Phloem Unloading in Developing Rice Caryopses and its Contribution to Non-Structural Carbohydrate Translocation from Stems and Grain Yield Formation.

Phloem unloading plays an important role in photoassimilate partitioning and grain yield improvements in cereal crops. The phloem unloading strategy and its effects on photoassimilate translocation and yield formation remain unclear in rice. In this study, plasmodesmata were observed at the interface between the sieve elements (SEs) and companion cells (CCs), and between the SE-CC complex and surrounding parenchyma cells (PCs) in phloem of the dorsal vascular bundle in developing caryopses. Carboxyfluorescein (CF) signal was detected in the phloem of caryopses, which showed that CF was unloaded into caryopses. These results indicated that the SE-CC complex was symplasmically connected with adjacent PCs by plasmodesmata. Gene expression for sucrose transporter (SUT) and cell wall invertase (CWI), and OsSUT1 and OsCIN1 proteins were detected in developing caryopses, indicating that rice plants might actively unload sucrose into caryopses by the apoplasmic pathway. Among three rice recombinant inbred lines, R201 exhibited lower plasmodesmal densities at the boundaries between cell types (SE-CC, SE-PC and CC-PC) in developing caryopses than R91 and R156. R201 also had lower expression of SUT and CWI genes and lower protein levels of OsSUT1 and OsCIN1, as well as CWI activity, than R91 and R156. These data agreed with stem non-structural carbohydrate (NSC) translocation and grain yields for the three lines. The nitrogen application rate had no significant effect on plasmodesmal densities at the interfaces between different cells types, and did not affect CF unloading in the phloem of developing caryopses. Low nitrogen treatment enhanced expression levels of OsSUT and OsCIN genes in the three lines. These results suggested that nitrogen application had no substantial effect on symplasmic unloading but affected apoplasmic unloading. Therefore, we concluded that poor symplasmic and apoplasmic unloading in developing caryopses might result in low stem NSC translocation and poor grain yield formation of R201.

Multivalent SnO2 quantum dot-decorated Ti3C2 MXene for highly sensitive electrochemical detection of Sudan I in food.

Quantum dots functionalization has been proven to be a simple modification strategy for improving the electroanalytical performance of two-dimensional electrode materials by increasing the specific surface area and active reaction sites. Herein, a new electrochemical sensing platform was fabricated by SnO2 quantum dot-functionalized Ti3C2 MXene (Ti3C2-SnO2QDs) for the highly sensitive detection of Sudan I in food. Ti3C2-SnO2QDs were prepared via in situ synthesis, which can control the nucleation and growth of SnO2QDs, resulting in the well-dispersed SnO2QDs with 2-3 nm size on the intersheet surface of MXene. Moreover, the formation of Ti3C2-SnO2QDs can effectively restrict the aggregation of Ti3C2 and improve the stability of SnO2QDs in aquatic environment. The prepared nanocomposite can be used as an improved modified material to further increase the electrocatalytic performance and electrochemical signal of Sudan I on the surface of a glassy carbon electrode. Under optimized conditions, the proposed analytical method displayed a linear dependence for Sudan I concentration ranging from 0.008 to 10 µM with a detection limit of 0.27 nM (S/N = 3) by electrochemical cyclic voltammetry. This sensor with excellent selectivity, reproducibility and accuracy was quantitatively validated in commercial ketchup and chili powder. This Ti3C2-SnO2QDs-based Sudan I sensor is expected to expand the application of MXene nanocomposites in electrochemical analysis and is envisioned as a promising candidate for monitoring illegal food additives in real food samples.

SnO2 quantum dots-functionalized Ti3C2 MXene nanosheets for electrochemical determination of dopamine in body fluids.

A novel SnO2 quantum dots (SnO2QDs)-functionalized Ti3C2 MXene nanocomposite was prepared via in situ synthesis method, resulting in well-regulated the nucleation and growth of SnO2QDs to evenly distribute onto MXene nanosheets. Ultra-small size SnO2QDs decorated on the surface of Ti3C2 MXene nanosheets can effectively prevent the restacking of MXene and remarkably increase the electroactive surface area of the electrode, which can further increase electrocatalytic activity toward dopamine. Then, an ultrasensitive electroanalytical method based on SnO2QDs-functionalized Ti3C2 MXene nanocomposite for dopamine monitoring was developed, and the effects of experimental condition were investigated systematically. Under optimized conditions, the prepared sensor presented a linear dependence for dopamine in the concentration range from 0.004 to 8.0 µM with the detection limit of 2.0 nM (S/N = 3). Moreover, it selectively perceived dopamine in presence of physiological interferents in urine and serum samples with excellent linearities (correlation coefficients higher than 0.9920). The relative recoveries were in the range 97.67-105.3% and 103.0-106.8%, while the limits of quantitation were 10.12 nM and 9.62 nM in urine and serum sample, respectively, demonstrating the method suitability for dopamine sensing and being envisioned as a promising candidate for neurotransmitter monitoring in biological diagnosis.

Low Nitrogen Application Enhances Starch-Metabolizing Enzyme Activity and Improves Accumulation and Translocation of Non-structural Carbohydrates in Rice Stems.

More than 4 billion inhabitants in Asia depend on rice for 35-60% of the calories consumed in their diets, but new rice cultivars frequently do not reach expected yields because of poor rice grain filling. Here, we quantified the activities of enzymes involved in starch metabolization in rice to investigate the mechanisms regulating the accumulation and translocation of stem non-structural carbohydrates (NSC) under different levels of nitrogen fertilizer application. A pot experiment was conducted using two rice cultivars, Liangyoupeijiu (LYPJ) and Shanyou63 (SY63), under high and low nitrogen applications. Compared with high nitrogen application (HN), low nitrogen application (LN) increased stem NSC concentration before the heading stage and NSC translocation during the grain filling stage; concomitantly, LN significantly shortened the active grain filling period and increased the grain filling rate in superior spikelets. Compared with the LYPJ cultivar, SY63 exhibited a higher grain weight, higher grain filling percentage, and higher stem NSC concentration before heading and greater NSC translocation after heading. During the period between panicle initiation and heading, the activities of adenosine diphosphate-glucose pyrophosphorylase (AGP), starch synthase (StS), and starch branching enzyme (SBE), all enzymes involved in starch synthesis, increased under the LN treatment and positively correlated with increases in stem NSC. During grain filling, the activities of enzymes involved in starch-to-sucrose conversion [α-amylase, ß-amylase, and sucrose phosphate synthase (SPS)] increased under the LN treatment and positively correlated with stem NSC remobilization. Overall, the investigated enzymes exhibited higher activities in SY63 than in LYPJ. Our results suggest that low nitrogen increases the activities of AGP, StS, SBE, α-amylase, ß-amylase, and SPS, leading to increased accumulation and remobilization of stem starch and NSC in SY63. We conclude that calculated reductions in nitrogen application and the choice of an appropriate cultivar may improve rice grain yields via enhanced stem NSC accumulation and translocation, thereby reducing the costs and increasing the sustainability of rice production.