Table 1.

Root tropism sensor regions, signaling mechanism, and action regions in Arabidopsis thaliana.

Tropism		Sensor region	Signalling	Action region
Gravitropism	primary	Columella S1 and S2[1]	Cholodny-Went[2]	Basal TZ[3]
	secondary	TZ or EZ[4]	Likely not C-W[5]	Apical TZ[4]
Hydrotropism		EZ[6]	Likely not C-W[7]	EZ cortex[6, 3]
Phototropism	BL neg.	Likely EZ or root cap[8, 9, 10]	Likely not C-W[11]	EZ[10]
	RL pos.	Possibly root cap[12, 13]	Unknown	EZ[12]
	BL pos.	Unknown	Unknown	Unknown
Halotropism		Unknown	Cholodny-Went[14]	Likely EZ* [15, 16]
Thigmotropism		Strongest in root cap[17]	C-W [18]	EZ [17]
(Electrotropism† )	primary	Unknown	Unknown	TZ[19]
	secondary	Unknown	Unknown	EZ[19]
(Thermotropism† )		Unknown	Unknown	Unknown
Oxytropism		Unknown	Possibly C-W[20]	Unknown
(Phonotropism† )		Unknown	I.a. Ca2+ [21]	Unknown

*Despite a lack of direct reporting, the action region of halotropism is likely in the EZ, as it is a Cholodny-Went tropism.

^†Likely not a sensu stricto tropism.

^[1](Blancaflor et al., 1998), ^[2](Geisler et al., 2014), ^[3](Krieger et al., 2016), ^[4](Wolverton et al., 2002), ^[5](Wolverton et al., 2011), ^[6](Dietrich et al., 2017), ^[7](Shkolnik et al., 2016), ^[8](Sakamoto and Briggs, 2002), ^[9](Briggs and Christie, 2002), ^[10](Mullen et al., 2002), ^[11](Kimura et al., 2018), ^[12](Kiss et al., 2003b), ^[13](Salisbury et al., 2007), ^[14](Galvan-Ampudia et al., 2013), ^[15](van den Berg et al., 2016), ^[16](Yokawa et al., 2014), ^[17](Massa and Gilroy, 2003), ^[18](Lee et al., 2020), ^[19](Wolverton et al., 2000), ^[20](Eysholdt-Derzsó and Sauter, 2017), ^[21](Rodrigo-Moreno et al., 2017).