Credit: CEPHAS/KEVIN JUDD

Pinot noir is an important grape variety that is used for making wine, including champagne. It is derived from an ancient strain that has been cultivated for maybe 2,000 years. Cultivated grapevines (Vitis vinifera) are usually reproduced from cuttings, so all individuals are genetically identical. But sometimes mutations arise, and long ago this resulted in the generation of another champagne grape variety, Pinot Meunier (pictured here), from Pinot noir. Pinot Meunier plants are genetically indistinguishable from Pinot noir in most cells, but their outer layer, the 'L1' epidermal cell layer, is different — meaning that Pinot Meunier has a furry surface on its leaves whereas Pinot noir does not. Elsewhere in this issue (Nature 416, 847–850; 2002), Paul K. Boss and Mark R. Thomas describe the precise mutation that causes this difference. Surprisingly, it is the grapevine equivalent of the 'dwarfing' mutations used to increase wheat yields during the green revolution.

Boss and Thomas started by producing grapevines that carried the mutation in all their cells, not just their skin: using tissue culture, the authors regenerated whole plants from Pinot Meunier L1 cells. As well as having hairy leaves, these plants were semi-dwarfed — they were shorter and stockier than usual. This provided a clue about which gene might be mutated. Sure enough, it turned out to be the grapevine equivalent of the gene that, when mutated, causes dwarfing in wheat. Gene sequencing showed that the gene had a similar mutation in the L1-cell plants and in dwarfed wheat.

What does the gene do? It was first identified and cloned from the thale cress Arabidopsis thaliana, and named GA INSENSITIVE because of the effect on the plant of mutating it. It is a regulatory gene that normally keeps a brake on plant growth; the brake is released by gibberellic acid (GA). Thus the plant can regulate its growth by controlling the production and location of this hormone. Some mutations in the gene disrupt the encoded protein so that gibberellic acid no longer releases the brake on growth. This means that the brake is permanently on, and the plant is smaller. Of course this only occurs if all cells are mutant; if the mutation is limited to the epidermis the only change seen is increased hairiness — gibberellic acid presumably also suppresses hair growth.

There was one more surprise from Boss and Thomas's study: in the L1-cell plants, tendrils were replaced by flowering stems. Normally, a new shoot produces several bunches of flowers (and thus grapes) opposite the first few leaves, and tendrils opposite leaves that form later. The tendrils anchor the vine as it grows in search of light. But in the semi-dwarfed mutant, flowering stems continued to form in place of tendrils. Presumably, the explanation is again that the plants cannot respond correctly to gibberellic acid. Normally, this hormone may ensure that the later-arising structures become tendrils rather than flowering stems. Gibberellic acid was known to influence flowering in other plant species, but this function is apparently new. Interestingly, tendrils in some plants such as peas have a different origin — they are modified leaflets rather than flowering stems. New knowledge about hormone-response genes may allow us to fine-tune both the vegetative architecture of grapevines, and how many bunches of grapes they produce.