Abstract
In many glandular structures, departure from the cell is only one step in the process of exudate release to the plant surface. Here the set of events that lead nectar to the external environment is presented and discussed mainly for stomata-free nectaries. After being synthesized, the nectar or some of its component needs to be released to the environment where it performs its functions. Nectar precursors derived from cell metabolism need to cross several barriers, such as the cell membrane and cell wall, in order to become nectar. Then the nectar must cross the cuticle or pass through stomata in order to be offered to plant mutualists. Release through stomata is a simple mechanism, but the ways by which nectar crosses the cuticle is still controversial. Hydrophilic pathways in the cuticle and repetitive cycles of rupture or cuticle detachment are the main routes for nectar release in stomata-free nectaries. In addition to nectar, there are other exogenous secretions that must leave the protoplast and reach the plant surface to perform their function. The ways by which nectar is released discussed herein are likely relevant to understanding the release of other hydrophilic products of the secretory process of plants.
Highlights
For many secretory structures, once the secreted substances cross the cell wall they remain inside the gland or beneath a protective cuticle
Hydrophilic pathways in the cuticle and repetitive cycles of rupture or cuticle detachment are the main routes for nectar release in stomata-free nectaries
The question arises: how do substances produced by secretory cells, like nectar, reach the exterior of the plant? In plant secretory systems, if secretory substances are discharged outside of the cell, they are considered to be extracellular secretions
Summary
Once the secreted substances cross the cell wall they remain inside the gland or beneath a protective cuticle. A cell-cycle model was recently proposed to explain how secretory products cross the plant cell wall (Paiva 2016), some doubt regarding the way secretory substances reach the surface of the outer cell wall remain. This is true when there is a cuticle that has to be traversed and, in part, results from the complexity of the cuticle, which is often erroneously interpreted. Some possible ways are proposed by which nectar could move from secretory cells towards the gland surface and perform its function, in stomata-free nectaries (mostly extrafloral)
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