Evidence for functional heterogeneity of sieve element–companion cell complexes in minor vein phloem of Alonsoa meridionalis

Two modes of phloem loading have been proposed, apoplastic and symplastic, depending on the structure of sieve element-companion cell complexes (SE-CCCs) in minor vein phloem. Species are usually classified as either apoplastic or symplastic loaders although the cytology of SE-CCCs in minor veins of the majority of plants indicates that both mechanisms can be simultaneously involved in phloem loading. The functions of structurally different SE-CCCs in minor veins of the stachyose-translocating plant Alonsoa meridionalis were examined. A stachyose synthase gene, AmSTS1, was expressed in intermediary cells but not in the ordinary companion cell of the same vein. In contrast, sucrose transporter AmSUT1 protein was present in ordinary companion cells but not in the neighbouring intermediary cells. These data reveal the principles of phloem sap formation in A. meridionalis and, probably, in many other dicots. The two types of SE-CCCs within one and the same minor vein load different carbohydrates, using contrasting mechanisms for their delivery into the phloem. Lateral sieve pores in the minor vein phloem lead to mixing of the carbohydrates soon after loading. While symplastic and apoplastic pathways can function simultaneously during phloem loading, they are separated at the level of different SE-CCCs combined in phloem endings.