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Expeditious industrialization and anthropogenic activities have resulted in large amounts of heavy metals (HMs) being released into the environment. These HMs affect crop yields and directly threaten global food security. Therefore, significant efforts have been made to control the toxic effects of HMs on crops. When HMs are taken up by plants, various mechanisms are stimulated to alleviate HM stress, including the biosynthesis and transport of auxin in the plant. Interestingly, researchers have noted the significant potential of auxin in mediating resistance to HM stress, primarily by reducing uptake of metals, promoting chelation and sequestration in plant tissues, and mitigating oxidative damage. Both exogenous administration of auxin and manipulation of intrinsic auxin status are effective strategies to protect plants from the negative consequences of HMs stress. Regulation of genes and transcription factors related to auxin homeostasis has been shown to be related to varying degrees to the type and concentration of HMs. Therefore, to derive the maximum benefit from auxin-mediated mechanisms to attenuate HM toxicities, it is essential to gain a comprehensive understanding of signaling pathways involved in regulatory actions. This review primarily emphases on the auxin-mediated mechanisms participating in the injurious effects of HMs in plants. Thus, it will pave the way to understanding the mechanism of auxin homeostasis in regulating HM tolerance in plants and become a tool for developing sustainable strategies for agricultural growth in the future. • Auxin homeostasis orchestrates plant tolerance to HMs through different mechanisms. • HM stress alters auxin level through regulating signaling and biosynthesis network. • Auxin mediated phytohormonal tuning involved in defense against HM stress. [ABSTRACT FROM AUTHOR]

• Zn impacts Arabidopsis root growth with dose-dependency. • Zn regulates auxin biosynthesis and transport in roots. • Plant response to zinc excess needs auxin biosynthesis and effective PAT apparatus to remodel root system. Zinc is an essential element for plant; however, in excess it could cause adverse effects on plant growth and productivity. Plants have evolved various regulatory mechanisms to optimize their growth and development at different zinc concentrations. In this work, we elucidated the role of auxin homeostasis and transport in the dose-dependent responses to zinc in Arabidopsis thaliana. The results showed that zinc had dose-dependent effects on both vegetative and reproductive growth of Arabidopsis. At the concentration of 50, 100 and 350 μM, zinc significantly affected the length of primary root and the size of root meristem. This root response to Zn were associated with differentially regulation of endogenous auxin, as demonstrated by the DR5:GUS reporter line and in vivo hormone measurements. Exogenous IAA could alleviate the Zn-induced short-root phenotype by affecting auxin accumulation in plants, while Arabidopsis mutants with deficient auxin synthesis or transport mechanisms exhibited hypersensitivity to Zn. In conclusion, the effect of zinc on root growth of Arabidopsis thaliana is regulated by differences in auxin synthesis and transport. [ABSTRACT FROM AUTHOR]