Toshiyuki Takai

Demand for rice (Oryza sativa L.) is rapidly increasing in sub-Saharan Africa; however, its productivity is constrained by nutrient-poor paddy soils, particularly those deficient in N and P. Improving root system architecture offers potential to enhance nutrient uptake efficiency and increase yield under such conditions. This study aimed to identify the most effective root phenotype for enhancing grain yield under the nutrient-deficient soil conditions in Madagascar. A high-yielding indica cultivar—IR64—and 12 near-isogenic and pyramiding lines carrying single or multiple quantitative trait loci for root growth angle, length, volume, and thickness were evaluated across several nutrient-deficient paddy fields with varying nutrient gradients with depth. A pyramiding line combining DRO1 and qsor1, which confer deeper and shallower rooting traits, respectively, exhibited significantly higher grain yield than IR64 at the yield levels of 2.7–5.1 t ha−1. This dimorphic root system expanded root distribution into both topsoil and subsoil, enhanced N and P uptake, and increased overall grain yield by 16–23% relative to IR64. These findings suggest that root system architecture expansion into multiple soil layers represents a promising ideotype for improving rice performance in nutrient-deficient paddy fields and may serve as a key breeding target to boost rice production in sub-Saharan Africa.

Tillering, also known as shoot branching, is a fundamental trait for cereal crops such as rice to produce sufficient panicle numbers. Effective tillering that guarantees successful panicle production is essential for achieving high crop yields. Recent advances in molecular biology have revealed the mechanisms underlying rice tillering; however, in rice breeding and cultivation, there remain limited genes or alleles suitable for effective tillering and high yields. A recently identified quantitative trait locus (QTL) called MORE PANICLES 3 (MP3) has been cloned as a single gene and shown to promote tillering and to moderately increase panicle number. This gene is an ortholog of the maize domestication gene TB1, and it has the potential to increase grain yield under ongoing climate change and in nutrient-poor environments. This review reconsiders the potential and importance of tillering for sustainable food production. Thus, I provide an overview of rice tiller development and the currently understood molecular mechanisms that underly it, focusing primarily on the biosynthesis and signaling of strigolactones, effective QTLs, and the importance of MP3 (TB1). The possible future benefits in using promising QTLs such as MP3 to explore agronomic solutions under ongoing climate change and in nutrient-poor environments are also highlighted.

Rice (Oryza sativa L.) is one of the most important cereals, which provides 20% of the world's food energy. However, its productivity is poorly assessed especially in the global South. Here, we provide a first study to perform a deep-learning-based approach for instantaneously estimating rice yield using red-green-blue images. During ripening stage and at harvest, over 22,000 digital images were captured vertically downward over the rice canopy from a distance of 0.8 to 0.9 m at 4,820 harvesting plots having the yield of 0.1 to 16.1 t·ha^-1 across 6 countries in Africa and Japan. A convolutional neural network applied to these data at harvest predicted 68% variation in yield with a relative root mean square error of 0.22. The developed model successfully detected genotypic difference and impact of agronomic interventions on yield in the independent dataset. The model also demonstrated robustness against the images acquired at different shooting angles up to 30° from right angle, diverse light environments, and shooting date during late ripening stage. Even when the resolution of images was reduced (from 0.2 to 3.2 cm·pixel^-1 of ground sampling distance), the model could predict 57% variation in yield, implying that this approach can be scaled by the use of unmanned aerial vehicles. Our work offers low-cost, hands-on, and rapid approach for high-throughput phenotyping and can lead to impact assessment of productivity-enhancing interventions, detection of fields where these are needed to sustainably increase crop production, and yield forecast at several weeks before harvesting.

SUMMARY: Improving crop yield potential through an enhanced response to rising atmospheric CO₂ levels is an effective strategy for sustainable crop production in the face of climate change. Large‐sized panicles (containing many spikelets per panicle) have been a recent ideal plant architecture (IPA) for high‐yield rice breeding. However, few breeding programs have proposed an IPA under the projected climate change. Here, we demonstrate through the cloning of the rice (Oryza sativa) quantitative trait locus for MORE PANICLES 3 (MP3) that the improvement in panicle number increases grain yield at elevated atmospheric CO₂ levels. MP3 is a natural allele of OsTB1/FC1, previously reported as a negative regulator of tiller bud outgrowth. The temperate japonica allele advanced the developmental process in axillary buds, moderately promoted tillering, and increased the panicle number without negative effects on the panicle size or culm thickness in a high‐yielding indica cultivar with large‐sized panicles. The MP3 allele, containing three exonic polymorphisms, was observed in most accessions in the temperate japonica subgroups but was rarely observed in the indica subgroup. No selective sweep at MP3 in either the temperate japonica or indica subgroups suggested that MP3 has not been involved and utilized in artificial selection during domestication or breeding. A free‐air CO₂ enrichment experiment revealed a clear increase of grain yield associated with the temperate japonica allele at elevated atmospheric CO₂ levels. Our findings show that the moderately increased panicle number combined with large‐sized panicles using MP3 could be a novel IPA and contribute to an increase in rice production under climate change with rising atmospheric CO₂ levels.

Phosphorus (P) deficiency is a major yield constraint for lowland rice production in the tropics. As P-fertilizer resources are finite, alternative fertilizer management is needed for sustainable rice production. We examined whether farmyard manure (FYM), a major nutrient source for smallholder farms, can overcome issue in typical P-deficient lowlands in the central highlands of Madagascar. A multi-location trial in sites varying in altitude and soil P availability, clarified that the effect of both FYM and mineral P fertilizer application on grain yield greatly increased at higher elevation and when the soil oxalate-extractable P content was −1. The yield increase was attributable to improved grain fertility, probably because FYM and mineral P applications decreased days to flowering and avoided low temperatures at late growth stages. Nutrient uptake assessment clarified that despite its relatively low P content, FYM had an equivalent effect on plant P uptake to those of mineral P fertilizer. We concluded that FYM application was effective in low-P availability soils at high altitude, as alternative of mineral P fertilizer. Further monitoring is required to assess the effect of consecutive FYM use on grain yield and plant nutrient uptake in the context of cold stress induced by P deficiency. Graphical Abstract