Nitrogen Deficiency Accelerates Rice Leaf Senescence Through ABA Signaling and Sugar Metabolic Shifts

Nitrogen (N) deficiency is one of the critical factors that induce leaf senescence by integrating with abscisic acid (ABA) metabolism, which results in a shortened leaf photosynthetic period and markedly lowered grain yield. However, the metabolic pathway by which ABA signaling participates in the regulation of senescence-associated change in sugar metabolism and its relationship with N allocation in plant tissues are not well understood. In this paper, the effect of supply level on leaf C/N allocation and its relation to ABA signalling, sugar metabolism, and N assimilation were investigated by using two rice genotypes subjected to four N treatments. Results indicated that N-deficiency markedly induced PYR1-like (PYL) expression and ABA biosynthesis, consequently leading to the activation of ABA signaling. The increased ABA concentration in leaf tissues triggered the catabolic pathways of sugar and N metabolisms, resulting in the reduced photosynthetic pigments and intensified oxidative damage in N-deficient leaves. ABA signaling induced by N-deficiency upregulates the expression of senescence-associated genes (SAGs) and C/N allocation by mediating several senescence-promoting factors, such as NAC, bZIP, and WRKY TFs, along with the suppression of PP2Cs. Therefore, N-deficiency impairs chlorophyll biosynthesis and triggers chlorophyll degradation to accelerate the timing and rate of leaf senescence. This metabolic network could provide helpful information for understanding the regulatory mechanism of leaf senescence in relation to sugar signaling, N-assimilation and N-use efficiency.