Karrikins, redox and plant abiotic stress tolerance: A focal review

Karrikins (KARs) are bioactive butenolide molecules that were initially discovered as germination stimulants in the smoke generated from burned plant materials. The past decades have witnessed the unraveling of KARs as a group of potential plant hormones that impact various aspects of plant physiology, from regulation of light responses and controlling root morphology, to modulation of secondary metabolism. KARs share a similar structure with strigolactones (SLs), and they activate similar signaling pathways where the interaction of α/β hydrolases with the F-box protein MORE AXILLARY GROWTH 2 (MAX2) is involved. Upon binding with KARs, the α/β hydrolase KAI2 (KARRIKIN INSENSITIVE 2) interacts with MAX2, which in turn activates the downstream signals by promoting the degradation of SMAX1/SMXL2 (SUPPRESSOR OF MAX2 1/SMX1-LIKE). Recent accumulating evidence has proved that KARs act as a protective agent regulating plant adaptation to abiotic stresses, particularly drought, salinity, and extreme temperature; however, these functions have not been widely perceived. This review revisits the knowledge of the characteristics and signals transduction of KARs in plants, with a focus on the interplay of KARs and redox signals in modulating plant tolerance to abiotic stresses. This collection of information is valuable for better understanding plant-environment interactions and will help create stress-tolerant crops and achieve the goal of agricultural and ecological sustainability