Breaking the barriers of grain-filling using novel mutant resources

Despite significant progress since the First Green Revolution, the yield of rice has almost plateaued in recent years. Unless revolutionary innovations are found, it will be impossible to meet the demands of a global population of almost 10 billion by 2050. The incorporation of C4 photosynthetic pathway in rice and the modification of panicle/spikelet architecture have been considered two promising strategies to increase rice yield. However, these strategies simply cannot guarantee success, because both merely increase either the source or sink size/activity, whereas yield is a very complex outcome of source-sink relationship. Notably, despite a decades-long effort of the New Plant Type (NPT) initiatives of the International Rice Research Institute (IRRI) turned out to be unsuccessful due to the poor biomass and poor grain-filling of NPT lines. Similarly, poor grain-filling of inferior spikelets also remains a problem of super-rice varieties. Therefore, a quantitative understanding of source-sink relationships during grain-filling is very critical for future increments in rice productivity. Otherwise “NPT history” will repeat itself.

We have isolated a novel rice double-grain mutant with triple distinct phenotypes: a single spikelet possessing (i) two awns, (ii) two lemmas, (iii) two grains. Moreover, both grains are almost fully filled. Therefore, the double-grained spikelet is significantly heavier than the wild type. We named the mutant double-grain-filled (dgf1), since grain-filling of the dgf1 mutant is not limited by the hull size, thus the mutant spikelet shows the open hull phenotype. The primary objective of this proposal is to identify the genetic basis of the double-grain-filled (dgf1) mutant phenotype via a BSA-seq approach instead of conventional map-based cloning and subsequent characterization of the regulation and/or evolution of the respective gene using routine molecular techniques.

Grain-filling depends on three major factors: the source, sink and flow. Recently, a rice ADP-glucose pyrophosphorylase (AGPase) double mutant (agpl1agpl3) was reported where the flow factor (temporary carbon storage and its remobilization in culm) was almost absent without implications for grain-filling. Therefore, development of a triple mutant (dgf1agpl1agpl3) and comparisons of grain-filling patterns, differential gene expressions, and hormonal contents among these mutants (dgf1, agpl1agpl3, dgf1agpl1agpl3) will generate valuable information on grain-filling regulations and help to quantify the contributions of the source, sink and flow. Consequently, the outcome of this research will not only improve our understanding of the genetic control of double- grain formation, but will also provide opportunity to identify the bottlenecks of the grain-filling process.