NEW IV MUTANTS OF PEA ARE DEFICIENT IN PHYTOCHROME-B

The Iv-1 mutant of pea (Pisum sativum L.) is deficient in responses re gulated by phytochrome B (phyB) in other species but has normal levels of spectrally active phyB. We have characterized three further Iv mut ants (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 li ght. The phyB apoprotein present in the Iv-1 mutant was undetectable i n 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 mut ation results specifically in the loss of normal biological activity o f this phytochrome. Red-light-pulse and fluence-rate-response experime nts 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 elongati on can be mediated by a lower level of phyB than the LFR for leaflet e xpansion. The retention of a strong response to continuous low-fluence -rate R in all four Iv mutants suggests that there may be an additiona l phytochrome controlling responses to R in pea. The kinetics of phyto chrome destruction and reaccumulation in the Iv mutant indicate that p hyB may be involved in the light regulation of phyA levels.