Multitude of Long-Distance Signal Molecules Acting Via Phloem

Abstract

As sessile organisms, plants use long-range signalling between organs in order to adapt to their environment. The phloem is an important pathway for such long-distance communication. It transports signals that trigger systemic defence responses to wounding, herbivory and infection by plant pathogens. It also plays a pivotal role for developmental transitions, such as floral induction and tuberization, in response to stimuli perceived by the leaves, and physiological adaptation to nutrient deprivation. The signals involved in these processes include hormones, metabolites, proteins and RNAs, transported by mass flow with the phloem translocation stream. Faster signals, such as electropotential waves, can be propagated by the phloem plasma membrane. Most recent studies showed that these signalling pathways can recruit combinations of signal molecules, and that additional steps, such as molecular ‘hopping’ and amplification, may occur within the phloem tissue. This provides a basis to explain how plants cope with multiple environmental stimuli to confer long-lasting effects against stresses and maintain plant growth and development.