Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada.
Am J Bot. 2012 Jan;99(1):101-7. doi: 10.3732/ajb.1100136. Epub 2011 Dec 14.
The soil-inhabiting insect-pathogenic fungus Metarhizium robertsii also colonizes plant roots endophytically, thus showing potential as a plant symbiont. Metarhizium robertsii is not randomly distributed in soils but preferentially associates with the plant rhizosphere when applied in agricultural settings. Root surface and endophytic colonization of switchgrass (Panicum virgatum) and haricot beans (Phaseolus vulgaris) by M. robertsii were examined after inoculation with fungal conidia.
We used light and confocal microscopy to ascertain the plant endophytic association with GFP-expressing M. robertsii. Root lengths, root hair density, and lateral roots emerged were also observed.
Initially, M. robertsii conidia adhered to, germinated on, and colonized roots. Furthermore, plant roots treated with Metarhizium grew faster and the density of plant root hairs increased when compared with control plants. The onset of plant root hair proliferation was initiated before germination of M. robertsii on the root (within 1-2 d). Plants inoculated with M. robertsii ΔMAD2 (plant adhesin gene) took significantly longer to show root hair proliferation than the wild type. Cell free extracts of M. robertsii did not stimulate root hair proliferation. Longer-term (60 d) associations showed that M. robertsii endophytically colonized cortical cells within bean roots. Metarhizium appeared as a mycelial aggregate within root cortical cells as well as between the intercellular spaces with no apparent damage to the plant.
These results suggest that M. robertsii is not only rhizosphere competent but also displays a beneficial endophytic association with plant roots that results in the proliferation of root hairs.
土壤中昆虫病原真菌绿僵菌也会内生性地定殖于植物根部，因此具有成为植物共生体的潜力。绿僵菌在土壤中并非随机分布，而是在农业环境中应用时优先与植物根际相关联。在接种绿僵菌分生孢子后，研究了绿僵菌对柳枝稷（Panicum virgatum）和菜豆（Phaseolus vulgaris）根表面和内生的定殖。
我们使用荧光显微镜和共聚焦显微镜来确定 GFP 表达的绿僵菌与植物的内生关联。还观察了根长、根毛密度和侧根的出现。
最初，绿僵菌分生孢子附着在、萌发在和定殖在根上。此外，与对照植物相比，用绿僵菌处理的植物根生长更快，植物根毛密度增加。当绿僵菌在根上萌发（在 1-2 天内）之前，植物根毛增殖就开始了。与野生型相比，用绿僵菌Δ MAD2（植物黏附素基因）接种的植物需要更长的时间才能显示根毛增殖。绿僵菌的无细胞提取物不能刺激根毛增殖。在较长时间（60 天）的关联中，绿僵菌内生性地定殖于菜豆根的皮层细胞。绿僵菌呈菌丝体聚集在根皮层细胞内以及细胞间隙中，对植物没有明显的损伤。
这些结果表明，绿僵菌不仅具有根际能力，而且与植物根部具有有益的内生关联，导致根毛增殖。
