Abstract
Sieve elements (SEs) represent one of the most specialized cell types in plants, yet several fundamental aspects of their structure, regulation, and physiological integration remain incompletely understood. Despite recent advances that refined our understanding of SE function, persistent questions and controversies remain concerning key aspects of SE biology, such as the disputed presence of a proteolytic machinery and a cytoskeleton, the functions of plastids and mitochondria, the roles of sieve-element reticulum (SER) and its membrane contact sites, the mechanisms and significance of sieve-element occlusion by SEOs and SEORs and phloem protein (PP) families, the nature and operation of Ca 2+ channels involved in sieve-tube occlusion and electrical signal propagation, and the mechanisms that compensate for SE enucleation. Here, we examine current concepts of SE biology, with particular emphasis on unresolved, conflicting, and emergent issues. We highlight experimental and conceptual approaches that may help resolve outstanding questions and to test hypotheses derived from existing data sets. Rather than providing a comprehensive catalogue of phloem functions, this review aims to delineate key conceptual bottlenecks and to outline future directions required to achieve a mechanistic understanding of SE function within the living plant. To this end, several hypothetical models of SE-functioning are included for further exploration.