Petiole hyponasty: an ethylene-driven, adaptive response to changes in the environment

BACKGROUND: Many plant species can actively reorient their organs in response to dynamic environmental conditions. Organ movement can be an integral part of plant development or can occur in response to unfavourable external circumstances. These active reactions take place with or without a directional stimulus and can be driven either by changes in turgor pressure or by asymmetric growth. Petiole hyponasty is upward movement driven by a higher rate of cell expansion on the lower (abaxial) compared with the upper (adaxial) side. Hyponasty is common among rosette species facing environmental stresses such as flooding, proximity of neighbours or elevated ambient temperature. The complex regulatory mechanism of hyponasty involves activation of pathways at molecular and developmental levels, with ethylene playing a crucial role. SCOPE: We present current knowledge on the mechanisms that promote hyponasty in the context of other organ movements, including tropic and nastic reactions together with circumnutation. We describe major environmental cues resulting in hyponasty and briefly discuss their perception and signal transduction. Since ethylene is a central agent triggering hyponasty, we focus on ethylene in controlling different stages during plant development and summarize current knowledge on the relationship between ethylene and cell growth.