<i>MORE PANICLES 3</i>, a natural allele of <i>OsTB1/FC1</i>, impacts rice yield in paddy fields at elevated <scp>CO₂</scp> levels

<jats:title>SUMMARY</jats:title><jats:p>Improving crop yield potential through an enhanced response to rising atmospheric CO<jats:sub>2</jats:sub> levels is an effective strategy for sustainable crop production in the face of climate change. Large‐sized panicles (containing many spikelets per panicle) have been a recent ideal plant architecture (IPA) for high‐yield rice breeding. However, few breeding programs have proposed an IPA under the projected climate change. Here, we demonstrate through the cloning of the rice (<jats:italic>Oryza sativa</jats:italic>) quantitative trait locus for <jats:italic>MORE PANICLES 3</jats:italic> (<jats:italic>MP3</jats:italic>) that the improvement in panicle number increases grain yield at elevated atmospheric CO<jats:sub>2</jats:sub> levels. <jats:italic>MP3</jats:italic> is a natural allele of <jats:italic>OsTB1/FC1</jats:italic>, previously reported as a negative regulator of tiller bud outgrowth. The temperate <jats:italic>japonica</jats:italic> allele advanced the developmental process in axillary buds, moderately promoted tillering, and increased the panicle number without negative effects on the panicle size or culm thickness in a high‐yielding <jats:italic>indica</jats:italic> cultivar with large‐sized panicles. The <jats:italic>MP3</jats:italic> allele, containing three exonic polymorphisms, was observed in most accessions in the temperate <jats:italic>japonica</jats:italic> subgroups but was rarely observed in the <jats:italic>indica</jats:italic> subgroup. No selective sweep at <jats:italic>MP3</jats:italic> in either the temperate <jats:italic>japonica</jats:italic> or <jats:italic>indica</jats:italic> subgroups suggested that <jats:italic>MP3</jats:italic> has not been involved and utilized in artificial selection during domestication or breeding. A free‐air CO<jats:sub>2</jats:sub> enrichment experiment revealed a clear increase of grain yield associated with the temperate <jats:italic>japonica</jats:italic> allele at elevated atmospheric CO<jats:sub>2</jats:sub> levels. Our findings show that the moderately increased panicle number combined with large‐sized panicles using <jats:italic>MP3</jats:italic> could be a novel IPA and contribute to an increase in rice production under climate change with rising atmospheric CO<jats:sub>2</jats:sub> levels.</jats:p>