Karrikins: Smoke-Derived Phytohormones from Stress Alleviation to Signaling

Plant-derived smoke is a source of several butenolide compounds collectively termed, karrikins (KARs). These have been recognized as a recent class of plant growth regulators associated with diverse phenomena especially seed germination, seedling vigor and other photo-morphogenetic responses. Role of KARs in improving the performance of plants exposed to different environmental constraints has been recognized recently. KARs mitigate oxidative stress in plants caused by salinity, drought, shade and heavy metals by modulating antioxidative metabolism (SOD, POX, GR, APX) and up-regulating the expression of various stress related genes in plants. Such a modulation involves an interplay with different transcription factors and endogenous plant growth regulators predominantly auxins, gibberellins, ethylene and abscissic acid. The present article is an attempt to gain a comprehensive understanding regarding the KARs-induced plant responses. Moreover, role of KARs in plant abiotic stress tolerance, perception of KARs in plants, expression of different enzymes and genes involved in abiotic stress mitigation, their cross-talk with other endogenous hormones under normal and chellanging environmental circumstances.