Significance of minor-vein anatomy to carbohydrate transport

Abstract

Plant species which translocate distinct combinations of carbohydrates in the phloem were investigated to assess whether differences in minor-vein anatomy were associated with differences in carbohydrate composition of the phloem sap. In Vicia faba L., a species in which the minor-vein companion cells are modified into transfer cells, sucrose alone was found to be the translocated form of carbohydrate. In Vicia, phloem transport of sucrose was inhibited by pretreatment of leaves with p-chloromercuribenzenesulfonic acid (PCMBS), a known inhibitor of the sucrose carrier. In contrast, in Ocimum basilicum L., a species in which the minor-vein companion cells are of the symplasmically linked intermediary cell type, both sucrose- and raffinose-family oligosaccharides were exported in the phloem. In this species, no PCMBS sensitivity was observed for phloem transport of either sucrose- or raffinose-family oligosaccharides, although a PCMBS-sensitive sucrose carrier was detected in leaf tissues. This carrier did not appear to be involved in phloem loading, rather, it appeared that phloem loading occurred via the symplasm in this species. In the polyoltranslocating species Petroselinum crispum L., the same insensitivity to PCMBS was seen, suggesting that symplasmic phloem loading also occurred. The companion cells were symplasmically connected to the surrounding bundle-sheath cells by numerous “H”-shaped plasmodesmata but were not intermediary cells, and no raffinose oligosaccharides were exported by Petroselinum. Taken together, the data indicate that apoplasmic transport may be responsible for phloem loading in species in which sucrose alone is exported. However, in those plant species in which a combination of sucrose and any other carbohydrate, including the polyols, is translocated, symplasmic phloem loading may predominate.