Plant halotropism: mechanisms, signaling networks, and adaptive significance

Soil salinization is one of the most pressing abiotic stresses currently confronting global agriculture, posing a severe threat to crop yields and global food security. As soil salinity is not uniform, plant roots actively adjust their direction of growth to evade high-salinity regions; a phenomenon called halotropism. The core molecular mechanism underlying this process relies on the asymmetric redistribution of auxin. This paper systematically reviews the current knowledge of the physiological and molecular mechanisms regulating root halotropism, compares the divergent halotropic strategies adopted by different plant types, and discusses the prospects of applying this knowledge to the genetic improvement of crop salt tolerance. Among others, the key topics covered include: polar auxin transport and the role of phospholipase D (PLD) in regulation of endocytosis of the PIN2 proteins; abscisic acid (ABA) - mediated reorganization of microtubule and anisotropic cell wall expansion; and the role of ROS and Ca2+ signaling in regulating halotropism. The phenomenon of a "positive root halotropism" observed in halophytes and its functional role is also discussed in the context of improving salinity stress tolerance in crops to deal with the issue of global soil salinization under current climate scenarios.