The mitochondrial test allows us to discern the resistant mitochondria from the susceptible ones after the mutagenesis experience (R. Cassini et al., 1977). This test is performed by the help of the three following parameters: (1) the respiratory control ratio (RCR), (2) the ADP/O ratio which estimates the energy production, and (3) the stimulating coefficient (SC) determined by the ratio of the oxygen uptake in the presence of NADH after adding toxin to that before adding toxin, or (state 2 + toxin)/state 2.
We examined a lot of progenies (about 150). The test is accurate and rapid. The mitochondria by their quantitative response to the toxins of Helminthosporium maydis and Phyllosticta maydis and to the insecticide methomyl allow us to point out some differences between progenies which we might have disregarded. Moreover, the mitochondrial test is applied as soon as the maize seed germinates. This screening among the mutagenesis progenies led us to notice an intermediary resistance level. So, we can now distinguish between: (1) fully resistant progenies which behave like the resistant control with the normal "N" cytoplasm (the RCR is untouched and there is no stimulation of the NADH oxidation, SC = 1); (2) fully susceptible progenies which behave like the susceptible control with the Texas "T" cytoplasm [there is no coupling, RCR = 1 and the SC is maximum (Fig. 1)]; (3) intermediary resistant progenies (RI) according to SC and RCR. Combinations between these two values are possible from RCR = 1 to RCR maximum whereas SC maximum runs to SC = 1.
For attempting to understand what this intermediary resistance level means, we compared the intermediary progenies with mixed mitochondria extracted from seedlings carrying the N cytoplasm or the T cytoplasm. The mixture was made in large proportions. For each proportion (from 0% N:100% T to 100% N:0% T), we measured the RCR, the ADP/O and the SC ratios.
We have seen clearly that the mutagenesis progenies do not behave like the experimental mixtures of resistant and susceptible mitochondria. The progenies cannot be classed in front of the same mixture proportion for the three mitochondrial parameters (RCR, ADP/O and SC) (Fig. 2). Two hypotheses can answer to this observation: either there are interactions between the resistant and the susceptible mitochondria within the intermediary progenies, or we are in the presence of a new kind of mitochondria.
Till now, the resistance acquisition at the plant level was always associated with a fertility return. We made an extended study of the mitochondria in a family which kept the Texas male sterility after mutagenesis. This study was performed over several generations. We showed that some resistance level was present by the M6 generation (Table 1).
Table 1. Resistance level acquisition within family
The mutagenesis experience has proved that it is possible to break the phenotypic binding group between Texas male sterility and susceptibility. Moreover, there is no correlation between the different susceptibility levels to the three toxic substances. This is providing a lot of applications since we can now expect to find a propitious association between resistance and male sterility at the plant level.
Michele Paillard and A. Bervillé
Return to the MNL 54 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page