Cytokinin signalling regulates organ identity via the AHK4 receptor in Arabidopsis

Abstract

Mutual interactions of the phytohormones, cytokinins and auxin determine root or shoot identity during postembryonic organogenesis in plants. However, our understanding of the role of hormonal metabolism and perception during early stages of cell fate reprogramming is still elusive. Here we show that auxin activates root formation, whereas cytokinins mediate early loss of the root identity, primordia disorganisation and initiation of shoot development. Exogenous and endogenous cytokinins influence the initiation of newly formed organs, as well as the pace of organ development. The process of shoot apical meristem establishment is accompanied by accumulation of endogenous cytokinins, differential regulation of genes for individual cytokinin receptors, strong activation of AHK4-mediated signalling and induction of the shoot-specific homeodomain regulator The last is associated with upregulation of isopentenyladenine-type cytokinins, revealing higher shoot-forming potential when compared with-zeatin. Moreover, AHK4-controlled cytokinin signalling negatively regulates the root stem cell organiser in the root quiescent centre. We propose an important role for endogenous cytokinin biosynthesis and AHK4-mediated cytokinin signalling in the control of-induced organ identity.

Cell-surface receptors enable perception of extracellular cytokinins

“…The transgenic line TCSn::GFP 11 was used for all cell-sorting experiments, TCSn::GFP and TCS::GFP 28 were employed in protoplast and seedling treatment experiments, and the ecotype Col-0 was used for extraction of apoplast/symplast and for feeding experiments with labelled cytokinins. Crossed lines ahk/TCSn::GFP were published previously 26 and the mutant lines cre1-2 (ahk4 ) [14], ahk2-5, ahk3-7, ahk2-5 ahk3-7 (ahk2,3), ahk2-5 cre1-2 (ahk2,4 ) and ahk3-7 cre1-2 ( ahk3,4 ) 37 were crossed also with TCS::GFP. Homozygous lines were then identified and used in confocal experiments.…”

“…iP had the strongest effect on meristematic stele cell initials, whereas tZ response was maximal in the transition zone in the stele. Although the exact biological role for cytokinins in the stele initials remains to be determined, TCS signal in those cells was absent in ahk4 mutant roots 26 (Supplementary Fig. 6c,d) providing genetic evidence for AHK4 being essential in cytokinin perception in the stele.…”

“…Finally, we tested the capacity of each of the three AHK receptors individually to respond to extracellular and free cytokinins, using TCSn::GFP lines mutated in the other two AHKs 26 . Although absolute signal strength was diminished in these mutant lines as shown elsewhere 26,28 , they all retained responsiveness to free IP and tZ (Fig. 4a,b).…”

Revealing the full-length transcriptome of caucasian clover rhizome development

“…T3 is closely linked to ETH and CTK-mediated pathways, which may be responsible for root swelling. It has been demonstrated that auxin activates root formation and that cytokinins mediate root identity, early primordial disintegration and early loss of bud development initiation [58]. CTKs are conducive to rhizome enlargement but not to rhizome induction.…”

Revealing the full-length transcriptome of caucasian clover rhizome development

“…T3 is closely linked to ETH and CTKmediated pathways, and it may be responsible for root swelling. It has been demonstrated that auxin activates root formation and that cytokinins mediate root identity, early primordial disintegration and early loss of bud development initiation [65]. CTKs are conducive to rhizome enlargement but not to rhizome induction.…”