Table 1.

The identified and/or putative cytokinin transporters^∗.

Transporter family	Name	Substrates (organism for detection)	Physiological functions in plant	Subcellular localization	Expression pattern	References
ENT	AtENT1	Adenosine in yeast	Unknown	Plasma membrane or vacuolar membrane		Li and Wang, 2000; Sun et al., 2005; Jaquinod et al., 2007
	AtENT8/SOI33	iPR in seedlings	Reduced sensitivity of soi33-1 to iPR; Overexpression increases sensitivity to iPR	Unknown	Seedling, hypocotyls, flowers, midribs, abscission zones, siliques	Sun et al., 2005
	AtENT3	iPR in seedlings; adenosine in yeast; nucleoside in Xenopus oocytes	Reduced uptake efficiency of nucleoside-type CK (iPR), seedlings	Unknown	Unknown	Li et al., 2003; Sun et al., 2005
	AtENT6	iPR, tZR and adenosine in yeast	Unknown	Plasma membrane	Vascular tissues of root, leaf and flower, and cotyledons, stomata	Wormit et al., 2004; Hirose et al., 2008
	OsENT2	iPR, tZR, adenosine and uridine in yeast	Unknown	Unknown	Vascular tissues of leaf and root	Hirose et al., 2005
PUP	AtPUP1	Hypoxanthine, zeatin, kinetin, caffeine in yeast (proton-gradient dependent symporter)	Unknown	Unknown	Leaf mesophyll cells, stems, flowers, siliques, and hydathodes, stigma	Gillissen et al., 2000; Bürkle et al., 2003
	AtPUP2	Adenine, tZ, cZ, iPA, kinetin, BAP and tZR in yeast (proton-gradient dependent symporter)	Unknown	Unknown	Vascular tissues of leaf, stem, particularly in phloem, pollen	Bürkle et al., 2003
	AtPUP14	tZ, iPA, BA in protoplast and microsomes of plant. (ATP- dependent)	CK signaling, morphogenesis in embryos, roots and the shoot apical meristem	Plasma membrane	Various organ and tissues, including seedlings, embryos and mesophyll cells	Zürcher et al., 2016
	OsPUP7	Caffeine in yeast	Plant height, seed size and flowering time. Higher iP and iPR levels in mutant plant	Endoplasmic reticulum	Vascular bundle of roots, culms and leaves, hull vein and flower, stigma, style, stamens	Qi and Xiong, 2013; Kirschner et al., 2018; Xiao et al., 2018
	OsPUP4/BG3	Unknown	Seed size	Plasma membrane	Parenchyma cells near vascular tissue	Xiao et al., 2018
ABCG	AtABCG14	tZ-type CKs	Reduced root-to-shoot translocation in mutant. Shoot and root growth, nitrogen signaling	Plasma membrane	Vascular tissue in root (mainly) and shoot, leaf midribs, veins, mature anthers	Ko et al., 2014; Zhang et al., 2014; Poitout et al., 2018

^∗iPA or iP, isopentenyladenine; iPR, isopentenyladenosine; BAP or BA, benzylaminopurine or benzyladenine; tZ, trans-zeatin; tZR, trans-zeatin riboside.