Root-to-shoot signaling in plant adaptation to soil salinity

Soil salinity is a major threat to sustainability and profitability of agricultural production systems and food security of future generations. Plants respond to salinity-induced constraints by activating numerous adaptive responses that operate in a strict tissue- and cell-specific manner and require coordination at the whole-plant level. Central to this process is the root-to-shoot signaling. Being the first organ to sense saline conditions in the rhizosphere, roots produce various signals that are then propagated through the plant, enabling the coordination and integration of physiological processes across different organs and tissues. These signals can be of different nature and include physical (electric and hydraulic signals; propagating reactive oxygen species and Ca2+ waves), chemical (hormones, photoassimilates, and nutrients), and molecular (peptides, proteins, and miRNAs) signals. Each category of long-distance signals has its own origin, transport mechanism, target tissue(s), function, and interaction with other signals. In this work, we summarize the current knowledge of such long-distance signaling in plants grown under saline conditions, with a specific focus on halophytes-naturally 'salt-loving' plants. Our aim is to reveal specific signaling traits that confer salinity stress tolerance that can then be used as new targets in breeding programs aimed to improve salinity stress tolerance in crops.