Kinetic parameters of RNA synthesis in isolated mitochondria of different genotypes

The analysis of mitochondrial RNA-synthesizing systems in different maize genotypes may be important for elucidation of the role of the mitochondrial genome in molecular-genetic mechanisms responsible for commercially essential traits of maize plants. In this connection it appears reasonable to investigate the kinetics of mtRNA synthesis by isolated mitochondria from maize plants differing in the level of adaptability. The mitochondria were isolated from 3-day-old etiolated maize seedlings by a standard method of differential centrifugation. Mitochondrial protein was determined by the Lowry method. The functional activity of the isolated mitochondria (rates of mitochondrial respiration in the 3d and 4th metabolic states, respiration control) was estimated during kinetic experiments by a polarographic method with a Clark electrode. The synthesis of mtRNA was registered with labelled 3H-UTP (specific radioactivity was 560 TBq.mol-1). DNA-dependent mtRNA synthesis was determined by incorporation of labelled ribonucleosidetriphosphate into acid-insoluble mitochondrial material. The mtRNA synthesis was highly sensitive to specific inhibitors of mitochondrial transcription (ethidium bromide, actinomycin D). The kinetic data were obtained at least from 3-4 experiments.

The kinetics of mtRNA synthesis was registered within the first 30 minutes after mitochondrial isolation since it was possible to eliminate in these conditions the changes in mitochondrial genome activity following uncoupling of oxidation and phosphorylation as a result of organelle aging.

Depending on the rate of mtDNA transcription, two groups of inbreds may be identified (Fig.): 1, W64A and A344 + / + inbreds showing a low rate of mtRNA synthesis in their mitochondria and 2, Sg25 inbred and dwarf mutant A344 sin/sin demonstrating a high level. The first group of inbreds had higher kernel yield than the second. The inbreds of the first group differed in ecological plasticity: the W64A inbred exhibited high plasticity while the A344 + / + exhibited low plasticity. These inbreds showed relatively constant variation of the kernel yield in a wide range of environment. The yield potential of the Sg25 and induced mutant A344 sin/sin was genetically limited, i.e. the yield gain significantly decreased after reaching 25 q/ha level. Therefore, the high level of mtRNA synthesis may be assumed to result from inhibiting effect of nuclear genes. The Sg25 inbred with high general combining ability produced crosses of an extensive type (bi=0.24, s2di = 3.45).

The W64A x Sg25 cross was intermediate according to a rate of mtDNA transcription. It showed: 1, low kernel yield potential, 2, high level of long-term mean kernel yield in rainfed conditions, 3, high stable ecological response (bi = 1. 16, s2di = 4.9). For W64A x V 158 and W64A x A344, bi was 1.24 and 0.35, s2di was 42.5 and 26.7, respectively.

We propose that the rate of mtDNA transcription is negatively associated with a genotype's ability to maintain the constant value of variation in the adaptation response to a wide range of environment.

Figure 1.

Yu.M. Konstantinov1, G.N. Lutsenko1, V.A. Podsosonny1 and A.S. Mashnenkov

1Siberian Inst. Plant Physiol. Biochein., Irkutsk

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