Phytochrome A is required for light-inhibited germination of Aethionema arabicum seed

O_LIThe germination of most seeds is influenced by the duration, intensity, and quality of light. The seeds of the model plant Arabidopsis are positive photoblastic and require light to germinate. The germination of negative photoblastic seeds is inhibited by white light. The molecular mechanisms that regulate negative photoblastic germination are unknown due to the lack of a suitable model plant. C_LIO_LIWe identified an accession with negative photoblastic germination in Aethionema arabicum that grows in semi-arid natural habitats. In a forward genetic screen, we identified a mutant - koyash2 (koy2) - that is defective in negative photoblastic germination. There is a nonsense mutation in the gene encoding the phytochrome A photoreceptor in the koy2 mutant. C_LIO_LIHere we show that phytochrome A is required for negative photoblastic germination. The defective negative photoblastic phenotype of the koy2 mutant is the result of defective inhibition of germination by the phytochrome A mediated high-irradiance response. C_LIO_LIThis is the first example of phytochrome A-mediated response controlling negative photoblastic seed germination in white, red, far-red, and blue light. We speculate that genetically encoded variation in phytochrome A-mediated germination responses is responsible for local adaptation of Ae. arabicum throughout the Irano-Turanian region. C_LI One sentence summaryIdentification and characterization of a phytochrome A null mutant demonstrates an active role of phytochrome A in light-inhibited seed germination in Aethionema arabicum, a negative photoblastic Mediterranean plant.