Transcriptomic analysis reveals the importance of sugar metabolism and export for positive mycorrhizal growth responses in soybean

To elucidate molecular mechanisms controlling differential growth responses to root colonization by arbuscular mycorrhizal (AM) fungi varying in colonization and cooperative behavior, transcriptome analyses were carried out using two soybean genotypes and three AM inocula. The results showed that inoculation by cooperative Rhizophagus irregularis (Ri) or less-cooperative Glomus aggregatum with high AM colonization (Ga-H) significantly promoted plant growth compared with inoculation by Glomus aggregatum with low AM colonization (Ga-L). A large number of soybean genes, especially those involved in metabolism, transport and regulation of lipids were significantly up-regulated by these three AM inoculations compared with un-inoculated control plants. However, genes encoding two SWEET transporters and one invertase were only up-regulated in response to Ri and Ga-H inoculation, which coincided with positive growth responses in these two inoculation treatments. Promoter analyses in transgenic soybean plants further demonstrated that GUS driven by the GmSWEET6 promoter was highly expressed in arbuscule-containing cortical cells. Additionally, Ri and Ga-H inoculation increased the contents of sucrose, glucose and fructose in both shoots and roots. These results imply that positive mycorrhizal growth responses in plants may mostly be due to the stimulation of photosynthate metabolism and transport by AM fungal inoculum with high colonization capabilities. Overall design: Transcriptome analyses were carried out using three AM inocula and a NM as control, every treatment had three replicates.