Synchrotron X-ray Fluorescence Microscopy unveils selenium distribution and a phloem-sink hypothesis in Neptunia amplexicaulis

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, N. heliophila. Synchrotron X-ray Fluorescence Microscopy (XFM) 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-hr period. Se distribution in N. amplexicaulis resembled that of N. heliophila, albeit at orders of magnitude higher prevailing Se concentrations. In both species, Se concentration was 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, though Se transport through the xylem should not be overlooked. Future work should focus on elucidating the subcellular and detailed phloem sink organ Se distribution.