Arsenite alters auxin transport and MAP kinase-mediated signaling in Arabidopsis thaliana L. root

Root is the plant organ in direct contact with the soil, and the later contains many pollutants as heavy metals and metalloids. Arsenite (As [III]) is the most toxic form of inorganic arsenic (As), a metalloid affecting plant growth and development. Little is known about the signaling mechanism by which plants respond to the toxic effect of As [III]. In this report, a pharmacogenetic strategy was used to show that As [III] inhibits root growth in Arabidopsis thaliana L., arresting cell division and elongation, as well as driving cell damage in primary root. Since N-1-naphthylphthalamic acid (NPA; auxin transport inhibitor) enhances the inhibitory primary root effects of As [III], it was hypothesized that auxins are involved in the response. This hypothesis was verified using the pin2 mutant, which is hypersensitive to the As [III] effect and accumulates auxins in the RAM, as well as evaluating the expression levels of the IAA17 gene, which regulates auxin-induced gene expression and whose expression was inhibited by As [III]. Besides, a MAPK cascade was implicated in the signaling pathway activated by As [III], since GUS reporter gene expression of pMPK6::UidA and pMPK3::UidA was up-regulated in a dependent manner on As [III] concentration, although, most likely due to functional redundancy, mpk6-2 and mpk3-1 mutants behave like a wild-type during As [III] treatments. In summary, our results suggest that As [III] inhibits cell division and expansion, causing cell damage on primary root tip and blocking auxin transport, through a MAP kinase module.