THE PHLOEM AND THE SYMPLASTIC PATHWAY DURING SINK ORGAN FORMATION

Abstract

The vascular bundle of plants is a highly specialized tissue that allows both communication among different organs and the transport of various substances over long distances. Water and minerals flow through the xylem, while water, sucrose, nucleic acids and proteins are distributed through the phloem. The phloem presents structural bridges called plasmodesmata, involved in the transport of signalling molecules related to cell-to-cell communication. Plasmodesmata can form specialized regions known as symplastic fields or symplastic domains, which are characterized for being semi-isolated or completely isolated from the rest of the tissues. Vascular tissue also participates in the formation of new organs from pericycle cells such as lateral roots, nodules, nematode feeding sites and tubers. In this review we discuss general aspects of vascular tissue, the importance of phloem as a molecular transport tissue, its role in the formation of important sink organs such as seeds and roots, as well as in the development of heterologous structures as nodules and nematode feeding sites (NFS). In addition, the influence of certain pathogens during their infection over the nutrient transport through the symplastic pathway is discussed.