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.
The release of heavy metals in the environment is a serious threat causing health hazards to living beings. Hence, it is essential to remediate chemical contamination for a safe environment. Bioremediation is considered one of the most cost-effective and sustainable agricultural techniques, in contrast with other conventional methods to reduce chromium toxicity in agricultural lands polluted with chromium, as it is a natural way to mitigate the toxic effects of hexavalent chromium with simultaneous amelioration in the growth of plants. In the current study, an attempt was made to reduce toxicity of chromium by using six plant growth-promoting chromium-resistant bacteria (Bacillus pumilus (ALa), Bacillus atrophaeus (BL2), Bacillus cereus (AR), Staphylococcus lentus (E3), T2aii and W6ii) for enhancing the growth of Zea mays L. in soil contaminated with chromium. In this regard, a pot experiment was conducted with pre-germination and post-germination inoculation treatments to Zea mays seeds in the presence of chromium stress, i.e., 200, 400, and 600 µg/ml. Our results have shown that toxicity of chromium caused a reduction in photosynthetic pigments and protein content together with reduction in growth parameters of plants, while treatments with chromium-resistant PGPB significantly enhanced chromium tolerance in treated plants compared with non-inoculated treatments in the presence of chromium stress. The present investigation suggests that applying post-germination inoculation treatments is an effective technique for improved plant growth and heavy metal alleviation in metal-contaminated soil. Thus, our current work revealed an incentive approach toward the green revolution in the age of industrialization by exploring beneficial chromium-tolerant auxin-producing microbes.
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