Long-distance phloem transport of glucosinolates in Arabidopsis

Glucosinolates are a large group of plant secondary metabolites found mainl y in the order Capparales, which includes a large number of economically im portant Brassica crops and the model plant Arabidopsis. In the present stud y, several lines of evidence are provided for phloem transport of glucosino lates in Arabidopsis. When radiolabeled p-hydroxybenzyl-glucosinolate (p-OH BG) and sucrose were co-applied to the tip of detached leaves, both tracers were collected in the phloem exudates at the petioles. Long-distance trans port of [C-14]p-OHBG was investigated in wild-type and transgenic 35S::CYP7 9A1 plants, synthesizing high amounts of p-OHBG, which is not a natural con stituent of wild-type Arabidopsis. In both wild-type and 35S::CYP79A1 plant s, radiolabeled p-OHBG was rapidly transported from the application site in to the whole plant and intact p-OHBG was recovered from different tissues. The pattern of distribution of the radioactivity corresponded to that expec ted for transport of photoassimilates such as sucrose, and was consistent w ith translocation in phloem following the source-sink relationship. Radiola beled p-OHBG was shown to accumulate in the seeds of wild-type and 35S::CYP 79A1 plants, where p-OHBG had been either exogenously applied or endogenous ly synthesized from Tyr in the leaves. p-OHBG was found in phloem exudates collected from cut petioles of leaves from both wild-type and 35S::CYP79A1 plants. Phloem exudates were shown to contain intact glucosinolates, and no t desulphoglucosinolates, as the transport form. It is concluded that intac t glucosinolates are readily loaded into and transported by the phloem.