Building lateral roots is an important ability for plants to exploit the undersoil environment, particularly when parts of it are unfavourable. Auxin is a significant hormone in regulating lateral root formation, but there may be other systems that can trigger the formation of lateral roots. In a new paper Ziping Chen and colleagues explore plant reactions to melatonin and examine what mechanisms interact to create new roots.
While melatonin was first discovered in animals, Chen et al. note that it has also been known in plants since 1995. We know it has a role in plants when combatting biotic and abiotic stresses, including work by some of the authors of the Annals paper elsewhere on salt tolerance and cadmium. It is also known to interact with auxin to regulate roots in rice, but the mechanism has remained unknown.
Chen and colleagues applied melatonin in solutions of various strengths to measure how three-day-old alfalfa seedlings responded. Unsurprisingly there were a variety of responses, but the strongest solutions did not have the greatest effect. Following this, the team tested to see if there would be a similar effect if the plant created its own hormone. They used transgenic Arabidopsis plants that overexpressed alfalfa MsSNAT to produce more melatonin than usual. The authors showed that the transgenic plants developed more lateral roots than wild-type plants.
They also followed up on prior work showing that ROS (Reactive Oxygen Species) signalling was vital to grow lateral roots. They found that the exogenous melatonin was inducing H2O2 generation in plants, and this appears to be the same with endogenous melatonin. Further, when they removed H2O2 from the plant, they found that the lateral root formation decreased.
The formation of roots by endogenous melatonin is a new finding with pharmacological and genetic evidence revealing that endogenous melatonin-triggered LR formation was H2O2-dependent.