NEW IV MUTANTS OF PEA ARE DEFICIENT IN PHYTOCHROME-B

The Iv-1 mutant of pea (Pisum sativum L.) is deficient in responses regulated by phytochrome B (phyB) in other species but has normal levels of spectrally active phyB. We have characterized three further Iv mutants (Iv-2, Iv-3, and Iv-4), which are all elongated under red (R) and white light but are indistinguishable from wild type under far-red light. The phyB apoprotein present in the Iv-1 mutant was undetectable in all three new Iv mutants. The identification of allelic mutants with and without phyB apoprotein suggests that Lv may be a structural gene for a B-type phytochrome. Furthermore, it indicates that the Iv-1 mutation results specifically in the loss of normal biological activity of this phytochrome. Red-light-pulse and fluence-rate-response experiments suggest that Iv plants are deficient in the low-fluence response (LFR) but retain a normal very-low-fluence-rate-dependent response for leaflet expansion and inhibition of stem elongation. Comparison of Iv alleles of differing severity indicates that the LFR for stem elongation can be mediated by a lower level of phyB than the LFR for leaflet expansion. The retention of a strong response to continuous low-fluence-rate R in all four Iv mutants suggests that there may be an additional phytochrome controlling responses to R in pea. The kinetics of phytochrome destruction and reaccumulation in the Iv mutant indicate that phyB may be involved in the light regulation of phyA levels.