Gravitropic mutants have been identified that affect starch accumulation, such as those affecting the "PGM1" (which encodes the enzyme phosphoglucomutase) gene in "Arabidopsis", causing plastids - the presumptive statoliths - to be less dense and, in support of the starch-statolith hypothesis, less sensitive to gravity. Other examples of gravitropic mutants include those affecting the transport or response to the hormone auxin. In addition to the information about gravitropism which such auxin-transport or auxin-response mutants provide, they have been instrumental in identifying the mechanisms governing the transport and cellular action of auxin as well as its effects on growth.