Decrease of cytokinin flux from roots enhances degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase: a mechanism of the accelerated decrease of leaf photosynthesis with senescence under soil moisture stress in rice (Oryza sativa L.)

Background and Aims Leaf senescence is accelerated by soil moisture stress during reproductive growth in rainfed paddy rice under drought and in irrigated paddy rice under intermittent irrigation for saving water or mitigating methane emissions. Leaf senescence decreases leaf photosynthetic rate (A(n)) and grain yield. We aimed to elucidate the mechanisms underlying the A(n) decrease under soil moisture stress.Methods A(n), leaf content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), synthesis and degradation of Rubisco and cytokinin flux from roots were compared between plants grown in moisture-deficient soil (DR-plants) and flooded or wet soil (FL- or WE-plants, respectively) during senescence in pot-grown rice (Oryza sativa L.).Results The decreases in A(n) and Rubisco content were larger in the DR-plants than in the FL-plants. A(n) was closely correlated with Rubisco content during moisture stress treatment. The larger decrease of Rubisco content in the DR-plants was from increased Rubisco degradation rather than decreased synthesis. The amount of cytokinins transported from roots to shoots was smaller in the DR-plants. The application of 6-benzylaminopurine to leaves of the DR-plants suppressed Rubisco degradation. In a wilty mutant with impaired leaf hydraulic conductance, leaf senescence was significantly higher in the DR-plants than in the FL-plants, although leaf water potential of both groups decreased similarly under sunny conditions.Conclusion The main cause of an A(n) decrease with senescence in rice under soil moisture stress was the decrease of cytokinin flux from roots to shoots and enhanced Rubisco degradation.