Karrikins enhance light responses during germination and seedling development in Arabidopsis thaliana

Karrikins are a class of seed germination stimulants identified in smoke from wildfires. Microarray analysis of imbibed Arabidopsis thaliana seeds was performed to identify transcriptional responses to KAR(1) before germination. A small set of genes that are regulated by KAR(1), even when germination is prevented by the absence of gibberellin biosynthesis or light, were identified. Light-induced genes, putative HY5-binding targets, and ABRE-like promoter motifs were overrepresented among KAR(1)-up-regulated genes. KAR(1) transiently induced the light signal transduction transcription factor genes HY5 and HYH. Germination of afterripened Arabidopsis seed was triggered at lower fluences of red light when treated with KAR(1). Light-dependent cotyledon expansion and inhibition of hypocotyl elongation were enhanced in the presence of germination-active karrikins. HY5 is important for the Arabidopsis hypocotyl elongation, but not seed germination, response to karrikins. These results reveal a role for karrikins in priming light responses in the emerging seedling, and suggest that the influence of karrikins on postfire ecology may not be limited to germination recruitment.