Abstract
Self-incompatibility (SI) of the Brassicaceae family can be overcome by CO2 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 F1 hybrid seed production; however, the molecular mechanism by which CO2 alters the SI pathway has not been elucidated. In this study, to obtain new insights into the mechanism of CO2-induced SI breakdown, the focus was on two inbred lines of Brassica rapa (syn. campestris) with different CO2 sensitivity. Physiological examination using X-ray microanalysis suggested that SI breakdown in the CO2-sensitive line was accompanied by a significant accumulation of calcium at the pollen–stigma interface. Pre-treatment of pollen or pistil with CO2 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 F1 progeny of a CO2-sensitive×CO2-insensitive cross suggested that CO2 sensitivity is a semi-dominant trait in these lines. Analysis of F2 progeny suggested that CO2 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 CO2 treatment. No QTL was detected at the loci previously shown to affect SI stability, suggesting that CO2 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 CO2-sensitive SI in F1 hybrid breeding.
Highlights
Self-incompatibility (SI) is a widespread genetic system in many flowering plants which serves to prevent self-fertilization and maintain genetic diversity
Total genomic DNA was extracted from young leaves of two parental lines and F2 progeny using the cetyltrimethylammonium bromide (CTAB) method (Murray and Thompson, 1980)
Based on reciprocal cross results, the CO2 sensitivity trait may be controlled by genes expressed in the female organ (Fig. 8)
Summary
Self-incompatibility (SI) is a widespread genetic system in many flowering plants which serves to prevent self-fertilization and maintain genetic diversity. When a compatible pollen grain lands on the stigma, it swells and a pollen tube is allowed to grow, whereas when self-pollen attaches to the stigma, SP11/SCR binds to the extracellular domain of SRK of the same S-haplotype (Takayama et al, 2001), which triggers an SI signalling pathway to reject selfpollen. Another stigmatically expressed gene located at the S locus, S locus glycoprotein (SLG) (Nasrallah et al, 1987; Takayama et al, 1987), has been shown to enhance the recognition process between self-pollen and stigma
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