Multiple signals regulate the growth and development of plant organs and enable their adaptation to environmental conditions
In complex multicellular organisms such as higher plants and animals, metabolism, growth, and development of the various organs are coordi-nated by the emission of signal compounds. In animals these signals can be hormones, which are secreted by glandular cells. Hormones are classi-fied in paracrine hormones, which function as signals to neighboring cells, and endocrine hormones, which are emitted to distant cells (e.g., via the blood circulation). Also in plants, signal compounds are released from cer-tain organs, often signaling to neighboring cells, but also to distant cells via the xylem or the phloem. All these plant signal compounds are termed phytohormones. Some of the phytohormones (e.g., brassinosteroids) resem-ble animal hormones in their structure, whereas others are structurally completely different. Like animal hormones, phytohormones also have many different signal functions. They control the adjustment of plant metabolism to environmental conditions, such as water supply, tempera-ture, and day length, and regulate plant development. Light sensors includ-ing phytochromes, which recognize red and far-red light, and cryptochromes and phototropin monitoring blue light, control the growth and the differen-tiation of plants depending on the intensity and quality of light.
The signal transduction chain between the binding of a certain hormone to the corresponding receptor and its effect on specific cellular targets, such as the transcription of genes or the activity of enzymes, is now known for many animal hormones. In contrast, signal transduction chains have not been fully resolved for any of the phytohormones or light sensors. However, partial results indicate that certain components of the signal transduction chain in plants may be similar to those in animals. The phytohormone recep-tors and light sensors apparently act as a multicomponent system, in which the signal transduction chains are interwoven to a network.