ARSENATE INDUCED CHLOROSIS 1/ TRANSLOCON AT THE OUTER ENVOLOPE MEMBRANE OF CHLOROPLASTS 132 Protects Chloroplasts from Arsenic Toxicity.

Arsenic (As) is highly toxic to plants and detoxified primarily through complexation with phytochelatins (PCs) and other thiol compounds. To understand the mechanisms of As toxicity and detoxification beyond PCs, we isolated an arsenate-sensitive mutant of Arabidopsis (Arabidopsis thaliana), arsenate induced chlorosis1 (aic1), in the background of the PC synthase-defective mutant cadmium-sensitive1-3 (cad1-3). Under arsenate stress, aic1 cad1-3 showed larger decreases in chlorophyll content and the number and size of chloroplasts than cad1-3 and a severely distorted chloroplast structure. The aic1 single mutant also was more sensitive to arsenate than the wild type (Columbia-0). As concentrations in the roots, shoots, and chloroplasts were similar between aic1 cad1-3 and cad1-3 Using genome resequencing and complementation, TRANSLOCON AT THE OUTER ENVOLOPE MEMBRANE OF CHLOROPLAST132 (TOC132) was identified as the mutant gene, which encodes a translocon protein involved in the import of preproteins from the cytoplasm into the chloroplasts. Proteomic analysis showed that the proteome of aic1 cad1-3 chloroplasts was more affected by arsenate stress than that of cad1-3 A number of proteins related to chloroplast ribosomes, photosynthesis, compound synthesis, and thioredoxin systems were less abundant in aic1 cad1-3 than in cad1-3 under arsenate stress. Our results indicate that chloroplasts are a sensitive target of As toxicity and that AIC1/Toc132 plays an important role in protecting chloroplasts from As toxicity.

Rice nitrogen use efficiency does not link to ammonia volatilization in paddy fields.

Ammonia (NH3) volatilization is a major pathway of nitrogen (N) losses from paddy fields, and could be potentially mitigated by cultivation of high nitrogen use efficiency (high-NUE) rice cultivars. However, the relationship between NUE and NH3 volatilization has not been validated under field conditions. A field experiment was conducted to evaluate the impact of four rice cultivars with different NUE [Wuyunjing 23 (W23), Zhendao 11 (Z11), Wuyujing 3 (W3), and Aoyusi 386 (A386)] on NH3 volatilization, as well as the related mechanisms. Two high-NUE rice cultivars W23 and Z11 was not more effective in reducing total NH3 volatilization from the paddy field compared to cultivar A386 with the lowest NUE. Cultivar A386 had 12.7-17.8% and 35.7-54.1% lower NH3 volatilization than other three rice cultivars at tillering fertilization stage (TFS) and panicle fertilization stage (PFS), respectively, mainly due to its greater shoot N accumulation, root biomass and volume at TFS and its greater shoot biomass, leaf area index and shoot N accumulation at PFS. There was no significant difference in NH3 volatilization among W23, Z11 and W3 at TFS. However, premature senescence phenomenon at later growth stages of A386 eventually led to its lowest NUE among the four rice cultivars. Our results suggest that NUE of rice does not link to NH3 volatilization from paddy fields. In order to make high-NUE rice cultivars also effective in mitigating NH3 volatilization, future breeding works should aim to improve N uptake capability and canopy structure at early tillering and panicle development stages while prevent premature senescence of rice plants to maintain high yields.

Producing cadmium-free Indica rice by overexpressing OsHMA3.

BACKGROUND: Considerable proportions of rice grains produced in some areas in southern China contain high concentrations of cadmium (Cd), leading to unsafe levels of dietary Cd intake. Cultivars of Indica rice, widely grown in southern China, are particularly prone to high Cd accumulation in the grain. Effective methods are needed to decrease Cd accumulation in Indica rice. METHODS: OsHMA3, encoding a tonoplast Cd transporter, was overexpressed in an elite Indica rice cultivar (Zhongjiazao 17) driven by CaMV 35S promoter. The effects on Cd translocation, accumulation and tolerance, as well as on the agronomic traits and micronutrient concentrations were evaluated. RESULTS: OsHMA3 overexpression markedly decreased Cd translocation from roots to shoots and increased Cd tolerance. OsHMA3 overexpression decreased Cd concentrations in brown rice by 94-98%, to levels just above the detection limit, when rice plants were grown in two Cd-contaminated paddy soils. OsHMA3 overexpression generally had no significant effect on grain yield and the concentrations of the essential micronutrients including zinc, iron, copper and manganese in field trials. CONCLUSION: Overexpression of OsHMA3 is a highly effective method to reduce Cd accumulation in Indica rice, producing rice grains that were almost Cd free with little effect on grain yield or essential micronutrient concentrations.