Divergent biochemical and enzymatic properties of oxalate oxidase isoforms encoded by four similar genes in rice

Application of RNA sequencing to understand the response of rice seedlings to salt-alkali stress

BACKGROUND

Salt-alkali stress represents one of the most stressful events with deleterious consequences for plant growth and crop productivity. Despite studies focusing on the effects of salt-alkali stress on morphology and physiology, its molecular mechanisms remain unclear. Here, we employed RNA-sequencing (RNA-seq) to understand how Na₂CO₃ stress inhibits rice seedling growth.

RESULTS

Na₂CO₃ stress significantly inhibited the growth of rice seedlings. Through RNA-seq, many differentially expressed genes (DEGs) were shown to be potentially involved in the rice seedling response to salt-alkali stress. After 1-day and 5-day treatments, RNA-seq identified 1780 and 2315 DEGs in the Na₂CO₃-treated versus -untreated rice seedling shoots, respectively. According to the gene ontology enrichment and the Kyoto Encylopedia of Genes and Genomes annotation of DEGs, the growth-inhibition processes associated with salt-alkali stress involve a myriad of molecular events, including biosynthesis and metabolism, enzyme activity, and binding, etc. CONCLUSION: Collectively, the transcriptome analyses in the present work revealed several potential key regulators of plant response to salt-alkali stress, and might pave a way to improve salt-alkali stress tolerance in rice.

The Biochemical Properties of Manganese in Plants

Manganese (Mn) is an essential micronutrient with many functional roles in plant metabolism. Manganese acts as an activator and co-factor of hundreds of metalloenzymes in plants. Because of its ability to readily change oxidation state in biological systems, Mn plays and important role in a broad range of enzyme-catalyzed reactions, including redox reactions, phosphorylation, decarboxylation, and hydrolysis. Manganese(II) is the prevalent oxidation state of Mn in plants and exhibits fast ligand exchange kinetics, which means that Mn can often be substituted by other metal ions, such as Mg(II), which has similar ion characteristics and requirements to the ligand environment of the metal binding sites. Knowledge of the molecular mechanisms catalyzed by Mn and regulation of Mn insertion into the active site of Mn-dependent enzymes, in the presence of other metals, is gradually evolving. This review presents an overview of the chemistry and biochemistry of Mn in plants, including an updated list of known Mn-dependent enzymes, together with enzymes where Mn has been shown to exchange with other metal ions. Furthermore, the current knowledge of the structure and functional role of the three most well characterized Mn-containing metalloenzymes in plants; the oxygen evolving complex of photosystem II, Mn superoxide dismutase, and oxalate oxidase is summarized.

Engineering a New Chloroplastic Photorespiratory Bypass to Increase Photosynthetic Efficiency and Productivity in Rice

Over the past few years, three photorespiratory bypasses have been introduced into plants, two of which led to observable increases in photosynthesis and biomass yield. However, most of the experiments were carried out using Arabidopsis under controlled environmental conditions, and the increases were only observed under low-light and short-day conditions. In this study, we designed a new photorespiratory bypass (called GOC bypass), characterized by no reducing equivalents being produced during a complete oxidation of glycolate into CO₂ catalyzed by three rice-self-originating enzymes, i.e., glycolate oxidase, oxalate oxidase, and catalase. We successfully established this bypass in rice chloroplasts using a multi-gene assembly and transformation system. Transgenic rice plants carrying GOC bypass (GOC plants) showed significant increases in photosynthesis efficiency, biomass yield, and nitrogen content, as well as several other CO₂-enriched phenotypes under both greenhouse and field conditions. Grain yield of GOC plants varied depending on seeding season and was increased significantly in the spring. We further demonstrated that GOC plants had significant advantages under high-light conditions and that the improvements in GOC plants resulted primarily from a photosynthetic CO₂-concentrating effect rather than from improved energy balance. Taken together, our results reveal that engineering a newly designed chloroplastic photorespiratory bypass could increase photosynthetic efficiency and yield of rice plants grown in field conditions, particularly under high light.

The oxalyl-CoA synthetase-regulated oxalate and its distinct effects on resistance to bacterial blight and aluminium toxicity in rice

Oxalic acid is widely distributed in biological systems and known to play functional roles in plants. The gene AAE3 was recently identified to encode an oxalyl-CoA synthetase (OCS) in Arabidopsis that catalyses the conversion of oxalate and CoA into oxalyl-CoA. It will be particularly important to characterise the homologous gene in rice since rice is not only a monocotyledonous model plant, but also a staple food crop. Various enzymatic and biological methods have been used to characterise the homologous gene. We first defined that AAE3 in the rice genome (OsAAE3) also encodes an OCS enzyme. Its K_m for oxalate is 1.73 ± 0.12 mm, and V_m is 6824.9 ± 410.29 U·min^-1 ·mg protein^-1 . Chemical modification and site-directed mutagenesis analyses identified thiols as the active site residues for rice OCS catalysis, suggesting that the enzyme might be regulated by redox state. Subcellular localisation assay showed that the enzyme is located in the cytosol and predominantly distributed in leaf epidermal cells. As expected, oxalate levels increased when OCS was suppressed in RNAi transgenic plants. More interestingly, OCS-suppressed plants were more susceptible to bacterial blight but more resistant to Al toxicity. The results demonstrate that the OsAAE3-encoded protein also acts as an OCS in rice, and may play different roles in coping with stresses. These molecular, enzymatic and functional data provide first-hand information to further clarify the function and mechanism of OCS in rice plants.

Increase of Fungal Pathogenicity and Role of Plant Glutamine in Nitrogen-Induced Susceptibility (NIS) To Rice Blast

Highlight  Modifications in glutamine synthetase OsGS1-2 expression and fungal pathogenicity underlie nitrogen-induced susceptibility to rice blast. Understanding why nitrogen fertilization increase the impact of many plant diseases is of major importance. The interaction between Magnaporthe oryzae and rice was used as a model for analyzing the molecular mechanisms underlying Nitrogen-Induced Susceptibility (NIS). We show that our experimental system in which nitrogen supply strongly affects rice blast susceptibility only slightly affects plant growth. In order to get insights into the mechanisms of NIS, we conducted a dual RNA-seq experiment on rice infected tissues under two nitrogen fertilization regimes. On the one hand, we show that enhanced susceptibility was visible despite an over-induction of defense gene expression by infection under high nitrogen regime. On the other hand, the fungus expressed to high levels effectors and pathogenicity-related genes in plants under high nitrogen regime. We propose that in plants supplied with elevated nitrogen fertilization, the observed enhanced induction of plant defense is over-passed by an increase in the expression of the fungal pathogenicity program, thus leading to enhanced susceptibility. Moreover, some rice genes implicated in nitrogen recycling were highly induced during NIS. We further demonstrate that the OsGS1-2 glutamine synthetase gene enhances plant resistance to M. oryzae and abolishes NIS and pinpoint glutamine as a potential key nutrient during NIS.