Since the advent of semidwarf rice in the 1960s that enabled a Green Revolution, further raising the genetic yield ceiling has been a breeding priority to ensure the food security of consumers by the middle of the present century. Grain development in rice panicles is position-specific. Grains that are disadvantaged for spatial and temporal location in the asynchronous pattern of panicle structure have poor endosperm starch synthesis and loose quality at maturity. The heterogeneous panicle architecture embodying the difference in grain occurrence between spikelets is a trait of resilience in traditional rice that had encountered uncertain weather conditions throughout its antiquity extending over thousands of years. Homogeneity in spikelet development would have been disastrous for seed production in a situation where inclement weather coincides with sensitive stages of development like anthesis leading to complete decimation of all reproductive units at one go. To break the yield barrier, breeders at the moment have increased the spikelet number per panicle in the new large-panicle rice, but there is no accrued benefit on yield due to poor filling.
The improvement of spikelet number per panicle without matching extension of panicle size increased spikelet density per unit area and made the distribution pattern relatively homogeneous in place of the heterogeneous pattern that marginalized the recursive dominance of spikelets in a panicle. The increase in spikelet density significantly accentuated competition for grain development, where spikelets located on the secondary branches in the basal part of the panicle were discriminated. The inferior type spikelets, not being able to grow according to size, accumulated soluble assimilates at the cost of starch synthesis, while superior spikelets on primary branches enjoyed the freedom of space for optimum grain filling. It implies that there could be hormonal factors involved in the route for inter-spikelet communication deciding the extent of competition between individual spikelets for growth. In several instances, spikelet-specific variation in hormonal homeostasis has been explored, which includes the expression of regulatory genes for ethylene, ABA, and cytokinin synthesis and action. However, knowledge is scant about their role or combinatorial effects in controlling endosperm cell metabolism and the trade-off between grain number and grain filling. The amelioration of the trade-off holds great potential to break the genetic yield ceiling of rice panicle and is possible by allelic, chemical, and biotechnological manipulations of spikelet position-specific hormone level or action.
We invite articles addressing the following themes of rice panicle:
1. Assessment of rice gene pool to discover physiological linkage in a trade-off between grain number and filling including quantitative analysis of the relationship between panicle architecture and resource allocation
2. Optimization of source-sink relations and source-to-sink translocation to improve grain filling by genetic and environmental manipulation
3. Characterization of genes controlling endosperm-specific starch synthesis
4. Carbon and nitrogen dynamics in relation to the determination of grain filling
5. Regulation of spikelet development and grain filling by hormones and identification of hormone-responsive genes controlling spikelet-specific development under stresses
6. Genetic control of primary, secondary branching, and panicle architecture
7. Manipulation of MADS-box genes and multigrain spikelets
8. Control of viviparous germination of rain-soaked rice seeds
9. Ideotype plant and panicle traits to improve rice yield under climate change
Since the advent of semidwarf rice in the 1960s that enabled a Green Revolution, further raising the genetic yield ceiling has been a breeding priority to ensure the food security of consumers by the middle of the present century. Grain development in rice panicles is position-specific. Grains that are disadvantaged for spatial and temporal location in the asynchronous pattern of panicle structure have poor endosperm starch synthesis and loose quality at maturity. The heterogeneous panicle architecture embodying the difference in grain occurrence between spikelets is a trait of resilience in traditional rice that had encountered uncertain weather conditions throughout its antiquity extending over thousands of years. Homogeneity in spikelet development would have been disastrous for seed production in a situation where inclement weather coincides with sensitive stages of development like anthesis leading to complete decimation of all reproductive units at one go. To break the yield barrier, breeders at the moment have increased the spikelet number per panicle in the new large-panicle rice, but there is no accrued benefit on yield due to poor filling.
The improvement of spikelet number per panicle without matching extension of panicle size increased spikelet density per unit area and made the distribution pattern relatively homogeneous in place of the heterogeneous pattern that marginalized the recursive dominance of spikelets in a panicle. The increase in spikelet density significantly accentuated competition for grain development, where spikelets located on the secondary branches in the basal part of the panicle were discriminated. The inferior type spikelets, not being able to grow according to size, accumulated soluble assimilates at the cost of starch synthesis, while superior spikelets on primary branches enjoyed the freedom of space for optimum grain filling. It implies that there could be hormonal factors involved in the route for inter-spikelet communication deciding the extent of competition between individual spikelets for growth. In several instances, spikelet-specific variation in hormonal homeostasis has been explored, which includes the expression of regulatory genes for ethylene, ABA, and cytokinin synthesis and action. However, knowledge is scant about their role or combinatorial effects in controlling endosperm cell metabolism and the trade-off between grain number and grain filling. The amelioration of the trade-off holds great potential to break the genetic yield ceiling of rice panicle and is possible by allelic, chemical, and biotechnological manipulations of spikelet position-specific hormone level or action.
We invite articles addressing the following themes of rice panicle:
1. Assessment of rice gene pool to discover physiological linkage in a trade-off between grain number and filling including quantitative analysis of the relationship between panicle architecture and resource allocation
2. Optimization of source-sink relations and source-to-sink translocation to improve grain filling by genetic and environmental manipulation
3. Characterization of genes controlling endosperm-specific starch synthesis
4. Carbon and nitrogen dynamics in relation to the determination of grain filling
5. Regulation of spikelet development and grain filling by hormones and identification of hormone-responsive genes controlling spikelet-specific development under stresses
6. Genetic control of primary, secondary branching, and panicle architecture
7. Manipulation of MADS-box genes and multigrain spikelets
8. Control of viviparous germination of rain-soaked rice seeds
9. Ideotype plant and panicle traits to improve rice yield under climate change
