Annals of Botany

CHS in CMS – the initial effect of orf138/?

Cytoplasmic male sterility (CMS) is a very useful tool in plant breeding, eliminating the possibility of selfing when outcrossing is required. For example, in radish (Raphanus sativa), CMS is caused by the mitochondrial (‘cytoplasmic’) gene orf128 but fertility may be restored by the nuclear Rf gene. This system, known as the Ogura cytoplasm, is the only CMS system available in cultivated radish and is thus widely used in its breeding and that of other members of the Brassicaceae. Despite this, relatively little is known about the way in which orf128 affects pollen development or viability and it is this deficiency that has been addressed by Yang et al. at Kyoto (pp. 483–489). Light and electron microscopic investigation of gametogenesis revealed that pollen development was normal until the tetrad stage. However, at this point things started to go wrong. Cell division continued in tapetal cells causing over-proliferation of the tapetum. This effectively inhibited further development of the microspores. The microspores did not gain colour and exine formation was perturbed. The failure in colour development was confirmed by the absence of UV-absorbing compounds, consistent with a deficiency in biosynthesis of flavanoid compounds. Assay by real-time PCR of mRNAs encoding enzymes involved in synthesis of flavanoids showed that most were unaffected in CMS plants. However, one – that encoding chalcone synthase (CHS), a key regulatory enzyme in flavanoid biosynthesis – was very markedly reduced in abundance. Thus the nuclear CHS gene was down-regulated in the presence of the mitochondrial gene orf128. Further, normal patterns of CHS expression were observed in plants in which fertility had been restored by the nuclear Rf gene. CHS has previously been shown to be involved in nucleus-based male sterility in radish and is known to be essential for pollen development in several other species, but now we know that CHS is involved in sterility in a CMS plant.