Root-associated endophytic fungi can create symbiotic relationships with trees to enhance stress tolerance, but the underlying mechanisms, especially with regard to waterlogging tolerance, remain unclear. This study aimed to elucidate the effects of Funneliformis mosseae and Serendipita indica on the growth, root cross-section structure, and root transcriptional responses of peach under waterlogging stress, with a focus on polyamine and proline metabolism. Genes and transcription factors associated with secondary cell wall biosynthesis were selected, and their expression profiles were analyzed. Funneliformis mosseae significantly increased the height, stem diameter and leaf number of peach seedlings subjected to 2 weeks of waterlogging stress, whereas S. indica only significantly improved stem diameter. Both fungal species substantially increased root diameter, stele diameter, the number of late metaxylem inside the stele and late metaxylem diameter, thus improving aeration within inoculated roots under waterlogging stress. Transcriptomic analysis of waterlogged roots identified 5425 and 5646 differentially expressed genes following inoculation with F. mosseae and S. indica, respectively. The arginine and proline metabolism and arginine biosynthesis pathways were enriched following fungal inoculations. Both fungi reduced the conversion of glutamate and ornithine for proline synthesis. However, S. indica promoted peptide-to-proline conversion by up-regulating the expression of PIPs. Although both fungi promoted the expression of genes involved in arginine and ornithine synthesis pathway, only F. mosseae led to increased levels of arginine and ornithine. Additionally, F. mosseae promoted the accumulation of putrescine and maintained polyamine homeostasis by down-regulating PAO2 and SAMDC. Moreover, F. mosseae facilitated the metabolism of cadaverine. In conclusion, both F. mosseae and S. indica formed symbiotic relationships with peach plants, with F. mosseae primarily improving polyamine accumulation and S. indica predominantly facilitating proline accumulation for enhanced waterlogging resistance.

Environmentally friendly arbuscular mycorrhizal fungi (AMF) in the soil can alleviate host damage from abiotic stresses, but the underlying mechanisms are unclear. The objective of this study was to analyze the effects of an arbuscular mycorrhizal fungus, Paraglomus occultum, on plant growth, nitrogen balance index, and expressions of salt overly sensitive genes (SOSs), plasma membrane intrinsic protein genes (PIPs), and tonoplast intrinsic protein genes (TIPs) in leaves of tomato (Solanum lycopersicum L. var. Huapiqiu) seedlings grown in 0 and 150 mM NaCl stress. NaCl stress severely inhibited plant growth, but P. occultum inoculation significantly improved plant growth. NaCl stress also suppressed the chlorophyll index, accompanied by an increase in the flavonoid index, whereas inoculation with AMF significantly promoted the chlorophyll index as well as reduced the flavonoid index under NaCl conditions, thus leading to an increase in the nitrogen balance index in inoculated plants. NaCl stress regulated the expression of SlPIP1 and SlPIP2 genes in leaves, and five SlPIPs genes were up-regulated after P. occultum colonization under NaCl stress, along with the down-regulation of only SlPIP1;2. Both NaCl stress and P. occultum inoculation induced diverse expression patterns in SlTIPs, coupled with a greater number of up-regulated TIPs in inoculated versus uninoculated plants under NaCl stress. NaCl stress up-regulated SlSOS2 expressions of mycorrhizal and non-mycorrhizal plants, while P. occultum significantly up-regulated SlSOS1 expressions by 1.13- and 0.45-fold under non-NaCl and NaCl conditions, respectively. It was concluded that P. occultum inoculation enhanced the salt tolerance of the tomato, associated with the nutrient status and stress-responsive gene (aquaporins and SOS1) expressions.

The role of arbuscular mycorrhizal fungi in sweet oranges is well known, but the function of their secondary metabolite, especially the easily extractable glomalin-related soil protein (EE-GRSP), an active fraction of glomalin, is still unclear. The proposed study aimed to analyze the field response of foliar application of exogenous EE-GRSP on tree mycorrhizal development and fruit quality of two sweet orange (Citrus sinensis L. Osbeck) varieties viz., Lane Late Navel (LLN) and Rohde Red Valencia (RRV). Application of EE-GRSP significantly increased the root mycorrhizal colonization and soil mycorrhizal hyphal length in both the sweet orange varieties. The external quality of fruits (fruit weight, polar diameter, and equatorial diameter) also improved in response to foliar application of EE-GRSP in both sweet orange varieties. However, EE-GRSP treatment showed no change in fruit soluble solid content, while it increased the Vc content, solids-acid ratio, fructose, glucose, and sucrose content of sarcocarp in the two sweet oranges varieties. The LLN variety treated with EE-GRSP recorded significantly higher N, P, K, Fe, and Si content of sarcocarp as a mark of nutritional quality, while the RRV variety treated with EE-GRSP displayed a higher concentration of nutrients like Cu, Fe, Si, and Zn in the sarcocarp as compared with the corresponding non-treated control. To the best of our knowledge, this is the first report regarding the improvement in fruit quality of late-ripening sweet oranges (especially LLN) in response to foliar application of EE-GRSP as another potential biostimulant.