Mechanism of boron facilitate root cell wall iron reutilization in iron deficient

Objectives In order to explore the effect of exogenous boron on the phenotype and iron content under iron deficiency, the mechanism of iron redistribution in Arabidopsis was revealed to provide a new strategy for alleviating iron-insufficient symptom in plants.

Methods Hydroponic culture experiments were carried out using seedlings of the wild type Arabidopsis (Columbia ecotype), which were cultured in the whole nutrient solution for three weeks before treatment. The treatment solution included insufficient (−Fe) and normal iron supplies (+Fe), and each was added with 100 and 1000 μmol/L of H₃BO₃ to form 6 treatments. After 7 days’ treatments, the shoots and roots of the seedling were harvested for the determination of total, soluble and cell wall Fe contents; the root tips were cut off for the measurement of NO content and the extraction of root RNA to detect the iron-related genes expression.

Results Under − Fe+1000 μmol/L H₃BO₃ treatment, the soluble Fe contents in roots and shoots were 1.56 and 2.65 times of those without the addition of exogenous boric acid, the symptom of iron deficiency in new leaves was alleviated significantly. The analysis of cell wall contents and cell wall iron concentrations showed that the cell wall Fe contents, hemicelluloses Fe contents and hemicelluloses contents were decreased by 60%, 52% and 53% respectively with the higher boron supplies (1000 μmol/L H₃BO₃) compared with the no exogenous boric acid treatment. Also compared with − Fe+100 μmol/L H₃BO₃ condition, these indexes were reduced by 41%, 41% and 43% which means the presence of a cell wall Fe-reutilization phenomenon is much more obvious with the increasing boron concentrations. After the measurement of total iron contents and iron related genes expression, these three genes related to the long-distance transport of Fe, such as FRD3 (FERRIC REDUCTASE DEFECTIVE3), YSL2 (YELLOW STRIPE-LIKE), and NAS1 (NICOTIANAMINE SYNTHASE1) can be up-regulated by 1.44, 1.15 and 0.75 times under 1000 μmol/L H₃BO₃ treatment with the increasing total Fe contents in plants, however, there was almost no differences between − Fe and − Fe+100 μmol/L H₃BO₃ treatments. The results of the endogenous NO contents in root tips indicated that boron could affect the metabolism of endogenous nitric oxide (NO) and NO content was about 1.5 times of that without the supply of boric acid, suggesting that the signal molecule NO might be involved in this process.

Conclusion Boron mainly increases the release of cell wall iron in Arabidopsis by altering the cell wall hemicellulose contents and the iron contents in hemicellulose, and then the soluble iron contents are raised to improve the growth of plants in iron deficient condition. Overall, under iron deficiency, exogenous boron (1000 μmol/L H₃BO₃) can alleviate the insufficient iron symptoms through facilitating the iron reutilization in Arabidopsis thaliana while there is almost no effect on the redistribution of iron under 100 μmol/L H₃BO₃ treatment.