The LRR receptor kinases are only one of many classes of transmembrane receptor serine/threonine kinases in plants. There are at least six additional families, each with its own characteristic set of extracellular domains. The lectin receptor kinases, for example, have extracellular domains that bind carbohydrate signal molecules. The Arabidopsis genome encodes over 300 receptor serine/threonine kinases, which makes them the largest family of receptors known in plants. Many are involved in defense responses against pathogens.
Ethylene Blocks the Degradation of Specific Transcription Regulatory Proteins in the Nucleus
Various plant growth regulators (also called plant hormones) help to coordinate plant development. They include ethylene, auxin, cytokinins, gibberellins, and abscisic acid, as well as brassinosteroids. Growth regulators are all small molecules made by most plant cells. They diffuse readily through cell walls and can either act locally or be transported to influence cells further away. Each growth regulator can have multiple effects. The specific effect depends on environmental conditions, the nutritional state of the plant, the responsiveness of the target cells, and which other growth regulators are acting.
Ethylene is an important example. This small gas molecule (Figure 15–69A) can influence plant development in various ways; it can, for example, promote fruit ripening, leaf abscission, and plant senescence. It also functions as a stress signal in response to wounding, infection, flooding, and so on. When the shoot of a germinating seedling, for instance, encounters an obstacle, ethylene promotes a complex response that allows the seedling to safely bypass the obstacle (Figure 15–69B and C).
Plants have various ethylene receptors, which are located in the endoplasmic reticulum and are all structurally related. They are dimeric, multipass transmembrane proteins, with a copper-containing ethylene-binding domain and a domain that interacts with a cytoplasmic protein called CTR1, which is closely related
Figure 15–69 The ethylene-mediated triple response that occurs when the growing shoot of a germinating seedling encounters an obstacle underground.
(A) The structure of ethylene. (B) In the absence of obstacles, the shoot grows upward and is long and thin. (C) If the shoot encounters an obstacle, such as a piece of gravel in the soil, the seedling responds to the encounter in three ways. First, it thickens its stem, which can then exert more force on the obstacle. Second, it shields the tip of the shoot (at top) by increasing the curvature of a specialized hook structure. Third, it reduces the shoot’s tendency to grow away from the direction of gravity, so as to avoid the obstacle. (Courtesy of Melanie Webb.) in sequence to the Raf MAP kinase kinase kinase discussed earlier (see Figure 15–49). Surprisingly, it is the empty receptors that are active and keep CTR1 active. By an unknown signaling mechanism, active CTR1 stimulates the ubiquitylation and degradation in proteasomes of a nuclear transcription regulator called EIN3, which is required for the transcription of ethylene-responsive genes. In this way, the empty but active receptors keep ethylene-response genes off. Ethylene binding inactivates the receptors, altering their conformation so that they no longer activate CTR1. The EIN3 protein is no longer ubiquitylated and degraded and can now activate the transcription of the large number of ethylene-responsive genes (Figure 15–70).
Regulated Positioning of Auxin Transporters Patterns Plant Growth