Efficient iron plaque formation on tea (Camellia sinensis) roots contributes to acidic stress tolerance

Tea plants grow in acidic soil, but to date, their intrinsic mechanisms of acidic stress tolerance have not been elucidated. Here, we assessed the tea plant response to growth on NH₄ ⁺ nutrient media having different pH and iron levels. When grown in standard NH₄ ⁺ nutrient solution (iron insufficient, 0.35 mg L^-1 Fe²⁺ ), tea roots exhibited significantly lower nitrogen accumulation, plasma membrane H⁺ -ATPase activity, and protein levels; net H⁺ efflux was lower at pH 4.0 and 5.0 than at pH 6.0. Addition of 30 mg L^-1 Fe²⁺ (iron sufficient, mimicking normal soil Fe²⁺ concentrations) to the NH₄ ⁺ nutrient solution led to more efficient iron plaque formation on roots and increased root plasma membrane H⁺ -ATPase levels and activities at pH 4.0 and 5.0, compared to the pH 6.0 condition. Furthermore, plants grown at pH 4.0 and 5.0, with sufficient iron, exhibited significantly higher nitrogen accumulation than those grown at pH 6.0. Together, these results support the hypothesis that efficient iron plaque formation, on tea roots, is important for acidic stress tolerance. Furthermore, our findings establish that efficient iron plaque formation is linked to increased levels and activities of the tea root plasma membrane H⁺ -ATPase, under low pH conditions.