger in volume and heavier in weight (Mena-Violante 2006). AM fungi had reduced the negative impact of water stress on the Chile Ancho. The Chile Ancho was subjected to a variety of soil moisture and results show the inoculated plants produced greater dry biomass and chlorophyll content than non-inoculated Chile anchos in well water environment. This is interesting because it brings further evidence that AM help the plant in a variety of environments and not just water-stressed environments.
Arbuscular mycorrhizae that are an association with plant roots not only enhanced growth and mineral elemental uptake but may also confer greater resistance to drought in many plant varieties.
References:
Al-Karaki, G., McMichael, B. & Zak, J. Mycorrhiza (2004) 14: 263. https://doi.org/10.1007/s00572-003-0265-2
Mena-Violante, H.G., Ocampo-Jiménez, O., Dendooven, L. et al. Mycorrhiza (2006) 16: 261. https://doi.org/10.1007/s00572-006-0043-z
Pictures from Google and Lecture slides
