These interactions result in a synergistic increase in auxin efflux. Tryptophan is the starting molecule in the synthesis of tryptamine, serotonin, auxin, kynurenines, and melatonin. This process requires modification of the auxin transporters (PIN proteins). Synthesis in the leaf of a plant hormone called auxin also ceases. It is classified as an auxin plant hormone derivative. These proteins are ubiquitinated by SCFTIR1, or SCF in complex with the auxin receptor TIR1. It is a naturally occurring polar auxin transport inhibitor. Gautheret and Nobecourt were able to maintain callus cultures of carrot using auxin hormone additions. Plants also sense gravity and respond through auxin pathways, resulting in gravitropism. Polyphenol degradation decreases, increasing auxin concentration. Cytokinins act in concert with auxin, another plant growth hormone. Fruits form abnormal morphologies when auxin transport is disturbed. The synthetic auxin 2,4-Dichlorophenoxyacetic acid or 2,4-D was one of the first commercial synthetic herbicides. Examples include abscisic acid, auxin, cytokinin, ethylene, and gibberellin. When added to rooting media together with auxin, phloroglucinol further stimulates rooting.