The barrier to radial oxygen loss protects roots against hydrogen sulphide intrusion and its toxic effect.

The root barrier to radial O 2 loss (ROL) is a key root trait preventing O 2 loss from roots to anoxic soils thereby enabling root growth into anoxic, flooded soils. We hypothesized that the ROL barrier can also prevent intrusion of hydrogen sulphide (H 2 S), a potent phytotoxin in flooded soils. Using H 2 S and O 2 sensitive microsensors, we measured the apparent permeance to H 2 S of rice roots, tested whether restricted H 2 S intrusion reduced its adverse effects on root respiration, and if H 2 S could induce the formation of a ROL barrier. The ROL barrier reduced apparent permeance to H 2 S by almost 99%, greatly restricting H 2 S intrusion. The ROL barrier acted as a shield towards H 2 S; O 2 consumption in roots with a ROL barrier remained unaffected at high H 2 S concentration (500 μM), compared to a 67% decline in roots without a barrier. Importantly, low H 2 S concentrations induced the formation of a ROL barrier. In conclusion, the ROL barrier plays a key role in protecting against H 2 S intrusion, and H 2 S can act as an environmental signalling molecule for the induction of the barrier. The study demonstrates the multiple functions of the suberized/lignified outer part of the rice root beyond that of restricting ROL.