Centre for Research in Agricultural Genomics CSIC-IRTA-UAB, Department of Molecular Genetics, Campus UAB, Edifici CRAG, Bellaterra (Cerdanyola del Vallès) 08193 Barcelona, Spain.
BMC Plant Biol. 2011 May 19;11:90. doi: 10.1186/1471-2229-11-90.
The arbuscular mycorrhizal (AM) symbiosis consists of a mutualistic relationship between soil fungi and roots of most plant species. This association provides the arbuscular mycorrhizal fungus with sugars while the fungus improves the uptake of water and mineral nutrients in the host plant. Then, the establishment of the arbuscular mycorrhizal (AM) symbiosis requires the fine tuning of host gene expression for recognition and accommodation of the fungal symbiont. In plants, calcium plays a key role as second messenger during developmental processes and responses to environmental stimuli. Even though calcium transients are known to occur in host cells during the AM symbiosis, the decoding of the calcium signal and the molecular events downstream are only poorly understood.
The expression of seventeen Calcium-dependent Protein Kinase (CPK) genes representative of the four distinct phylogenetic groups of rice CPKs was monitored during the presymbiotic phase of the AM symbiosis. Among them, OsCPK18 and OsCPK4, were found to be transcriptionally activated in response to inoculation with the AM fungus Glomus intraradices. OsCPK18 and OsCPK4 gene expression was also up-regulated by fungal-produced diffusible molecules. Laser microdissection revealed expression of OsCPK18 in cortical cells, and not in epidermal cells of G. intraradices-inoculated rice roots, suggesting a preferential role of this gene in the root cortex. Moreover, a plasma membrane localization of OsCPK18 was observed by transient expression assays of green fluorescent protein-tagged OsCPK18 in onion epidermal cells. We also show that the myristoylation site of the OsCPK18 N-terminus is required for plasma membrane targeting.
The rapid activation of OsCPK18 expression in response to AM inoculation, its expression being also induced by fungal-secreted signals, together with the observed plasma membrane localization of OsCPK18, points to a role for OsCPK18 in perception of the AM fungus. The OsCPK18 gene might be considered as a marker for the presymbiotic phase of the symbiotic process. These findings provide a better understanding of the signaling mechanisms operating during the AM symbiosis and will greatly facilitate their molecular dissection.
丛枝菌根（AM）共生由土壤真菌与大多数植物根系之间的互利关系组成。这种共生关系为丛枝菌根真菌提供了糖分，而真菌则改善了宿主植物对水和矿物养分的吸收。然后，建立丛枝菌根（AM）共生需要宿主基因表达的精细调节，以识别和容纳真菌共生体。在植物中，钙作为发育过程和对环境刺激反应的第二信使发挥着关键作用。尽管已知钙瞬变发生在宿主细胞中，但钙信号的解码和下游的分子事件仍知之甚少。
监测了代表水稻 CPK 四个不同系统发育群的十七个钙依赖蛋白激酶（CPK）基因在 AM 共生的预共生阶段的表达。其中，OsCPK18 和 OsCPK4 被发现对丛枝菌根真菌 Glomus intraradices 的接种呈转录激活。真菌产生的可扩散分子也上调了 OsCPK18 和 OsCPK4 的基因表达。激光微切割显示 OsCPK18 在皮层细胞中表达，而不在接种 G. intraradices 的水稻根的表皮细胞中表达，这表明该基因在根皮层中具有优先作用。此外，通过在洋葱表皮细胞中瞬时表达绿色荧光蛋白标记的 OsCPK18，观察到 OsCPK18 的质膜定位。我们还表明，OsCPK18 N 端的豆蔻酰化位点对于质膜靶向是必需的。
OsCPK18 表达对 AM 接种的快速激活，以及其表达也被真菌分泌的信号诱导，加上观察到的 OsCPK18 质膜定位，表明 OsCPK18 在感知 AM 真菌中起作用。OsCPK18 基因可以被认为是共生过程预共生阶段的标志物。这些发现为理解 AM 共生过程中运行的信号机制提供了更好的理解，并将极大地促进它们的分子剖析。
