Role of IAA in the establishment of host-microbe symbiotic relationship for sustainable production of food

Dr. Pradeep Kumar

Assistant Professor

Department of Microbiology, Kalinga University, Raipur

World Population is increasing at a haphazard pace, which causing a scarcity of food.  The world population will warrant an increase in food and fiber crop production. However, Managing land and updating agriculture practices are not sufficient enough to meet the ever-increasing food demand due to the extensive pace of population growth versus crop production requires, and the main reason behind it is rapid urbanization. For fulfilling the food requirement, there some earlier approaches to using chemical fertilizer. But, chemical fertilizer has added burden in the ecosystem by bringing undesired effects besides the expense involved, like altering the pH of the soil, which affects many aspects of crop production, including nutrient availability, metal toxicity potential, and nitrogen fixation in crops. It also disturbs beneficial microbial ecosystems, increasing weeds, and even contributing to the release of greenhouse gases. To address this issue, an alternate approach to using Plant Growth promoting factors, and these factors are produced by many microorganisms designated as PGPRs (Plant Growth Promoting Rhizobacteria). These bacteria are present in the rhizosphere, which was first discovered by Lorenz Hiltner in 1904, i.e., the layer of soil near the roots, which is rich in bacterial than the surrounding bulk soil (Lugtenberg and Kamilova 2009). These rhizobacteria are root-associated bacteria that form a symbiotic relationship with many plants. These PGPRs have two significant relationship classes; rhizospheric and endophytic.

Nowadays, we have studied lots of endophytic microorganisms that provide these types of responses to the plants by producing auxin (Spaepen and Vanderleyden 2011). They also alter the physiology of plants by primary roots elongation, the proliferation of adventitious and lateral roots. They also enhance anchoring ability to obtain nutrients like ions, minerals, etc. from the soil and water by their environment due to which chances for survival increase and enhancement of overall plant growth (Ahmad et al. 2005). They also have unique in their adaptations to the particular environment which provided by the host plant, due to which they synthesize host-specific compounds using their molecular machinery. Indole-3-acetic acid (IAA) is a member of the auxin group, has been extensively studied by plant physiologists for their essential role in plants like apical growth, cell division, cell growth, root formation, and seedless fruit formation (Glick 1995). Microorganisms also produce IAA for their signaling purposes like quorum sensing. It is an essential trait in plant growth promoters. The release of L-tryptophan in the root exudates of the plant results in the synthesis of IAA by the rhizospheric bacteria by different tryptophan-dependent pathways (Figure 1) (Kravchenko et al. 2004).

Figure 1: Tryptophan dependant pathway for IAA biosynthesis

In a plant-microbe symbiotic relationship, bacterial IAA, within an ideal range, induces root hair formation, increasing the number and length of lateral and primary roots (Duca et al. 2014). Plant on the other side, provides exudates (containing nutrients) and shelter to the bacterial partner (Khan et al. 2016). However, signaling molecules are required before a mutually beneficial relationship between host and microbe is established. So, both plants and microbes secrete several secondary metabolites to initiate and later establish the symbiotic relationship. Chemical communication between plant roots and associated microbes is a significant prerequisite for the initiation of symbiosis (Mehmood et al. 2019). IAA is one of the signalling molecules released by endophytes, volatile compounds, iron carriers among others . Likewise, plant roots exude chemoattractant to attract beneficial microbes (Mehmood et al. 2019). Plant roots can produce several organic compounds like amino acids, phenols, flavonoids, sugars, organic acids, and vitamins as demonstrated in figure 2.

Figure 2: Helpful roles of IAA for plant and microorganisms

IAA is released by most of the ectomycorrhizal fungi. Slankis, 1973 established the critical role of fungal IAA in hyperauxiny of ectomycorrhizal fungi which induces the formation of mycorrhiza in small pine roots by exogenously supplied IAA and ectomycorrhizal fungi culture filtrates has been established. They concluded that fungal IAA is responsible for the morphology of mycorrhiza.

The significant role of IAA in symbiosis is clear from the studies in which IAA suppressors have been used to check colonization. Suppression of IAA secretion has been found to reduce the endophyte (Aspergillus nomius) colonization in maize roots by 67% (Mehmood et al. 2019). Inhibition of IAA production by foliar application of yucasin reduced endophyte (Aspergillus awamori Wl1) colonization by 52% and root application of the same by 66 % in maize roots which had effectively colonized maize roots otherwise.

References

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