Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation. They were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth. Cytokinins and auxins often work together, and the ratios of these two groups of plant hormones affect most major growth periods during a plant's lifetime. Cytokinins counter the apical dominance induced by auxins; they in conjunction with ethylene promote abscission of leaves, flower parts, and fruits.  The correlation of auxins and cytokinins in the plants is a constant (A/C = const.). [ citation needed ]
BL-binding to BRI1 is not sufficient to activate BRI1 directly. Instead, BL serves as “molecular glue” to induce heterodimerization of BRI1 and BAK1 via their ECDs (Santiago et al. 2013 ). This ECD interaction brings kinase domains of BRI1 and BAK1 into a close proximity, resulting in the dissociation of the negative regulators from the complex and the association of the BRI1-BAK1 kinases in the cytoplasm (Wang and Chory 2006 ). Transphosphorylation between BRI1 and BAK1 activates BRI1 and turns on downstream signaling cascade (Wang et al. 2005a , 2008 ; Yun et al. 2009 ; Yan et al. 2012 ). This is consistent with the “double-lock” model proposed earlier for BRI1-BAK1 complex formation (Jaillais et al. 2011a ; Li 2011 ). Yet, details of BRI1 activation still need to be clarified. One important process in BRI1 activation is the release of the BRI1 negative regulator, BKI1. BKI1 associates with the cytoplasmic domain of BRI1, inhibiting BRI1 kinase activity and blocking BRI1-BAK1 association (Wang and Chory 2006 ). It is proposed that BL-binding to BRI1 induces intracellular conformational change and kinase activation (Wang et al. 2008 ; Li 2011 ; Bücherl et al. 2013 ). Activated BRI1 phosphorylates BKI1 at Tyr211, Ser270, and Ser274 to trigger disassociation of BKI1 from PM (Jaillais et al. 2011b ; Wang et al. 2011 ). However, the crystal structures of BL-bound and BL-free BRI1 ECDs are found to be almost identical (She et al. 2011 ). Moreover, even BRI1 ECD-BL-BAK1 ECD formation is unable to cause striking conformational change in BRI1 ECD (Sun et al. 2013 ). It is more likely that the formation of BRI1-BL-BAK1 complex results in direct intracellular domain contact between BRI1 and BAK1, prior to BKI1 disassociation. BAK1 may indirectly remove BKI1 via activating BRI1, or directly phosphorylates BKI1 followed by the release of BKI1 from BRI1-BAK1 complex (Figure 1 ).