Fundamental molecular bases for differential transcriptional programs of top leaves during grain-filling in rice

Flag (FL) and second leaves (SL) in rice show differential aging patterns during monocarpic senescence. Coordination of aging programs in the top leaves is important for effective grain-filling. However, molecular bases for differential aging programs in the top leaves have not been systematically explored in rice. Here, we performed mRNA-sequencing of FL and SL at six time points during the grain-filling period. mRNA expression data revealed 6,365 genes showing aging-dependent expression changes in FL and/or SL. Of them, while 3047 genes showed shared aging-dependent expression patterns between FL and SL, 3058 genes showed differential expression patterns, which were classified into 5 major groups (G1-5) based on their differential expression patterns. Of the groups, G3 representing amino acid (AA) transport showed consistent differential age-dependent expression patterns in independent samples, whereas the other groups showed inconsistent differential expression patterns. Moreover, of AA transporters (AATs) in G3, long-distance AATs showed invariant differential age-dependent expression patterns after panicle removal, consistent to panicle removal-invariant differential nitrogen contents between FL and SL, known to be associated with protein concentration in grains. Our results suggest that long-distance AA transport is an invariant core transcriptional program of differential aging in rice top leaves for nitrogen remobilization during grain-filling.