The fluorescent probe HPTS as a phloem-mobile, symplastic tracer: an evaluation using confocal laser scanning microscopy

Confocal laser scanning microscopy (CLSM) has been used to evaluate the use of the fluorescent probe HPTS (8-hydroxypyrene-1,3,6-trisulphonic acid) as a symplastic tracer. HPTS-acetate was used to load HPTS into the phloem and its movement was followed in three systems where symplastic unloading has been proposed. In Arabidopsis root tips and Abutilon nectaries the intercellular distribution of HPTS differed markedly from that observed with 5-(and 6)-carboxyfluorescein (CF)- HPTS was observed in the nuclei and cytoplasm whilst CF was rapidly transferred into the vacuoles. In contrast, both HPTS and CF accumulated in the vacuoles of the vascular parenchyma and nucellus cells following unloading from the phloem of the developing barley caryopsis. The results indicate that HPTS has a number of advantages as a symplastic probe compared with CF. The findings are discussed in relation to the influence of vacuolar sequestration on dye distribution.