Table 2.

Secondary messengers and phytohormones (potentially) involved in Arabidopsis thaliana root tropisms. With the following abbreviations: inositol 1,4,5-trisphosphate (InsP₃), phospholipase Dζ2 (PLDζ2), phosphatidylinositol 3-phosphate (PtdIns3P), and phosphatidic acid (PA).

Tropism		Secondary messengers	Phytohormones
Gravitropism	primary	Ca2+ [1] , InsP3 [2], NO[3], pH[1] , PLDζ2[4], PtdIns3P[5], ROS[6]	Auxin[7], gibberellic acid[8]
	secondary	Unknown	Unknown
Hydrotropism		Ca2+ [9], PLDζ2[4], ROS[6]	ABA[10], auxin?[11, 12], brassinosteroids[13], cytokinin[14], ethylene?[15]
Phototropism	BL neg.	Ca2+?[16], Flavonoids[17]	Cytokinin[18]
	RL pos.	Unknown	Unknown
	BL pos.	Unknown	Unknown
Halotropism		Ca2+?[19], Flavonoids[20], H2O2?[19], PLDζ2[21]	Unknown
Thigmotropism		Ca2+ [22], pH[22], ROS[22, 23]	Auxin[24], ethylene[25]
(Electrotropism† )		Unknown	Unknown
(Thermotropism† )		Unknown	Unknown
Oxytropism		Unknown	Auxin?[26], ethylene[26]
(Phonotropism† )		Ca2+ [27], K+ [27], superoxide (O2 -)[27]	Unknown

^†Likely not a sensu stricto tropism.

^[1](Monshausen et al., 2011), ^[2](Perera et al., 2006), ^[3](París et al., 2018), ^[4](Taniguchi et al., 2010), ^[5](Joo et al., 2005), ^[6](Krieger et al., 2016), ^[7](Sato et al., 2015), ^[8](Löfke et al., 2013), ^[9](Takano et al., 1997), ^[10](Dietrich et al., 2017), ^[11](Shkolnik et al., 2016), ^[12](Kaneyasu et al., 2007), ^[13](Miao et al., 2018), ^[14](Saucedo et al., 2012), ^[15](Rowe et al., 2016), ^[16](Pedmale et al., 2010), ^[17](Silva-Navas et al., 2016), ^[18](Silva-Navas et al., 2016), ^[19](Shabala et al., 2015), ^[20](Petrella et al., 2018), ^[21](Galvan-Ampudia et al., 2013), ^[22](Monshausen et al., 2009), ^[23](Kurusu et al., 2013), ^[24](Lee et al., 2020), ^[25](Yamamoto et al., 2008), ^[26](Eysholdt-Derzsó and Sauter, 2017), ^[27](Rodrigo-Moreno et al., 2017).

The compounds thought to be involved on the basis of little or only indirect evidence, or with controversy, are followed by a question mark.