Ethylene inhibits rice root elongation in compacted soil via ABA- and auxin-mediated mechanisms

Intensive agriculture and changing tillage practices are causing soils to become increasingly compacted. Hard soils cause roots to accumulate the hormone ethylene, triggering reduced root elongation and increased radial swelling. We demonstrate that ethylene regulates these distinct root growth responses using different downstream signals, auxin, and abscisic acid (ABA). Auxin is primarily required to reduce cell elongation during a root compaction response, whereas ABA promotes radial cell expansion. Radial swelling was originally thought to aid root penetration in hard soil, yet rice ABA-deficient mutants disrupted in radial swelling of root tips penetrate compacted soil better than wild-type plants. The combined growth responses to auxin and ABA function to reduce the ability of roots to penetrate compacted soil.