Plant Transport and Physiology

Plants allow leaves to fall off in autumn

Directions: Answer the question based on the following passage.

A Season of mists and mellow fruitfulness it may be. But autumn is also the fall, and there is little fruitfulness to be had from leafless plants. This does not matter in nature. The winter dieback has a reason: the cost of maintain the leaves, and the risk of storm and frost damage, exceed the photosynthetic benefit to be from the pallid winter sunshine. But the inter ests of vegetables and the interests of their growers do not always coincide. The latter would like the former to flourish all year around.

Although artificial light can fool plants into thinking that autumn has not actually arrived, this is expensive in electricity. A new technique developed by Richard Amasino and Susheng Gan, of the University of Wisconsin-Madison, may fool plants on the cheap. They have devised a way to persuade their charges to keep their leaves even when the external signals are warning that winter is nigh. As a bonus, the method also seems to keep plants green and fresh for longer after they have been cut.

It is done by meddling with the genes. When a plant prepares to drop its leaves, it first evacuates as much protein from them as possible. Annual plants recycle this protein into their seeds; perennials transport it into storage in the trunk or stem. In nature, the biochemical changes that carry out this mobilization are governed by a genetic promoter: a DNA switch that starts up the genes that code for the enzymes required for the autumnal changes to take place.

Dr. Amasino and Mr. Gan have isolated this promoter and attached it to a new genetic partner. The gene they chose is for the enzyme that produces cytokinin, a hormone that invigorates many plant tissues, including the leaves. They have inserted their new gene promoter combination into tobacoo and arabidopsis (a weed much favoured by plant geneticists as a workhorse for experiments).

The idea was that, when autumnal changes switched on the natural promoter to start the process of leaf senescence, they would also switch on the synthetic promoter for the cytokinin gene, nullifying the message to close down the leaf. Cytokinin would be produced until it had managed to keep the leaves green and healthy. At that point it would shut itself off. In this way the system would be self regulating; the plant would never produce enough hormone to have side - effects beyond the leaves.

It worked, - Leaves from transgenic plants stayed green for the whole of a 20 weeks trail period, while control leaves gradually turned yellow. Even when picked, leaves from transgenic plants stayed green for around 40 days. Dr. Amasino and Mr. Gan see their discovery's first use as being to protect leaf vegetables such as lettuces and cabbages and to prolong their shelf life. But the new genetic combination could eventually be applied more widely. The added growth permitted by extra weeks or months of photosynthesis could boost yields of grain, and of flowers, form crops containing the hybrid gene. The transgenic tobacoo plants turned out to weight 50% more seed, than normal plants. Fruitful indeed.