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. 2015 Jul 1;142(13):2237–2249. doi: 10.1242/dev.117614

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© 2015. Published by The Company of Biologists Ltd

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

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Fig. 2. — Auxin-cytokinin cross-talk in meristems. (A) In the OC (red), WUS integrates cytokinin signalling (green+red) and controls stem cell (SC) activity together with HEC1. Cytokinin biosynthesis depends on two groups of enzymes, which are encoded by the LOG (LOG3, LOG4, LOG7) and IPT (IPT7) gene families. STM regulates cytokinin production by activating IPT7 expression, whereas cytokinin degradation is catalysed by CKX3 and CKX5. WUS facilitates cytokinin signalling by directly repressing the expression of type-A ARRs (ARR7). Auxin also modulates stem cell activity via regulation of type-A ARRs by the ARF transcription factor MP. In addition, it is responsible for determining the sites of primordia initiation, which involves PIN1, MP and AHP6. (B) The RAM is composed of different functional domains. Basally, formative stem cell divisions in the stem cell niche give rise to different cell lineages and, as the root grows, cells transit successively through a transit-amplifying zone, a transition zone and, finally, a differentiation zone in which they start elongating. The balance between cytokinin and auxin positions the transition zone and therefore controls progenitor cell entry into differentiation. Polar auxin transport (PAT) is regulated by PIN family auxin transporters. Auxin transport and signalling are tightly connected to the activity of PLT transcription factors. Cytokinin promotes cell entry into differentiation by dampening the accumulation of PIN1, PIN3 and PIN7. (C) Stem cell niche activity (SCN) in the root involves the concerted activity of PLT2, WOX5 and SCR. These genes are integrated in the hormonal transcriptional network and modulate the auxin-cytokinin balance. In addition to promoting stem cell fate through the positive regulation of PLT genes, auxin promotes distal stem cell differentiation by repressing WOX5 expression through ARF10 and ARF16. Cytokinin modulates auxin biosynthesis by repressing SCR and by signalling through the type-B ARR ARR1, which subsequently promotes auxin biosynthesis through its positive effect on ASB1 expression.