Zinc toxicity in plants: a brief overview on recent developments

Zinc (Zn) is a vital element for plant growth and development as it performs crucial physio-biochemical functions in several metabolic pathways. Yet, the excessive levels of Zn in the soil environment can be accumulated by plant parts and impose toxic effects on seed germination rate, growth, photosynthesis, CO2-assimilation rates, cellular respiration, nitrogen metabolism, mineral nutrients uptake, osmotic/water balance, energy metabolism (such as mitochondrial organization, nicotinamide, and NAD+ breakage), and extra production of reactive oxygen species by desynchronizing antioxidant defense machinery. The physiology of Zn phytotoxicity is largely dependent upon its uptake transport and accumulation mechanisms. Zn-uptake is mainly governed by heavy metals (HMs) protein transporters such as HMs ATPs and ZIP family, while transport is mediated by metal transporter proteins (MTPs) related to cation diffusion facilitator (CDF) protein family and Zinc induced facilitator (ZIF). There is growing concern regarding the Zn-induced toxic effects in plants for the sustainable development of agriculture. Thus, in this chapter, we highlighted the mechanisms of uptake and transport of Zn-homeostasis in plant tissues as well as morpho-physiological and biochemical/metabolic modifications against excessive Zn levels. The outcomes of the current overview can shed light on different molecular mechanisms utilized by plants to handle excessive Zn accumulation and its induced toxic effects that can help plant breeders to develop Zn-tolerant genotypes.