Table 1.

Classification, perception and functions of phytocytokines

Type	Phytocytokines	Receptor	Functions	Reference
Non-secreted peptides	Systemin	SYR1	Induction of proteinase inhibitors, extracellular alkalization, and ethylene emission, mediation of systemic defense response, defense against insect herbivory	Pearce et al. 1991; Ryan and Pearce 2003; Wang et al. 2018
	Pep1, Pep2, Pep3	PEPR1, PEPR2	Activation of PTI responses, and plant resistance to B. cinerea, P. syringae, and P. irregular, activation of ET, JA, and SA signaling pathways, mediation of systemic immunity	Huffaker et al. 2006, 2011; Liu et al. 2013; Ross et al. 2014; Yamaguchi et al. 2006, 2010
	ZIP1	Unknown	Activation of SA defense signaling, maize resistance against U. maydis and susceptibility to B. cinerea	Ziemann et al. 2018
	GmPep914, GmPep890	Unknown	Induction of extracellular alkalization and the expression of defense genes	Yamaguchi et al. 2011
Secreted peptides	PSK	PSKR1	Attenuation of PTI and SA signaling, activation of JA signaling, increase resistance to necrotrophic pathogens and susceptibility to biotrophic pathogens	Amano et al. 2007; Igarashi et al. 2012; Matsubayashi et al. 2002; Matsubayashi and Sakagami 1996; Mosher et al. 2013; Rodiuc et al. 2016; Zhang et al. 2018
	PSY1	PSY1R?
	PIP1, PIP2	RLK7	Activation of PTI responses and plant resistance to P. syringae and F. oxysporum	Hou et al. 2014
	IDA	HAE, HSL2	Regulation of plant resistance to P. syringae	Butenko et al. 2003; Patharkar and Walker 2016; Stenvik et al. 2008; Wang et al. 2017
	IDL6	HAE, HSL2	Suppression of PTI responses and SA signaling, attenuation plant resistance to P. syringae
	SCOOP12	MIK2	Activation of PTI responses, plant resistance to P. syringae and F. oxysporum, and susceptible to E. amylovora	Gully et al. 2019; Hou et al. 2021a, b; Rhodes et al. 2021
	RGF7/GLV4	RGI3/4	Activation of PTI responses	Matsuzaki et al. 2010; Ou et al. 2016; Stegmann et al. 2021; Wang et al. 2021; Whitford et al. 2012
	RGF9/GLV2	RGI3	Activation of PTI responses Promotion of FLS2 accumulation
	HypSys	Unknown	Induction of proteinase inhibitors, extracellular alkalization, and ethylene emission, activation of resistance to insect herbivory	Pearce et al. 2001a, b; Pearce and Ryan, 2003
	RALF1	FER-LLG	Attenuation of PTI, activation of JA signaling	Haruta et al. 2014; Li et al. 2015; Stegmann et al. 2017; Guo et al. 2018
	RALF17	Unknown	Activation of PTI responses
	RALF22, RALF23	FER-LLG	Attenuation of PTI signaling