Long days induce adaptive secondary dormancy in the seeds of the Mediterranean plant Aethionema arabicum.

Secondary dormancy is an adaptive trait that increases reproductive success by aligning seed germination with permissive conditions for seedling establishment. Aethionema arabicum is an annual plant and member of the Brassicaceae that grows in environments characterized by hot and dry summers. Aethionema arabicum seeds may germinate in early spring when seedling establishment is permissible. We demonstrate that long-day light regimes induce secondary dormancy in the seeds of Aethionema arabicum (CYP accession), repressing germination in summer when seedling establishment is riskier. Characterization of mutants screened for defective secondary dormancy demonstrated that RGL2 mediates repression of genes involved in gibberellin (GA) signaling. Exposure to high temperature alleviates secondary dormancy, restoring germination potential. These data are consistent with the hypothesis that long-day-induced secondary dormancy and its alleviation by high temperatures may be part of an adaptive response limiting germination to conditions permissive for seedling establishment in spring and autumn.

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 (Arabidopsis thaliana), 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 member of Brassicaceae. Their seeds respond to light with gene expression changes of key regulators converse to that of Arabidopsis, resulting in opposite hormone regulation and prevention of germination. However, the photoreceptors involved in this process in A. arabicum remain unknown. Here, we screened a mutant collection of A. arabicum and 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 were 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 2 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.