Manganese toxicity disrupts indole acetic acid homeostasis and suppresses CO₂assimilation reaction in rice plants

<jats:title>ABSTRACT</jats:title><jats:p>Despite the essentiality of Mn in terrestrial plants, its excessive accumulation in plant tissues causes growth defects, known as Mn toxicity. Mn toxicity can be divided into apoplastic and symplastic types depending on its onset. For growth defects, symplastic rather than apoplastic Mn toxicity is hypothesised to be more critical. However, details of the relationship between growth defects and symplastic Mn toxicity remains elusive. In this study, we aimed to elucidate the molecular mechanisms of symplastic Mn toxicity in rice plants. We found that under excess Mn conditions, CO<jats:sub>2</jats:sub>assimilation was inhibited by stomatal closure, and both carbon anabolic and catabolic activities were decreased. In addition to stomatal dysfunction, stomatal and leaf anatomical development were also altered by excess Mn accumulation. Furthermore, the indole acetic acid (IAA) concentration was decreased, and auxin-responsive gene expression analyses showed IAA-deficient symptoms in leaves due to excess Mn accumulation. These results suggest that excessive Mn accumulation causes IAA deficiency, and low IAA concentrations suppress plant growth by suppressing stomatal opening and leaf anatomical development for efficient CO<jats:sub>2</jats:sub>assimilation in leaves.</jats:p><jats:sec><jats:title>HIGHLIGHT</jats:title><jats:p>Increased Mn concentration lowers auxin concentrations in rice leaves, which suppresses photosynthesis by changing stomatal function and development.</jats:p></jats:sec>