Breeding better crops that grow faster is high on the agricultural agenda. Now scientists have discovered a novel way of speeding up plant growth rate using a naturally occurring plant protein that regulates cell division, according to today's edition of Nature magazine. The final size of the plants and other characteristics, however, are not altered - fields of colossal crops will not be on the horizon; the plants will just reach maturity earlier.
Plants need to respond rapidly to variations in their external and internal environment because when a location becomes unpleasant they cannot up and move to more satisfying surroundings. Instead they alter their architecture by growing more roots or leaves to make the most of available nutrients. But the exact way in which this floral foraging occurs is something of an enigma.
Researchers at the Institute of Biotechnology in Cambridge and Aventis CropScience in Belgium are investigating the growth and adaptability of plants and have found that cell division is a critical factor in determining how plants react to environmental changes. Cell division occurs mainly at the growing tip of the shoot and roots in special zones called meristems. It's the activity of these regions that determines the architecture and growth rate of all plants.
During the cell division cycle, a complex process unfolds: specific protein complexes orchestrate a cascade of chemical activity within the cell and interact with the cell's structural machinery. Regulatory proteins called cyclins are central to this intricate process in animals, plants and yeasts. They get together with their catalytic protein partner, called a cyclin-dependent kinase (CDK) which becomes activated at particular points in the cycle if specific signals indicate that the time is right to divide. These complexes are the guardians of the gate, patrolling proliferation.
The Cambridge team, led by Dr Jim Murray, discovered the CycD family of genes from a tiny plant known as Arabidopsis. Plant D-type cyclins respond to extra-cellular signals such as sugar availability and control the point during the cell cycle at which cells become committed to division.
They discovered that engineering tobacco, another favourite experimental plant, with the Arabidopsis CycD2 gene leads to faster growth, measured as biomass accumulation, height and root development, and improved vigour in the field.' Four weeks after germination the altered plants were up to twice the size of the control plants. The plants produce new leaf organs more rapidly and, because they reach maturity more quickly, they flower up to two weeks earlier.
The Arabidopsis CycD2 protein associates with a tobacco CDK partner, forming a biochemically active complex that can drive the cell cycle. Elevated levels of CycD2-CDK complexes enhance plant growth rate by accelerating the rate at which new cells are produced in the meristems.
P ainstaking studies, taken over a 20-hour period, show that the cell cycle is shorter and that there are larger numbers of faster cycling cells in the meristem than in the control plants. These faster rates of cell division in the meristems do not affect cell size or meristem structure, but result in more rapid production of new leaves.
"The way plants react to environmental influences by altering their growth rate has always been rather obscure," says Dr Murray. "This research points to cell division as playing a key role."
Plants were previously thought to grow at a maximum rate under given circumstances. Discovering that a naturally occurring plant regulatory protein can accelerate growth shows that growth is not intrinsically optimised and that cell division influences the activity of meristems and overall plant growth rate. This challenges the widely held view that cell division is not the major driving force for growth and only occurs to subdivide space.
Conventional breeding tries to improve crop performance by crossing closely related species but this random shuffling of genes can sometimes cause unwanted side effects. The fine-tuning of one gene appears to be a precise way of increasing growth rate without introducing detrimental side-effects from the gene pool.
Arabidopsis belongs to the Brassica family, an extensive plant group which includes all the vegetables children love to hate: cabbage, cauliflower and brussel sprouts. As many of its genes closely resemble those from oilseed rape and cereals, these findings have good implications for crop design, potentially shortening the time to harvest, giving greater flexibility of sowing time and faster crop rotation during a single growing season.
There are also environmental benefits. Since the plants show improved competition against weeds and grow more vigorously in the early stages of growth, this reduces the need for widespread crop spraying with herbicides. If this technology is transferable to trees, it could be beneficial for the forestry industry: to grow trees more quickly for timber production or for environmentally friendly energy production initiatives such as the Arable Biomass Renewable Energy (ARBOR) project in Yorkshire.
