Table 2 Evolutionary history, protein characteristics and proposed roles of cytokinin metabolism genes in angiosperms.

	tRNA-IPTs	ATP/ADP-IPTs	Ancient CKXs	Non-ancient CKXs
Products or substrates^a	cZs	iPs/tZs	Preferentially cZs	Preferentially iPs/tZs
Origin	Ancient IPT in the last common ancestor of eukaryotes	Class II tRNA-IPT in the common ancestor of angiosperms	Chlamydial CKX during the primary endosymbiosis	Ancient CKX in the last common ancestor of land plants
Expansion & functional divergence in angiosperms	No	Yes	No	Yes
Organ development of angiosperms	Mutations lead to small and often chlorotic plants⁵	Altered expression leads to plants with abnormal organ development⁵	Overexpression leads to no significant changes after root growth is restored two weeks after germination^13,b	Altered expression leads to plants with abnormal organ development^37,61
Responses to abiotic stresses	Slightly responsive or nonresponsive	Downregulated	Nonresponsive	Rapidly induced
Abiotic stress tolerance	Not reported	Knockout mutations lead to increased drought or salt tolerance¹²; stress-, maturation-, or senescence-induced overexpression of the Agrobacterium AMP-IPT leads to increased iPs/tZs and improved tolerance to drought, salt and heat stresses^12,62	Not reported	Overexpression leads to improved tolerance to drought, salt⁶², heat¹⁴, or oxidative stresses, and knockout mutations of an inflorescence meristem-specific CKX lead to improved salt tolerance⁶³
Proposed roles in angiosperms	Housekeeping	Regulatory	Housekeeping	Regulatory

^aProducts of IPTs or substrates of CKXs
^bPlants overexpressing the ancient AtCKX7 gene showed small shoots and early-terminating primary roots within two weeks after germination. Thereafter, root growth was restored, and no significant changes were observed in plant growth or developmental patterns at later stages¹³

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