Root halotropism and light as a stress factor in roots

Yokawa, Fasano, Kagenishi and Baluška (2014) summarise earlier work showing that salt-stressed Arabidopsis roots change growth direction to avoid high-salt areas. They credit Li and Zhang (2008) and Sun et al. (2008) with the salt avoidance tropism.¹ The response needs an ion gradient sensing pathway that controls PIN2 abundance, recycling and degradation.¹ Later work tied the behaviour to phospholipase D Zeta2 activity, which stimulates clathrin-mediated endocytosis of PIN2. The tropism was then named root halotropism.² The same authors note that PLDζ2 also acts in hydrotropism, where it suppresses root gravitropism through PIN2.²

Yokawa and colleagues stress that light-induced stimulation of root growth changes the physiology of the whole seedling. They summarise it as running through COP1 interactions with the actin cytoskeleton and through anthocyanin biosynthesis.³ Both routes need sucrose as an energy source.³ They report Meng (2014) on roots lacking the coactivator AN3. These roots had less anthocyanin and grew longer than wild type in light. In darkness they were shorter. AN3 binds the COP1 promoter and inhibits light-induced elongation.⁴ The authors also expect light to affect Casparian band formation and call that easy to test.³

Calvo and Baluška (2015) list halotropism among the many tropisms of roots. They say roots show more growth and movement responses than other plant parts.⁵ Calvo and Friston (2017) argue that halotropism is not a hard-wired salt-avoidance response. In their account, it lets seedlings bypass salt patches while weakening or inhibiting the response to gravity and light.⁶ They read this through active inference. Roots may track the salt gradient and turn from the source. The plant may choose to resample through root nutation rather than update its expectations. They compare this to saccades in animal vision.⁷