Tomato seed germination: regulation of different response modes by phytochrome B2 and phytochrome A

ABSTRACT Lycopersicon esculentum seeds germinate after rehydration in complete darkness. This response was inhibited by a far‐red light (FR) pulse, and the inhibition was reversed by a red light (R) pulse. Comparison of germination in phytochrome‐deficient mutants (phyA, phyB1, phyB2, phyAB1, phyB1B2 and phyAB1B2) showed that phytochrome B2 (PhyB2) mediates both responses. The germination was inhibited by strong continuous R (38 µmol m−2 s−1), whereas weak R (28 nmol m−2 s−1) stimulated seed germination. Hourly applied R pulses of the same photon fluence partially replaced the effect of strong continuous R. This response was called ‘antagonistic’ because it counteracts the low fluence response (LFR) induced by a single R pulse. This antagonistic response might be an adaptation to a situation where the seeds sit on the soil surface in full sunlight (adverse for germination), while weak R might reflect that situation under a layer of soil. Unexpectedly, the effects of continuous R or repeated R pulses were mediated by phytochrome A (PhyA). We therefore suggest that low levels of PhyA in its FR‐absorbing form (Pfr) cause inhibition of seed germination produced either by extended R irradiation (by degradation of PhyA‐Pfr) or by extended FR irradiation [keeping a low Pfr/R‐absorbing form (Pr) ratio].