IN-VITRO MORPHOGENIC CHARACTERISTICS OF PHYTOCHROME MUTANTS IN NICOTIANA-PLUMBAGINIFOLIA ARE MODIFIED AND CORRELATED TO HIGH INDOLE-3-ACETIC-ACID LEVELS

The involvement of indole-3-acetic acid (IAA) in the integration of th e light signal perceived by phytochrome during the morphogenesis of pl ants was investigated in Nicotiana plumbaginifolia Viviani. The chromo phore mutant pew1, deficient in all the phytochrome types, and the aur ea-like mutant pew2, which appears to be specifically deficient in phy tochromes expressed in darkness, were analysed for IAA-related morphog enic effects such as rooting, shooting and callus formation. We observ ed, in the absence of exogenously applied hormones, abundant root form ation by the pew2 mutant. The pew1 mutant exhibited callus formation i n the presence of gibberellins and cytokinins when the wild type did n ot. The previously described lethality of the double mutant pew1-pew2 was shown to be hormone-dependent since, in the light, exogenously app lied auxin and cytokinin (0.1 mg . 1(-1) each) led to plant regenerati on from calli and subsequent normal development. These observations su ggested an increase in the auxin/cytokinin ratio as a consequence of t he phytochrome mutations. We correlated these morphogenic characterist ics with high IAA levels in the mutants. The difference in IAA accumul ation in the two mutants indicates that among the different phytochrom es expressed by N. plumbaginifolia, the light-expressed isoforms play a major role in the control of IAA levels.