Bradyrhizobium japonicum, B. elkanii and B. diazoefficiens Interact with Rice (Oryza sativa), Promote Growth and Increase Yield.

Bradyrhizobium is a genus of plant growth-promoting rhizobacteria (PGPR) that have been studied for several decades mainly for the ability to fix diazotrophic nitrogen after having been established endosymbiotically inside root nodules of the legumes of Fabaceae. The aim of this work was to evaluate the capability of Bradyrhizobium to promote the growth of crops belonging to other families, in this case, rice (Oryza sativa), both in laboratory and in field trials. For laboratory test, surface-sterilized rice seeds were soaked with cultures of each strain and planted in pots. Plant length and dry weight were measured after 35 days. For the field test, rice seeds of varieties Yeruá La Plata and Gurí INTA were inoculated with the three best strains observed in the laboratory test and planted in plots. After 60 days of growth, plant length and dry weight were measured. At harvest time, we measured the dry weight of the aerial part, yield and thousand-grain weight. Inoculation with any of the three species described provoked significant increments compared to the uninoculated control at least in one of the parameters measured, both in the laboratory and in the field tests. Bradyrhizobium japonicum E109 was the strain that promoted rice growth the most in the lab while Bradyrhizobium elkanii SEMIA 587 was the strain that promoted rice growth the most in the field, with increments in yield of approximately 1000 kg/ha. Data obtained suggest that the Bradyrhizobium species promoted all rice growth and yield.

Field and genetic evidence support the photosynthetic performance index (PIABS) as an indicator of rice grain yield.

The rice breeding process for grain yield could be effectively enhanced by developing efficient tools that accelerate plant selection through the rapid determination of reliable predictors. In this study, we have described various associations between grain yield and photosynthetic parameters, which can be easily and quickly obtained using a non-invasive technique on the flag leaf during the anthesis stage. Among the analyzed photosynthetic parameters, the photosynthetic performance index (PIABS) stood out due to its strong association with grain yield. A genome-wide association analysis conducted on plants from a rice diversity panel at the tillering stage revealed the presence of a quantitative trait locus on chromosome 9. This locus was characterized by a group of candidate chloroplastic genes that exhibited contrasting haplotypes for PIABS. An analysis of these haplotypes revealed a clear division into two groups. One group consisted of haplotypes linked to high values of PIABS, which were predominantly associated with Japonica spp. subpopulations. The other group consisted of haplotypes linked to low values of PIABS, which were exclusively associated with Indica spp. subpopulations. Japonica spp. genotypes exhibited higher values in the yield component panicle weight compared with the Indica spp. genotypes. The findings of this study indicate that PIABS could serve as an early predictor of yield parameters during the tillering stage in rice breeding processes.

Patterns of physiological parameters and nitrogen partitioning in flag leaf explain differential grain protein content in rice.

The grain protein content (GPC) in rice is low, and more efforts with agronomic and molecular approaches were performed to increase them. However, the rice research focusing on the plant physiological behaviour that modulates the phenomenon of grain protein filling is very scarce. This work contains physiological parameters related to photosynthetic activity in the flag leaf in the grain filling period and N partitioning assays of high (Nutriar) and traditional (Camba) GPC cultivars. Results indicated a higher photosynthetic capacity, a better capacity to provide CO2 to the chloroplast and a healthier PSII structure in Camba relative to Nutriar. Chlorophyll fluorescence parameters decreased more steeply over time in the high protein variety, and a strong negative correlation was observed between GPC and PSII structure parameters. N content in the flag leaf at anthesis showed lower values and higher remobilisation during the grain filling period in Nutriar compared to Camba. The results of this work suggested that the inactivation of some PSII structures in higher GPC cultivars is associated with N remobilisation and would contribute to an increase in the free N available to be translocated to the grain.