The ability of Arabidopsis to respond to
geotropism is tied to the redistribution of auxin (Muday and
Rahman, 2008; Geisler et al., 2014), as this hormone regulates
root growth by targeting Aux/IAA (auxin/indoleacetic acid)
repressor proteins for degradation (Band et al., 2012) and the
asymmetry in the auxin distribution is established and maintained
by a spatio-temporal regulation of the PIN-FORMED (PIN) auxin
transporter activity (Baster et al., 2013).