Long-distance plant signaling pathways in response to multiple stressors: the gap in knowledge

Plants require the capacity for quick and precise recognition of external stimuli within their environment for survival. Upon exposure to biotic (herbivores and pathogens) or abiotic stressors (environmental conditions), plants can activate hydraulic, chemical, or electrical long-distance signals to initiate systemic stress responses. A plant's stress reactions can be highly precise and orchestrated in response to different stressors or stress combinations. To date, an array of information is available on plant responses to single stressors. However, information on simultaneously occurring stresses that represent either multiple, within, or across abiotic and biotic stress types is nascent. Likewise, the crosstalk between hydraulic, chemical, and electrical signaling pathways and the importance of each individual signaling type requires further investigation in order to be fully understood. The overlapping presence and speed of the signals upon plant exposure to various stressors makes it challenging to identify the signal initiating plant systemic stress/defense responses. Furthermore, it is thought that systemic plant responses are not transmitted by a single pathway, but rather by a combination of signals enabling the transmission of information on the prevailing stressor(s) and its intensity. In this review, we summarize the mode of action of hydraulic, chemical, and electrical long-distance signals, discuss their importance in information transmission to biotic and abiotic stressors, and suggest future research directions.