Effects of deficiency and excess of zinc on morphophysiological traits and spatiotemporal regulation of zinc-responsive genes reveal incidence of cross talk between micro- and macronutrients.

Zinc (Zn) is an essential micronutrient which affects plant growth and development in deficiency and can be toxic when present in excess. In Arabidopsis thaliana , different families of cation transporters play pivotal roles in Zn homeostasis. In the present study, we evaluated the effects of Zn in its deficiency (0 μM; Zn-) and excess (75 μM; Zn++) on various morphophysiological and molecular traits. Primary root length was reduced in Zn- seedlings, whereas there were significant increases in the number and length of lateral roots under Zn- and Zn++ conditions, respectively. Concentration of various macro- and microelements showed variations under different Zn regimes and notable among them was the reduced level of iron (Fe) in Zn++ seedlings compared to Zn+. Certain members of the ZIP family (ZIP4, ZIP9, and ZIP12) showed significant induction in roots and shoots of the Zn- seedlings. Their suppression under Zn++ condition indicated their transcriptional regulation by Zn and their roles in the maintenance of its homeostasis. Zn-deficiency-mediated induction of HMA2 in roots and shoots suggested its role in effluxing Zn into xylem for long-distance transport. Attenuation in the expression of Fe-responsive FRO2 and IRT1 in Zn- roots and their induction in Zn++ roots provided empirical evidence toward the prevalence of a cross talk between Zn and Fe homeostasis. Variable effects of Zn- and Zn++ on the expression of subset of genes involved in the homeostasis of phosphate (Pi), potassium (K), and sulfur (S) further highlighted the prevalence of cross talk between the sensing and signaling cascades of Zn and macronutrients. Further, the inducibility of ZIP4 and ZIP12 in response to cadmium (cd) treatment could be harnessed by tailoring them in homologous or heterologous plant system for removing pollutant toxic heavy metals from the environment.