Specific carotenoids and proteins as prerequisites for chromoplast tubule formation

Abstract

The effect of inhibitors of carotenoid and protein biosynthesis on the development of tubulous chromoplasts has been investigated inTropaeolum majus andImpatiens noli-tangere by biochemical methods and electron microscopy. (1) Under the influence of norflurazone, a potent inhibitor of carotenoid biosynthesis, colorless flowers are formed in both species, the cells of which contain globulous leukoplasts instead of tubulous chromoplasts. (2) Contrary to this, unpigmented areas of petals ofCaltha palustris (normally with globulous chromoplasts) andNarcissus pseudonarcissus (with membraneous chromoplasts) are not affected in their respective plastid fine structure by norflurazone treatment. (3) CPTA, also affecting carotenoid biosynthesis, leads to the formation of pale pink flowers inTropaeolum, their major pigment being lycopene (normally not present in flowers of this plant). Chromoplast tubules are not formed under the influence of CPTA. Instead, the flower plastids contain osmiophilic globules as well as lycopene crystals and lycopene containing membranes. (4) Cycloheximide, inhibiting cytoplasmic protein biosynthesis, does not affect carotenoid biosynthesis inTropaeolum. It nevertheless abolishes the formation of chromoplast tubules, and also the accumulation of a nucleus-encoded 30 kDa protein that appears to be a major and obligatory component of chromoplast tubules. After CHI treatment, chromoplasts contain globules instead of tubules. (5) Inhibitors of plastid transcription (e.g., rifampicin) or translation (chloramphenicol, lincomycin) do not affect chromoplast fine structure in flowers ofTropaeolum. (6) Tubulous chromoplasts of flowers and fruits of plants from 4 diverse angiosperm classes invariantly contain apolar carotenoids as their main pigments and are devoid of xanthophylls. (7) Taken together, the results of our study strongly suggest that apolar carotenoids as well as specific structural proteins are essential constituents of chromoplast tubules. The data presented agree with a previously proposed model of the molecular fine structure of chromoplast tubules.