Transcriptome Analysis of Cucumber Roots Reveals Key Cold-Resistance Genes Induced by AM Fungi

Cold stress is an important factor that limits the yield and quality of various agricultural plants worldwide. Symbiosis with arbuscular mycorrhizal fungi (AMF) could be an efficient way to help some plants resist cold stress. However, our understanding of the molecular mechanism concerning this particular process is limited. To determine the role of AMF in cold resistance, we performed transcriptome sequencing of cucumber roots with four treatments: NI (non-inoculated, normal temperature), NI + C (non-inoculated, cold stress), AMF (inoculated, normal temperature), and AMF + C (inoculated, cold stress). Nearly 67 million (87.5%) reads were mapped to the cucumber genome, whereas only 758,106 (0.91%) and 281,324 (0.34%) reads were mapped to the AMF genome for AMF and AMF + C, respectively. Compared to the NI + C, AMF + C had 2173 differentially expressed genes (DEGs), with 1128 up-regulated and 1045 down-regulated. Expression profiling for nine cucumber DEGs was further confirmed by RT-qPCR. Interestingly, we detected 180 cucumber DEGs, which were significantly down-regulated by cold stress but significantly up-regulated by AMF inoculation under cold stress. They were inferred as key cold-resistance genes induced by AMF. Third, functional enrichment analysis of cucumber DEGs between NI + C versus AMF + C indicated that these genes mainly participate at biological processes of oxidative metabolism and ion (nitrate and iron) uptake/transport. Together, our findings suggest that AMF-induced key genes significantly respond to cold stress and provide clues for further functional analyses of these genes to cold resistance.