hormone metabolism plant hormone metabolism plant growth regulator metabolism UPa:UPA00437 phytohormone metabolism PGR metabolism pathway Metabolism of plant hormones (plant growth regulators, PGRs, phytohormones). Plant hormones are internally-secreted chemicals in plants that are used for regulating the plant growth. According to a standard definition, plant hormones are signal molecules produced at specific locations, that occur in very low concentrations, and cause altered processes in target cells at other locations. It is accepted that there are five major classes of plant hormones: auxins, cytokinins(CK's), ethylene, gibberellins (GA's) and abscisic acid (ABA).