Control of tissue and organ growth in plants.

Plant organs grow to characteristic, species-specific sizes and shapes. At the cellular level, organ growth is initially characterized by cell proliferation, which gives way to cell expansion at later stages. Using mainly Arabidopsis thaliana as a model species, a number of factors have been isolated in recent years that promote or restrict organ growth, with the altered organ size being associated with changes in cell number, in cell size, or in both. However, cells in an organ do not appear to follow a strictly autonomous program of proliferation and expansion, and their behavior is coordinated in at least three different respects: normally sized organs can be formed consisting of altered numbers of cells with compensatory changes in the size of the individual cells, suggesting that cellular behavior is subject to organ-wide control; the growth of cells derived from more than one clonal origin is coordinated within a plant lateral organ with its different histological layers; and growth of cells in different regions of an organ is coordinated to generate a reasonably flat leaf or floral organ. Organ growth is strongly modulated by environmental factors, and the molecular basis for this regulation is beginning to be understood. Given the complexity of organ growth as a dynamic four-dimensional process, precise quantification of growth parameters and mathematical modeling are increasingly used to understand this fascinating problem of plant biology.