Put your best root forward: How hydrotropism cannibalizes gravitropism to orient root growth towards moisture

While root gravitropism (growth towards gravity) has an extended history of in-depth research, less is known about root hydrotropism (growth towards water) and about how these two potentially conflicting processes interact with each other. Since soil water distribution is often heterogeneous, plant roots must be able to sense moisture gradients and grow toward areas of higher water potential. This directional growth does not always align with the downward pull of gravity, indicating a potential conflict or interaction with the gravitropic response. Zhang et al. showed that hydrotropism co-opts gravitropism machinery in order to override the latter’s effects on root growth. Mutants of the hydrotropism regulator, MIZU-KUSSEI 1 (MIZ1) do not avert root growth away from sorbitol-infused agar that simulates osmotic stress. Using a clinorotation treatment that simulates microgravity by rotating plants, the authors found that miz1 mutants lose, at least in part, their ability to orient root growth towards gravity, demonstrating crosstalk between gravitropism and hydrotropism. Through a series of expression studies and drought-simulation experiments, the authors concluded that plant roots can perceive decreases in environmental moisture and hijack PIN-FORMED (PIN) auxin efflux transporters to temporarily attenuate gravitropism in favor of seeking water. (Summary by Marvin Jin @MarvinJYS) Proc. Natl. Acad. Sci. USA 10.1073/pnas.2427315122