Plant–Plant Competition Limits Arabidopsis Shoot Branching and Silique Production Independently of Soil Mineral Nutrients, Strigolactones and BRANCHED1

In natural environments, plants compete with neighbouring plants for resources such as light, water and nutrients. To detect neighbours, plants have evolved mechanisms that are poorly understood at the molecular-genetic level. This study examined the impact of competition on the growth and reproductive success of Arabidopsis thaliana grown in crowded settings together with conspecifics or with the grass Lolium perenne. Intraspecific and interspecific competition resulted in strongly reduced shoot branching and silique production. The reduction in shoot branching correlated with greatly altered gene expression in lateral buds, in particular of hormone- and defence-related genes, while it was independent of the hub transcription factor BRANCHED1. Mutants defective in strigolactone signalling retained a response to competition. Similarly, competitors unable to synthesize strigolactones caused a normal inhibitory effect, indicating that strigolactones are not required for a response of Arabidopsis to competition. Fertilization did not overcome the inhibitory effect of competition, showing that plants under competition did not experience a lack of mineral nutrients. When the roots of focal and competitor plants were separated by water-impermeable below-ground partitions, plants did not respond to competition. We suggest that below-ground communication, together with a sensing of soil volume, participates in the response to competition.