Colegio de Postgraduados, Edafología, km 36.5 Carr. Mexico-Texcoco, Montecillo, Estado de México 56230, Mexico.
U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Land Remediation and Pollution Control Division, Cincinnati, OH, United States.
Fungal Biol. 2014 May-Jun;118(5-6):444-52. doi: 10.1016/j.funbio.2014.03.002. Epub 2014 Mar 19.
The protective mechanisms employed by arbuscular mycorrhizal fungi (AMF) to reduce the toxic effects of arsenic on host plants remain partially unknown. The goal of this research was identifying the in situ localization and speciation of arsenic (As) in the AM fungus Rhizophagus intraradices [formerly named Glomus intraradices] exposed to arsenate [As(V)]. By using a two-compartment in vitro fungal cultures of R. intraradices-transformed carrot roots, microspectroscopic X-ray fluorescence (μ-XRF), and microspectroscopic X-ray absorption near edge structure (μ-XANES), we observed that As(V) is absorbed after 1 h in the hyphae of AMF. Three hours after exposure a decrease in the concentration of As was noticed and after 24 and 72 h no detectable As concentrations were perceived suggesting that As taken up was pumped out from the hyphae. No As was detected within the roots or hyphae in the root compartment zone three or 45 h after exposure. This suggests a dual protective mechanism to the plant by rapidly excluding As from the fungus and preventing As translocation to the plant root. μ-XANES data showed that gradual As(V) reduction occurred in the AM hyphae between 1 and 3 h after arsenic exposure and was completed after 6 h. Principal component analysis (PCA) and linear combination fitting (LCF) of μ-XANES data showed that the dominant species after reduction of As(V) by R. intraradices extra-radical hyphal was As(III) complexed with a reduced iron(II) carbonate compound. The second most abundant As species present was As(V)-iron hydroxides. The remaining As(III) compounds identified by the LCF analyses suggested these molecules were made of reduced As and S. These results increase our knowledge on the mechanism of As transport in AMF and validate our hypotheses that R. intraradices directly participates in arsenic detoxification. These fungal mechanisms may help AMF colonized plants to increase their tolerance to As at contaminated sites.
丛枝菌根真菌（AMF）用来减轻砷对宿主植物的毒性的保护机制在部分上仍然未知。本研究的目的是鉴定在暴露于砷酸盐[As(V)]的丛枝菌根真菌 Rhizophagus intraradices[以前称为 Glomus intraradices]中砷（As）的原位定位和形态。通过使用 R. intraradices 转化胡萝卜根的体外真菌培养的两室系统，微束 X 射线荧光（μ-XRF）和微束 X 射线吸收近边结构（μ-XANES），我们观察到 AMF 中的菌丝在 1 小时后吸收了 As(V)。暴露 3 小时后，注意到 As 的浓度降低，暴露 24 和 72 小时后，没有检测到可检测的 As 浓度，这表明吸收的 As 被从菌丝中泵出。暴露后 3 或 45 小时，在根室区域的根或菌丝中未检测到 As。这表明 AM 真菌通过迅速将 As 从真菌中排除并防止 As 向植物根部转运，为植物提供了双重保护机制。μ-XANES 数据显示，暴露于砷后 1 至 3 小时，AM 菌丝中逐渐发生 As(V)还原，6 小时后完成。μ-XANES 数据的主成分分析（PCA）和线性组合拟合（LCF）表明，R. intraradices 外生菌根菌丝还原 As(V)后的主要物种是与还原的铁（II）碳酸盐化合物配位的 As(III)。存在的第二种最丰富的 As 物种是 As(V)-铁氢氧化物。LCF 分析鉴定的其余 As(III)化合物表明这些分子由还原的 As 和 S 组成。这些结果增加了我们对 AMF 中 As 转运机制的了解，并验证了我们的假设，即 R. intraradices 直接参与砷解毒。这些真菌机制可能有助于丛枝菌根真菌定殖的植物在污染地点增加对 As 的耐受性。
