Realizing the yield potential of Narrow Leaf 1 (NAL1) in rice: The way forward

Yield, a key parameter targeted by breeders to increase rice productivity is a complex trait, governed by source sink interactions and also subject to genotype x environmental effects. Over the last two decades, QTL mapping and map-based cloning have identified several loci and genes related to yield in rice. Among them, a variant of Narrow Leaf 1 (NAL1), a gene conferring pleiotropic effects in rice, has been inadvertently selected during domestication to enhance yield in japonica rice. In this review, we synthesize recent literature on NAL1 in rice, including molecular function, association with auxin transport, associated interactome, regulation at transcriptional and post-transcriptional levels that impact the narrow leaf phenotype. Causes of NAL1 pleiotropic effects are also examined, in addition to trade-offs between yield and photosynthesis conferred by distinct NAL1 variants. Finally, we suggest that the distinct allelic variants of NAL1, leading to partial or full functionality, found in indica and japonica rice backgrounds respectively integrate source-sink interactions to optimize rice yield in a given eco-physiological context. To realize the benefits of the fully functional NAL1 in conferring yield benefits under field conditions, genotype background is crucial and a systems approach is essential to elucidate the causes for such differences. The way forward to enhancing yield in japonica rice (with fully functional NAL1) further by introgression of additional sink and source traits from indica rice is outlined.