Phytochromes mediate germination inhibition under red, far-red, and white light in Aethionema arabicum

The view on the role of light during seed germination stems mainly from studies with Arabidopsis , where light is required to initiate this process. In contrast, white light is a strong inhibitor of germination in other plants, exemplified by accessions of Aethionema arabicum , another Brassicaceae. Their seeds respond to light with gene expression changes of key regulators converse to Arabidopsis , resulting in antipodal hormone regulation and prevention of germination. The photoreceptors involved in this process in A. arabicum were unknown. Screening the first mutant collection of A. arabicum , we identified koy-1 , a mutant that lost light inhibition of germination, due to a deletion in the promoter of HEME OXYGENASE 1 , the gene for a key enzyme in the biosynthesis of the phytochrome chromophore. koy-1 seeds are unresponsive to red- and far-red light and hyposensitive under white light. Comparison of hormone and gene expression between wild type and koy- 1 revealed that very low light fluence stimulates germination, while high irradiance of red and far-red light is inhibitory, indicating a dual role of phytochromes in light-regulated seed germination. The mutation also affects the ratio between the two fruit morphs of A. arabicum , suggesting that light reception via phytochromes can fine-tune several parameters of propagation in adaptation to conditions in the habitat. One sentence summary Characterization of a phytochrome chromophore biosynthesis mutant demonstrates an active role of phytochromes in the light-inhibited seed germination in Aethionema arabicum .