The root:shoot ratio has long been known to be enhanced in plants under drought stress. Here we discovered that osmotic stress enhances long-distance sucrose transport to increase the root:shoot ratio in an abscisic-acid-dependent manner. The Arabidopsis sucrose transporters SWEET11 and 12, key players in phloem loading, are rapidly phosphorylated upon drought and abscisic acid treatments. The drought- and abscisic-acid-activated SnRK2 protein kinases phosphorylate the carboxy-terminal cytosolic regions of SWEET11 and 12. This phosphorylation enhances the oligomerization and sucrose transport activity of SWEETs, which results in elevated sucrose contents in roots and improved root growth under drought stress, leading to the enhanced root:shoot ratio of biomass and drought resistance. Notably, the expression of phospho-mimic SWEETs led to improved root growth even under non-stressed conditions. The phosphorylation of sucrose transporters provides an explanation for the long-standing observation that drought stress enhances the root:shoot ratio in plants and suggests a strategy for engineering drought-resistant crops.
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