Mechanisms of carbon allocation to stem-like axial sink organs

Plant leaves provide a source of organic carbon for distribution to non-photosynthetic sink organs, transported as sugars in the phloem. Besides terminal sinks at the ends of the phloem, carbon is also delivered laterally to axial sinks such as stems, petioles, or taproots, which surround phloem conduits. Here, we review the current understanding of the mechanisms involved in allocating sugars to axial sinks. Sugar unloading can occur via apoplastic or symplastic cellular paths, depending on species and development. We highlight the roles of sugar transporters, as well as sugar-cleaving enzymes, which contribute to maintaining sink strength by modulating local sugar gradients. Although the underlying transport machinery is broadly similar to that in terminal sinks, axial sinks may require specific regulatory mechanisms to balance competition with downstream terminal sinks, as well as solutions for storing high levels of sucrose effectively-mechanisms that remain largely uncharacterized. We highlight major knowledge gaps and challenges in research on axial sinks. Given the economic importance of axial sink crops (e.g. sugarcane, cassava), a better understanding of resource allocation has a large potential for improving yield through targeted manipulation of sugar transport.