Plants respond to environmental signals such as light, temperature, and chemicals through receptor-mediated cell signaling. Receptor-like kinases or RLKs make up the largest family of plant receptors. The most abundant RLKs are the serine/threonine kinase receptors.These receptors contain a cytoplasmic domain that phosphorylates serine and threonine residues. The light-responsive receptors or phytochromes are serine/threonine kinases inactivated by far-red light and activated by red light. Activated phytochromes initiate signaling cascades that regulate the expression of light-responsive genes according to light availability. For example, seed germination is inhibited in the dark and starts when light is available. Histidine kinase receptors, such as ethylene receptors, are another class of RLKs. In the absence of ethylene, the ethylene receptor activates a signaling cascade that prevents the expression of ethylene-responsive genes. Ethylene binding inactivates the receptor and allows these ethylene-responsive genes to be transcribed. This leads to physiological responses, such as fruit ripening.