Nectar Secretion: Its Ecological Context and Physiological Regulation

Abstract

Plants secrete nectar to attract pollinators and indirect defenders. The chemical contents of both floral and extrafloral nectar appear adapted to attract and nourish these two classes of animal mutualists. Being rich in sugars and amino acids, however, nectar also requires protection from nectar robbers and infecting micro-organisms. This role is mainly fulfilled by nectar proteins (nectarins) and by secondary compounds such as alkaloids. Although much on the chemical ecology of nectar and the phenotypic patterns of its secretion is known, we have only limited knowledge on the molecular control of the synthesis of nectar components and of nectar secretion. Likewise, carbohydrates are uploaded from the phloem into the nectariferous tissue, where they might move via an apoplastic or a vesicle-bound, symplastic pathway. Cell wall invertases play a central role in creating the required source–sink relations and controlling the sucrose/hexose ratio of nectars. No information exits on the sites of synthesis of non-carbohydrate nectar components such as proteins and alkaloids, although it appears likely that at least the bulk of nectarins is synthesized in the nectariferous tissue itself. Most of the common model species do not depend on nectar secretion, and it might be this fact that has hindered nectar research over the last 50 years. We recommend the use of contemporary “omics” techniques in comparative approaches to understand how plants synthesize and secrete nectar.