Isotope-based visualization of element distribution in phloem provides functional evidence for the operation of SOS1 Na+/H+ exchangers in mature zones of Arabidopsis root

Abstract

In the phloem, various solutes such as photosynthates, mineral nutrients, or toxic elements move from the source organs to sinks. One of them is the root. While this process may mediate significant quantities of solutes delivered to the root, it is unclear how the solutes are distributed along the root axis. To elucidate the distributing pattern of solutes, we visually analyzed the movement of solutes in Arabidopsis roots using a radioisotope imaging technique. The distribution patterns fell into four different groups: (1) root tip accumulation for 14C-photosynthates, 28Mg2+, 32PO43-, and 35SO42-; (2) homogenous distribution along the root axis for 42K+ and 137Cs+; (3) no detectable accumulation in the root for 45Ca2+ and 59Fe2+; and (4) transient accumulation in the root for 22Na+. The latter phenomenon was explored in more detail using Arabidopsis knockout mutants lacking functional sodium efflux transporter SOS1 (operating as Na+/H+ exchanger). By utilizing a non-invasive microelectrode MIFE ion flux measuring technique, we found that Na+ efflux was active in the mature root zone of wild-type Arabidopsis plants but not in root apex as initially thought, and that in sos1 mutants lacking functional Na+/H+ exchangers, shoot-derived 22Na+ remained in the root. These findings challenge the notion that Na+ exclusion via SOS1 is confined to the root apex and demonstrates the power of combining the radioisotope imaging technique and the MIFE ion flux measuring technique to study the kinetics of ion transport in the root and root-to-shoot communication.