硼促进缺铁条件下拟南芥根系细胞壁铁的再利用

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

【目的】为了探讨在缺铁条件下外源施加硼对植株表型以及体内铁含量的影响,揭示拟南芥在缺铁状态下体内铁的再分配机理,为缓解植株缺铁症状提供一个新的策略。【方法】以模式作物拟南芥(野生型)为供试材料进行了水培试验。供试营养液以正常铁浓度为加铁(+Fe)处理,不含铁营养液为缺铁(-Fe)处理,在两种铁营养液中分别加入H_3BO_3 100、1000μmol/L,共形成6个处理。拟南芥幼苗在全营养液中培养3周后,在处理溶液中培养7 d,收集根系和地上部,分别测定植株全铁、有效铁以及细胞壁吸附的铁含量;剪下根尖部位检测内源NO含量,提取根系RNA检测铁运输相关基因的表达量。【结果】在缺铁条件下,外源添加硼(1000μmol/L H_3BO_3)后植株根系和地上部有效铁含量分别是不加硼时的1.56倍和2.65倍,拟南芥新叶缺铁黄化的症状受到显著缓解。细胞壁组分含量分析结果表明,与不加硼相比,添加1000μmol/L H_3BO_3后植株根系细胞壁铁含量、半纤维铁含量以及半纤维素含量分别降低了60%、52%和53%,同时与100μmol/L H_3BO_3相比也分别降低了41%、41%和43%,说明随着外源添加硼浓度的增加,细胞壁以及细胞壁铁的解析作用也愈加明显。通过对植株不同部位总铁含量以及铁运输相关基因表达量分析后发现,只有在添加1000μmol/L H_3BO_3时缺铁胁迫下铁运输相关的3个基因才能受到显著诱导,具体表现为:与不加硼相比,1000μmol/L H_3BO_3处理后AtFRD3、AtYSL2和AtNAS1 3个基因的表达量分别上调了1.44、1.15和0.75倍,并且伴随着植株体内总铁含量的升高;而100μmol/L H_3BO_3浓度处理对铁相关基因的表达以及总铁含量的积累影响不大。最后,通过对根系内源NO含量的检测分析显示,硼可以影响内源NO的代谢,且外源施加硼后根系NO含量是不施加硼时的1.5倍,暗示信号分子NO可能参与这一过程。【结论】硼主要是通过改变细胞壁中的半纤维素含量和半纤维素上结合的铁含量来增加拟南芥根系细胞壁铁的释放,进而提高植株体内有效铁的含量,促进植株在缺铁的条件下正常生长。在缺铁的条件下,外源添加硼(1000μmol/L H_3BO_3)可以通过促进拟南芥植株体内铁的再利用机制来缓解植物缺铁症状,而添加100μmol/L H_3BO_3则对植株体内铁的再分配过程影响不大。

【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.