Table 1.

List of the plant SPX genes whose function has been analysed to some extent. N, cell nucleus; M, cell membrane; C, cytoplasm; +, increase; +(*), increase (except seeds); +(**), increase (except flowers and seeds); =, no difference; −, decrease; Pr, positive regulation; Pr*, positive regulation (except PvPDR2-like); Nr, negative regulation.

species	gene	protein location	expression after Pi starvation	regulation of PSI gene	main functional characteristics	source
Oryza sativa	OsSPX1	N	+	Nr	OsSPX1 can interact with OsPHR2 and acts as a negative regulator of OsPHR2. OsSPX1 regulates OsSPX2, 3 and 5 at the transcriptional level, and the repression of OsSPX1 results in excessive P accumulation in the shoot	Wang et al. [27,28]
	OsSPX2	N	+		OsSPX2 can interact with OsPHR2 and acts as a negative regulator of OsPHR2. PHR2, SPX1 and SPX2 constitute a regulatory feedback loop in P signalling	Wang et al. [28,45]
	OsSPX3	N/C	+	Nr	OsSPX3 plays an important role in OsIPS1/miR399-mediated long distance regulation on OsPHO2 and acts as a negative regulator of OsPHR2. OsSPX3 negatively regulates the root-to-shoot transportation of P. Overexpression of OsSPX3 inhibits plant growth, which is more severe under P-deficient conditions	Wang et al. [28] Shi et al. [46]
	OsSPX4	N/C	=		OsSPX4 can interact with OsPHR2 in the cytoplasm and inhibits translocation of PHR2 into the nucleus. OsSPX4 functions as a negative regulator of PHR2 and can affect the activity of OsPHR2, sequentially regulating downstream gene expression	Lv et al. [47]
	OsSPX5	N/C	+	Nr	OsSPX5 and OsSPX3 are paralogous genes. SPX3/5 proteins act as repressors of PHR2. Overexpression of SPX3 and SPX5 completely rescues the excessive shoot of P accumulation. SPX3/5 negatively regulates P transport from roots to leaves with redundant function	Shi et al. [46] Zhang et al. [43]
	OsSPX6		+		OsSPX6, as a paralogue of SPX3/5, may play a compensatory role	Shi et al. [46]
Arabidopsis thaliana	AtSPX1	N	+	Pr	AtSPX1 can interact with AtPHR1 and may act as a negative regulator of AtPHR1 in P concentration	Duan et al. [26] Qi et al. [48]
	AtSPX2	N	+		AtSPX2 can interact with AtPHR1 in the cell nucleus. AtSPX1 and AtSPX2 have functional redundancy with one another	Puga et al. [49]
	AtSPX3	M/C	+	Nr	Partial repression of AtSPX3 can exacerbate phosphate-deficiency symptoms, alter P allocation and enhance the expression of a subset of phosphate starvation responsive genes including AtSPX1	Duan et al. [26]
	AtSPX4	M	−		AtSPX4 can interact with AtPHR1 in the cytoplasm	Duan et al. [26]
Glycine max	GmSPX1	N/C	+(*)	Nr	GmSPX1 interacts with a newly identified P starvation-induced transcription factor GmMYB48, and this interaction may represent a potential suppressor of P signalling network in soya bean	Zhang et al. [43]
	GmSPX2, 4, 6, 9 and 10	N/C	+		—	Yao et al. [50]
	GmSPX3, 7 and 8	N	+		GmSPX3 overexpression results in increased P concentration in both leaf and root tissues under high P conditions, which correlates with elevated transcript levels of several PSI genes in the root hairs	Yao et al. [50]
	GmSPX5	N/C	+(**)		—	Yao et al. [50]
Phaseolus vulgaris	PvSPX1	N	+	Pr*	Overexpression of PvSPX1 results in increased P concentration in the roots, morphological change in root hairs, inhibition of main root growth, more numerous lateral roots and upregulated transcription of 10 PSR genes	Yao et al. [44]
	PvSPX2	N	+	Pr*	PvSPX2 participates in P signalling pathway in both shoot and root tissues. Overexpression of PvSPX2 results in increased transcription of several genes downstream from PvSPX1, suggesting that PvSPX2 might have a similar regulatory role as PvSPX1	Yao et al. [44]
	PvSPX3	N/C	+	=	PvSPX2 participates in P signalling pathway in both shoot and root tissues. PvSPX3 expression is less sensitive to P deficiency compared with that of PvSPX1 and PvSPX2	Yao et al. [44]
Triticum aestivum	TaSPX129		+		—	Fang et al. [51]
	TaSPX				TaSPX participates in high temperature-induced resistance to wheat stripe rust	Wei et al. [52]