Biological containment

Male sterility: Plants without pollen

---

Pollen is produced naturally by a plant’s male flower parts. Targeted pollen sterility can be an effective means of preventing the spread of the foreign genes introduced into GM plants. Various genetic approaches have been developed to produce "male-sterile" plants by suppressing pollen production.

No pollen, no spread: Pollen occurs in the male inflorescence, the filaments (anthers). If pollen production is prevented, the plants are male-sterile - they can no longer reproduce.

Two procedures are required to directly suppress pollen production: A gene present in the plant, the activity of which is required for the function or development of cells (or pollen), is "switched off". It would also be possible to introduce a new gene which disrupts important processes in the cell.  To ensure that this effect is limited exclusively to the cells involved in pollen production, the relevant gene constructs are linked to tissue-specific promoters . These cause the gene activity to be triggered only in specific cell types. The first concept for male-sterile plants was developed fifteen years ago. At that time scientists were concerned with using sterile plants to produce simple hybrid seed. Genetically modified oilseed rape, developed using this concept, has been grown in North America for several years. Today this concept can be used to produce pollen-sterile plants to prevent them reproducing and spreading in the environment. Further concepts are currently being developed.

Switching off an anther-specific enzyme

One new approach is the inhibition of the enzyme invertase, which is essential for pollen development. This anther-specific invertase breaks down sucrose (household sugar, a disaccharide) into the monosaccharides fructose and glucose, two important carbon sources for pollen development. Without the breakdown of sucrose, the pollen is deprived of its food source and is unable to develop fully: the plant is sterile.

Two methods can be used to limit or switch off invertase activity: The gene responsible for invertase can be switched off using the antisense technique, or alternatively the enzyme activity can be directly blocked using an invertase inhibitor.

 

(1) Antisense inhibition: The genetic blueprint for the production of a protein (in this case the enzyme invertase) is expressed at the corresponding site in the genome and a copy (messenger RNA or mRNA) is sent to the ribosomes, the protein factories in the cells. This process can be suppressed using special gene constructs. This involves exploiting a natural mechanism (RNA interference), which plants use to protect themselves against viruses, as a result of which the plant’s own, naturally occurring messenger RNA is regarded as "foreign" and destroyed. To ensure that this occurs only in cells responsible for pollen development, the antisense construct, in this case the invertase gene, is equipped with a tissue-specific promoter (see illustration above).

 

(2) Enzyme inhibitors: Enzyme activity can be inhibited by substances that have a chemical structure similar to the actual substrate - they too can bind to the enzyme, but are not processed. The relevant enzyme can be individually blocked using specific inhibitors.

To suppress pollen development, the pollen-specific invertase, for example, can be blocked with a substrate resembling sucrose. In this case the invertase is unable to convert sucrose because the inhibitor remains tightly bound to the enzyme. If the gene for the invertase inhibitor is introduced into a plant, it produces the inhibitor which then blocks the invertase: Pollen formation is suppressed (see illustration above).

 

In a current research project the two methods described are being applied to oilseed rape and maize to test their practical suitability.