Rice Biofortification With Zinc and Selenium: A Transcriptomic Approach to Understand Mineral Accumulation in Flag Leaves.

Human malnutrition due to micronutrient deficiencies, particularly with regards to Zinc (Zn) and Selenium (Se), affects millions of people around the world, and the enrichment of staple foods through biofortification has been successfully used to fight hidden hunger. Rice (Oryza sativa L.) is one of the staple foods most consumed in countries with high levels of malnutrition. However, it is poor in micronutrients, which are often removed during grain processing. In this study, we have analyzed the transcriptome of rice flag leaves biofortified with Zn (900 g ha-1), Se (500 g ha-1), and Zn-Se. Flag leaves play an important role in plant photosynthesis and provide sources of metal remobilization for developing grains. A total of 3170 differentially expressed genes (DEGs) were identified. The expression patterns and gene ontology of DEGs varied among the three sets of biofortified plants and were limited to specific metabolic pathways related to micronutrient mobilization and to the specific functions of Zn (i.e., its enzymatic co-factor/coenzyme function in the biosynthesis of nitrogenous compounds, carboxylic acids, organic acids, and amino acids) and Se (vitamin biosynthesis and ion homeostasis). The success of this approach should be followed in future studies to understand how landraces and other cultivars respond to biofortification.

Substitution of tyrosine residues at the aromatic cluster around the betaA-betaB loop of rubisco small subunit affects the structural stability of the enzyme and the in vivo degradation under stress conditions.

Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) plays a central metabolic role in photosynthetic eukaryotes, and its catabolism is a crucial process for the nutrient economy of higher plants. The rubisco holoenzyme is assembled from eight chloroplast-encoded large subunits and eight nuclear-encoded small subunits. We have identified a cluster of conserved tyrosines at the interface between subunits (comprising Y67, Y68, and Y72 from the betaA-betaB loop of the small subunit and Y226 from the large subunit) that may contribute to holoenzyme stability. To investigate the role of these tyrosines in rubisco structure and in vivo degradation, we have examined site-directed mutants of these residues (Y67A, Y68A, Y72A, and Y226L) in Chlamydomonas reinhardtii. Even if all mutant strains were able to grow photoautotrophically, they exhibited a reduction in rubisco activity and/or the level of expression, especially the Y67A and Y72A mutants. Besides, all mutant rubiscos were inactivated at a lower temperature than the wild type. The kinetics of proteolysis of the mutant enzymes with subtilisin revealed structural alterations, leading to facilitated disassembly (in the cases of Y67A and Y72A) or aggregation propensity (for Y68A and Y226L). When subjected to oxidative stress in vivo through exposure of liquid cultures to hydrogen peroxide, all mutant strains degraded rubisco at a faster rate than the wild type. These results demonstrate that the tyrosine cluster around the betaA-betaB loop of rubisco small subunit plays a stabilizing role by affecting the catalytic activity and the degradation rate of the enzyme in stressed cells.