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Plant environment is a complex system where coordinated interactive biology involving various metabolite products and other intermediates determines the overall development and growth of plant. Among the phytohormones, auxins play a fundamental role in various signaling pathways involving other hormones and metabolites affecting cell division and differentiation of plant tissues. Likewise, phenolics are the secondary metabolites secreted by plants that play a key role as defense agents during environmental stress conditions. Biosynthesis of auxins and phenolics follows different metabolic pathways, although shikimate pathway is considered as the root for the production of auxins and phenolics following the synthesis of their corresponding precursors. The interactions between these two compounds may have some physiological and biochemical alterations in plant metabolism, thus affecting plant biology. In addition, the role of soil microbiota is also evident to mediate the communicative behavior of both auxins and phenolics. Phenolic compounds may affect auxin transport and play its role in defense signaling of plants. Some representative examples regarding interactive biology of auxins and phenolic compounds under in vitro conditions are also discussed in this chapter.

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Phytohormones play a fundamental role in the development of plants. Among various phytohormones produced by the plants, Auxins act as a master hormone that plays a major role during plant development and differentiation through cell division. Besides plants, many rhizospheric microorganisms are also capable of producing auxins specifically indole-3-acetic acid (IAA), that act as signaling molecules for the regulation of gene expressions in plants. However, bacterial IAA is majorly linked with the modulation of plant roots architecture and developing positive plant-microbe interactions. Bacterial auxin modifies root morphology by enhancing root length, forming adventitious root and root hair, thereby, increasing surface area for water and nutrient absorption affecting various aspects of plant biology in a number of ways. Bacteria mostly utilize tryptophan, present in plant root exudates, to synthesize IAA that eventually helps bacteria to colonize roots by establishing beneficial associations with plant roots. Auxins also stimulate the formation of exopolysaccharides and biofilms that help bacterial root colonization. Auxins have given the survival benefit to rhizobacteria that make them more competent to establish symbiotic interaction with plants. Synergistic and antagonistic interactions of auxins (both interkingdom and Intrakingdom) with other phytohormones play a key role in plant development and growth improvement.

Phosphorous limits agricultural productivity due to its limited plant availability. Use of synthetic phosphate fertilizers disturbs soil fertility and ecosystem ecology as it contaminates environment. Plants have developed certain mechanisms to respond to P-scarcity, which involve release of specific chemical messengers through root exudates that attract rhizospheric phosphorbacteria to colonize plant root vicinity. Thus, use of phosphate-solubilizing bacteria/rhizobacteria (PSB/PSR) as biofertilizers is a safer approach toward sustainable agrobiology. These PSR are capable of solubilizing soil phosphate from insoluble to plant available form. Due to instability and slow movement of available phosphates in soils, they readily get incorporated with soil particles or chelates as metal complexes. In this scenario, PSR provide continuous chain of soluble phosphate to plants. PSR direct plant root system architecture toward available phosphate zones in soils. Moreover, there is an increased number of roots, root hair and lateral root, increase root absorbing surface area by increasing contact to soil particles. Hence, PSR-based root system morphology is a significant trait in measuring their agronomic efficiency. Moreover, PSB also possess phytostimulatory properties that significantly contribute to agricultural efficiency. Hence, the use of phosphate-solubilizing bacteria can improve crop productivity by increasing soil P-mobility and soil fertility.