Extracellular secretion of major latex-like proteins related to the accumulation of the hydrophobic pollutants dieldrin and dioxins in Cucurbita pepo

Abstract

Members of the Cucurbitaceae family, such as cucumber, pumpkin, and zucchini, show high accumulation of dieldrin and dioxins in their aboveground parts. Major latex-like proteins (MLPs) transport these hydrophobic compounds by forming complexes with them. The translocation mechanisms of MLPs in zucchini plants (Cucurbita pepo L. subspecies pepo) have not yet been clarified. MLP gene promoters in zucchini plants mainly showed the expression in roots, especially in the endodermis and stele. Immunostaining of MLPs in zucchini roots using specific antibodies revealed intracellular localization in cultivars accumulating lower levels of the compounds; however, in cultivars of high accumulation, MLPs accumulated in intercellular space. Transgenic tobacco plants transformed with MLP-PG1 showed intracellular localization of MLP-PG1 in the root cells, whereas MLP-GR1, which differs from MLP-PG1 by one amino acid, was detected intercellularly. Furthermore, clear difference was observed in the subcellular localization of MLPs in the soluble and membrane fractions between the low- and high-accumulating cultivars. Differences in the amino acid sequences of MLPs and subcellular localization are critical for the extracellular secretion of MLPs, where would determine the dieldrin and dioxin accumulation in zucchini cultivars.