Synchrotron X-ray fluorescence microscopy unveils selenium distribution and a phloem-sink hypothesis in <i>Neptunia amplexicaulis</i>

Neptunia amplexicaulis is a legume that grows on seleniferous soils in Queensland, Australia. It is one of the strongest known selenium (Se) hyperaccumulators, capable of accumulating up to 13,600 μg Se g-1 in its leaves. This study aimed to investigate the distribution and translocation of Se within N. amplexicaulis tissues compared to a non-accumulator Neptunia species, Neptunia heliophila. Synchrotron-based X-ray fluorescence microscopy was used to determine the in situ distribution of Se within the organs and tissues of N. amplexicaulis and N. heliophila. A pulse chase experiment was also undertaken to reveal Se translocation, using a control plant that was analyzed repeatedly after exposure to Se over a 74-h period. Selenium distribution in N. amplexicaulis resembled that of N. heliophila, albeit at orders of magnitude higher prevailing Se concentrations. In both species, Se concentrations were highest in the youngest/developing plant organs and preferentially localized in the vascular tissues, though Se was also strongly present in the xylem and pith of N. amplexicaulis. A phloem-sink model is proposed as the basis of Se distribution in N. amplexicaulis. Future studies should focus on elucidating the subcellular distribution of Se and on obtaining insights in the phloem loading and unloading processes of Se.