AUTHOR=Antoniadi Ioanna , Mateo-Bonmatí Eduardo , Pernisová Markéta , Brunoni Federica , Antoniadi Mariana , Villalonga Mauricio Garcia-Atance , Ament Anita , Karády Michal , Turnbull Colin , Doležal Karel , Pěnčík Aleš , Ljung Karin , Novák Ondřej 

TITLE=IPT9, a cis-zeatin cytokinin biosynthesis gene, promotes root growth

JOURNAL=Frontiers in Plant Science

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.932008

DOI=10.3389/fpls.2022.932008

ISSN=1664-462X

ABSTRACT=<p>Cytokinin and auxin are plant hormones that coordinate many aspects of plant development. Their interactions in plant underground growth are well established, occurring at the levels of metabolism, signaling, and transport. Unlike many plant hormone classes, cytokinins are represented by more than one active molecule. Multiple mutant lines, blocking specific parts of cytokinin biosynthetic pathways, have enabled research in plants with deficiencies in specific cytokinin-types. While most of these mutants have confirmed the impeding effect of cytokinin on root growth, the <italic>ipt29</italic> double mutant instead surprisingly exhibits reduced primary root length compared to the wild type. This mutant is impaired in <italic>cis</italic>-zeatin (<italic>c</italic>Z) production, a cytokinin-type that had been considered inactive in the past. Here we have further investigated the intriguing <italic>ipt29</italic> root phenotype, opposite to known cytokinin functions, and the (bio)activity of <italic>c</italic>Z. Our data suggest that despite the <italic>ipt29</italic> short-root phenotype, <italic>c</italic>Z application has a negative impact on primary root growth and can activate a cytokinin response in the stele. Grafting experiments revealed that the root phenotype of <italic>ipt29</italic> depends mainly on local signaling which does not relate directly to cytokinin levels. Notably, <italic>ipt29</italic> displayed increased auxin levels in the root tissue. Moreover, analyses of the differential contributions of <italic>ipt2</italic> and <italic>ipt9</italic> to the <italic>ipt29</italic> short-root phenotype demonstrated that, despite its deficiency on <italic>c</italic>Z levels, <italic>ipt2</italic> does not show any root phenotype or auxin homeostasis variation, while <italic>ipt9</italic> mutants were indistinguishable from <italic>ipt29</italic>. We conclude that IPT9 functions may go beyond <italic>c</italic>Z biosynthesis, directly or indirectly, implicating effects on auxin homeostasis and therefore influencing plant growth.</p>