The rice <i>HIGH‐TILLERING DWARF1</i> encoding an ortholog of Arabidopsis MAX3 is required for negative regulation of the outgrowth of axillary buds

<jats:title>Summary</jats:title><jats:p>Rice tillering is an important agronomic trait for grain production. The <jats:italic>HIGH‐TILLERING DWARF1</jats:italic> (<jats:italic>HTD1</jats:italic>) gene encodes an ortholog of Arabidopsis MAX3. Complementation analyses for <jats:italic>HTD1</jats:italic> confirm that the defect in <jats:italic>HTD1</jats:italic> is responsible for both high‐tillering and dwarf phenotypes in the <jats:italic>htd1</jats:italic> mutant. The rescue of the Arabidopsis <jats:italic>max3</jats:italic> mutant phenotype by the introduction of <jats:italic>Pro</jats:italic><jats:sub>35S</jats:sub><jats:italic>:HTD1</jats:italic> indicates HTD1 is a carotenoid cleavage dioxygenase that has the same function as MAX3 in synthesis of a carotenoid‐derived signal molecule. The <jats:italic>HTD1</jats:italic> gene is expressed in both shoot and root tissues. By evaluating <jats:italic>Pro</jats:italic><jats:sub><jats:italic>HTD</jats:italic>1</jats:sub>:GUS expression, we found that the <jats:italic>HTD1</jats:italic> gene is mainly expressed in vascular bundle tissues throughout the plant. Auxin induction of <jats:italic>HTD1</jats:italic> expression suggests that auxin may regulate rice tillering partly through upregulation of <jats:italic>HTD1</jats:italic> gene transcription. Restoration of dwarf phenotype after the removal of axillary buds indicates that the dwarfism of the <jats:italic>htd1</jats:italic> mutant may be a consequence of excessive tiller production. In addition, the expression of <jats:italic>HTD1</jats:italic>, <jats:italic>D3</jats:italic> and Os<jats:italic>CCD8a</jats:italic> in the <jats:italic>htd1</jats:italic> and <jats:italic>d3</jats:italic> mutants suggests a feedback mechanism may exist for the synthesis and perception of the carotenoid‐derived signal in rice. Characterization of <jats:italic>MAX</jats:italic> genes in Arabidopsis, and identification of their orthologs in pea, petunia and rice indicates the existence of a conserved mechanism for shoot‐branching regulation in both monocots and dicots.</jats:p>