白菜叶片对大气颗粒物中铅的吸收和富集机制

Chinese cabbage is one kind of popular vegetables. The accumulation of Pb in leaves has been concerned in recent years. The plants take up lead from soil via roots or foliar uptake by leaves from atmospheric fallouts. Foliar uptake pathway is more important when chinese cabbage is cultivated in the regions and seasons with heavy haze. The previous study showed that the concentrations of Pb in edible parts exceeded the safety limitation although the levels of Pb in soil is low. Moreover, the ratios of different stable isotopes of lead measured by ICP-MS indicated Pb in leaves mostly came from atmospheric fallouts. The machenisms of Pb foliar uptake by Chinese cabbage (Brassica rapa spp. pekinensis) exposed to atmospheric fallouts are still not clear. Two genotypes of Brassica rapa with high and low Pb concentrations were selected in the previous experiment to explore the characteristics of Pb foliar uptake during the whole growing period using ICP-MS and obtain the key growing stage for Pb foliar uptake. There are two pathways for foliar uptake of heavy metals: stomota route and non-stomota route. A serials experiments will be carried out to find the main route of Pb foliar uptake. In addition, the transfter of Pb from atmosphere to the plant leaf will be tracked via SEM-EDS, TEM-EDS and NanoSIMS technologies. Moreover, Two RNA libraries of the leaves of two genotypes from untreated Pb and treated Pb Brassica rapa will be constructed and sequenced using RNA-Seq approach to isolate the differentially-regulated genes involved Pb stress at mRNA level. This research would explore the machenisms of Pb foliar uptake by chinese cabbage exposed the atmospheric fallouts from the pathways of plant physiology and molecular biology, which would supply the useful information for genetic breeding of Brassica rapa with low level of Pb in leaves and screening the practical measurements to reduce Pb accumulation from atmosphereic fallouts in leaves of chinese cabbage.

白菜为大众喜爱的蔬菜，其可食部位铅污染问题日益受到关注。白菜可经根系从土壤中吸收铅，也可经叶片吸收大气中的铅，第二种途径在雾霾严重的季节和地区贡献更为突出。前期研究显示206个白菜株系叶片铅含量超标率>50%，且主要源于大气沉降，那么白菜叶片是如何吸收并富集大气颗粒物中铅的？本研究以前期筛选的铅高积累和低积累的两个白菜自交系为材料，采用ICP-MS探索整个生育期白菜叶片对铅的吸收特征并确定其吸收的关键生育期，验证气孔是否为叶片吸收颗粒物铅的主要途径，通过SEM-EDS、TEM-EDS和NanoSIMS追踪铅在大气-叶片中的行为，从生理角度探明白菜叶片对大气颗粒物铅的吸收和富集机制；在叶片吸收铅的关键生育期，采用RNA-Seq技术开展白菜叶片铅富集相关基因表达谱分析，初步探索白菜叶片吸收和富集铅的分子机制，为低铅白菜分子辅助育种和筛选阻遏铅沉降进入白菜叶片的可行性措施提供理论依据。

铅（Pb）易被大气沉降中细小颗粒物PM2.5吸附后形成污染物PM2.5-Pb，而PM2.5是形成雾霾的主要因素之一。众所周知，秋冬季是京津冀地区雾霾多发期，也是大白菜的种植季节，较大叶面积为白菜叶片接收大气沉降，以及吸收PM2.5-Pb提供了必备条件。研究表明，PM2.5-Pb是大白菜叶片富集Pb的主要贡献者，同位素结果显示其对收获期可食叶片中Pb的贡献率可达95%以上，且气孔是叶片吸收PM2.5-Pb的主要途径，并首次证实气孔孔径直接决定了叶片对颗粒物负载Pb的富集；其次，Pb在白菜叶片中亚细胞分布为细胞质>细胞壁>细胞器，结合NanoSIMS证实了Pb在叶肉组织中的分布特征；同时利用Leadmium™ Green AM dye也发现PM2.5负载的Pb可进入叶片表皮毛细胞中，并主要在表皮毛基部累积，明确了气孔和表皮毛均是调控大白菜叶片吸收和转运颗粒物负载Pb的重要途径；进一步结合RNA-Seq技术探明了PM2.5-Pb胁迫显著刺激了铅低积累大白菜叶片中主要参与编码细胞壁相关的XTHs和PMEs蛋白的基因发生差异表达，其作用于细胞壁的增厚、重构与生物合成，证明细胞壁增厚是其防御铅胁迫的重要屏障，可将PM2.5-Pb拦截在质外体，有效阻控Pb进入共质体。深入解析了阻控PM2.5-Pb进入大白菜叶片的转录机制，即在PM2.5-Pb胁迫下，ZIP1基因的表达显著下调，降低了Pb吸收能力。此外，功能基因CNGC19（蛋白定位于液泡膜负责离子从液泡外排）和ABCG36（蛋白位于细胞膜参与离子外排）均显著上调表达，形成了将Pb从液泡泵入胞质，再从胞质外排的闭合通路，实现了Pb的解毒。综上，相关功能基因在阻控大白菜叶片吸收PM2.5-Pb和降低Pb毒性方面均发挥着重要作用，同时为实现大白菜绿色清洁生产提供了重要支撑。