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Plant biology

Engineered male sterility

The phenomenon of ‘cytoplasmic male sterility’ in plants has long been exploited to enhance the productivity of certain crops. An innovative genetic-engineering system promises to widen applicability of the approach.

Among the main items on the wish-list of plant breeders are these. First, the ability to artificially suppress pollination and so prevent a plant's self-fertilization, thereby encouraging cross-pollination and higher-yielding seed through an effect known as ‘hybrid vigour’. Second, the ability to genetically engineer such suppression of male fertility into elements in the cytoplasm, rather than the nucleus — the result is transmission of desirable characteristics through genes in the female line, and those genes cannot ‘escape’ uncontrollably via pollen. Third, the ability to selectively restore male fertility. Although farmers want high-yielding hybrid seeds, for certain crops that seed has to produce fertile plants.

These aims can be achieved by exploiting a phenomenon known as ‘cytoplasmic male sterility’. As they describe in Plant Physiology, Ruiz and Daniell1 have demonstrated a promising new way of achieving all three goals. Their approach was tested in tobacco plants. It involves inserting a gene (phaA) from the bacterium Acinetobacter into plant chloroplasts, with — upstream of that gene — a ‘promoter’, psbA, and other regulatory elements.

The phaA gene encodes an enzyme, β-ketothiolase. The authors show that in their transgenic plants the enzyme accumulates in leaves and anthers, the pollen- producing structures, and alters the course of synthesis of fatty acids. A starting point in fatty-acid synthesis is acetyl-CoA, which under normal circumstances is converted to malonyl-CoA. However, β-ketothiolase overrides the usual enzyme involved, acetyl-CoA carboxylase, to produce acetoacetyl-CoA instead (Fig. 1a, b). Correct lipid metabolism is essential to the normal development of pollen, not least the pollen wall2,3. Ruiz and Daniell1 found that β-ketothiolase accelerates anther development and, among other consequences, causes the pollen grains to collapse — and thereby results in male sterility. But the transgenic plants were otherwise unaffected.

Figure 1: Engineering cytoplasmic male sterility with β-ketothiolase1.
figure1

a, In chloroplasts, acetyl-CoA, the substrate for the synthesis of fatty acids, is normally converted by acetyl-CoA carboxylase to yield malonyl-CoA. This pathway results in the correct development of anthers, pollen grains and seeds. b, In the transgenic chloroplasts, β-ketothiolase out-competes acetyl-CoA carboxylase for acetyl-CoA, with acetoacetyl-CoA being produced instead. The upshot is distorted anthers and failure of pollen development. Fertility in plants grown from the resulting hybrid seeds is restored under continuous illumination, with reversion to the normal pathway.

So much for producing male sterility. How about restoring it? The hybrid seed itself is valuable for growing certain ornamental species, for example, or for producing vegetables. But in cases such as oilseed rape, sunflower or maize, where the crop germinates from the second-generation seed of the hybrid plants, fertility has to be restored.

In some plants, the nuclear genome overrides cytoplasmic male sterility to restore male function, but this process often works inefficiently and has deleterious effects on plant growth because it interferes with general metabolism and development4,5. In the case investigated by Ruiz and Daniell1, no nuclear-encoded restoring factor is involved. This is where the psbA promoter and associated regulatory elements come in, because they confer light-sensitivity on the gene they control6.

The authors hypothesized that, even though both acetyl-CoA carboxylase and β-ketothiolase are controlled by light-inducible promoters, under continuous illumination the carboxylase would gain the upper hand, so restoring normal fatty-acid synthesis and male fertility. That turned out to be the case, at least to some extent. When grown under continuous illumination for 10 days, a sample of transgenic plants produced four flowers with viable pollen, and in due course viable seed.

Genetic transformation of chloroplasts in the cytoplasm has several advantages over nuclear transgenic technologies7. Apart from transgene containment8,9,10,11, those advantages include a comparatively high level of transgene expression, yielding proteins that are properly folded and fully functional; lack of side effects, such as stunting or other abnormalities12; and elimination of the laborious back-crossing that is needed with nuclear transformation to introduce cytoplasmic male sterility into élite plant lines. The new method is likely to be especially advantageous when applied to crop plants with longer generation times, such as cotton, maize and rice.

As to future research, it is not clear from Ruiz and Daniell's study how sterility was reversed by continuous illumination because the two competing enzymes are both light-regulated. That, then, is one aspect that calls for further investigation.

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Khan, M. Engineered male sterility. Nature 436, 783–785 (2005). https://doi.org/10.1038/436783a

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