Structural and Functional Characteristics of the Mutant Plastids from Extranuclear Variegated Forms of Sunflower

Abstract

The electron-microscopic analysis of extranuclear variegated forms of sunflower demonstrated that plastids from white leaf areas are practically devoid of inner membrane structures. The mutants and the initial (wild-type) green line were compared by the contents of chlorophyll (Chl), carotenoids, and 70S ribosomes and by the activity of Rubisco. A mutant Var10 line was used to demonstrate that the primary characteristic manifestations of Chl deficiency include the synchronous retardation of the synthesis of a specific Chl precursor, 5-aminolevulinate, a decrease in the Chl a/b ratio to the level below that in the wild type, and the reduction of photosystem (PS) I Chl luminance. The progress of photodestructive processes in the mutant aggravated the listed disturbances; as a result, PSII complexes and light-harvesting complexes gradually degraded. The manifestation of the trait of Chl-deficiency notably varied depending on plant growth conditions (primarily, temperature and illumination regimes). The dependence of this manifestation on light and temperature was observed only at the early stage of development of pigment-containing tissues. When plants terminated this stage under low irradiation, they did not become Chl-deficient, and when their leaves were subsequently transferred to the conditions promoting the maximum pigment anomaly, the destructive processes did not advance in the mutant tissue. The authors assume that the plastom mutations under study impair the control over the expression of the structural plastogenes that determine the synthesis of pigment and protein components of the photosynthetic apparatus, rather than directly damage to the primary structures of plastogenes.