Physiological and genetic analysis of CO(2)-induced breakdown of self-incompatibility in Brassica rapa

Self-incompatibility (SI) of the Brassicaceae family can be overcome by CO(2) gas treatment. This method has been used for decades as an effective means to obtain a large amount of inbred seeds which can then be used for F(1) hybrid seed production; however, the molecular mechanism by which CO(2) alters the SI pathway has not been elucidated. In this study, to obtain new insights into the mechanism of CO(2)-induced SI breakdown, the focus was on two inbred lines of Brassica rapa (syn. campestris) with different CO(2) sensitivity. Physiological examination using X-ray microanalysis suggested that SI breakdown in the CO(2)-sensitive line was accompanied by a significant accumulation of calcium at the pollen–stigma interface. Pre-treatment of pollen or pistil with CO(2) gas before pollination showed no effect on the SI reaction, suggesting that some physiological process after pollination is necessary for SI to be overcome. Genetic analyses using F(1) progeny of a CO(2)-sensitive×CO(2)-insensitive cross suggested that CO(2) sensitivity is a semi-dominant trait in these lines. Analysis of F(2) progeny suggested that CO(2) sensitivity could be a quantitative trait, which is controlled by more than one gene. Quantitative trait locus (QTL) analyses identified two major loci, BrSIO1 and BrSIO2, which work additively in overcoming SI during CO(2) treatment. No QTL was detected at the loci previously shown to affect SI stability, suggesting that CO(2) sensitivity is determined by novel genes. The QTL data presented here should be useful for determining the responsible genes, and for the marker-assisted selection of desirable parental lines with stable but CO(2)-sensitive SI in F(1) hybrid breeding.