Abstract
Spinach (Spinacia oleracea L.) is one of most important leafy vegetables because of its high nutritional value and high oxalate content, which can be toxic with negative effects on human nutrition. Ammonium and nitrate can effectively regulate oxalate accumulation, although the mechanisms underlying the oxalate biosynthesis and regulation are still undetermined in plants. In the present study, we identified 25 putative genes that are involved in the oxalate biosynthetic and degradation pathway, before analyzing the oxalate content and the expression levels of the corresponding proteins under normal growth conditions, with or without ammonium and nitrate treatments, using high and low oxalate-accumulated spinach genotypes. The two cultivars exhibited different profiles of total oxalate and soluble oxalate accumulation. The high oxalate concentrations in spinach were as a result of the high transcription levels of the genes that are involved in oxalate biosynthesis under normal growth conditions, such as SoGLO2, SoGLO3, three SoOXACs, SoMLS, SoMDH1, SoMDH2, and SoMDH4. The results revealed that the ammonium and nitrate were able to control the oxalate content in leaves, possibly because of the different transcription levels of the genes. The oxalate content is regulated by complex regulatory mechanisms and is varied in the different varieties of spinach. The results from this research may be used to assist the investigation of the mechanism of oxalate regulation and breeding for reduced oxalate content in spinach.
Highlights
Spinach (Spinacia oleracea L.) is widely cultivated as an economically important green leafy vegetable crop for consumption in both fresh and processed forms [1]
Three homologous genes of SoOXAC were found in spinach, and the differences in the expressions of SoOXAC genes could have helped to explain the differences in the oxalate concentrations between the cultivars, as the expression levels were three or four times higher in the SP14 compared with the low-oxalate cultivar SP104 (Figure 4). These results indicated that the high oxalate concentrations in SP14 could have been attributed to the high transcription levels of the genes that were involved in oxalate biosynthesis
We identified 25 putative genes that were involved in the oxalate biosynthetic and degradation pathway, in addition to analyzing the oxalate content and the expression levels of the corresponding proteins under normal growth conditions, with or without ammonium and nitrate treatments
Summary
Spinach (Spinacia oleracea L.) is widely cultivated as an economically important green leafy vegetable crop for consumption in both fresh and processed forms [1]. Spinach is produced in more than 50 countries, with production primarily occurring in China, USA, Japan, and Europe. Spinach is considered to be one of the healthiest vegetables in the human diet because of its high concentration of nutrients and health-promoting compounds, such as beta carotene (provitamin A), lutein, folate, vitamin C, calcium, iron, phosphorous, and potassium [2,3]. Spinach contains a relatively large amount of oxalic acid compared to most crops [4,5,6,7,8], which affects both taste and human health. Oxalic acid plays an important biological function in plants, such as calcium regulation in plant cells [9], protection against herbivory [10], protection against pathogen defense response [11], tissue strength, light gathering, ion balance (e.g., Na and K) [12,13,14], and reflection [15,16]
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