IntroductionPhosphorus (P) is one of the most important nutrient elements for plant growth and development. Under P starvation, arbuscular mycorrhizal (AM) fungi can promote phosphate (Pi) uptake and homeostasis within host plants. However, the underlying mechanisms by which AM fungal symbiont regulates the AM symbiotic Pi acquisition from soil under P starvation are largely unknown. Here, we identify a HLH domain containing transcription factor RiPho4 from Rhizophagus irregularis.MethodsTo investigate the biological functions of the RiPho4, we combined the subcellular localization and Yeast One-Hybrid (Y1H) experiments in yeasts with gene expression and virus-induced gene silencing approach during AM symbiosis.ResultsThe approach during AM symbiosis. The results indicated that RiPho4 encodes a conserved transcription factor among different fungi and is induced during the in planta phase. The transcription of RiPho4 is significantly up-regulated by P starvation. The subcellular localization analysis revealed that RiPho4 is located in the nuclei of yeast cells during P starvation. Moreover, knock-down of RiPho4 inhibits the arbuscule development and mycorrhizal Pi uptake under low Pi conditions. Importantly, RiPho4 can positively regulate the downstream components of the phosphate (PHO) pathway in R. irregularis.DiscussionIn summary, these new findings reveal that RiPho4 acts as a transcriptional activator in AM fungus to maintain arbuscule development and regulate Pi uptake and homeostasis in the AM symbiosis during Pi starvation.

Summary Arbuscular mycorrhizal (AM) fungi can form mutualistic endosymbiosis with > 70% of land plants for obtaining fatty acids and sugars, in return, AM fungi promote plant nutrients and water acquisition to enhance plant fitness. However, how AM fungi orchestrate its own signaling components in response to drought stress remains elusive. Here, we identify a transcription factor containing C2H2 zinc finger domains, RiMsn2 from Rhizophagus irregularis. To characterize the RiMsn2, we combined heterologous expression, subcellular localization in yeasts, and biochemical and molecular studies with reverse genetics approaches during the in planta phase. The results indicate that RiMsn2 is highly conserved across AM fungal species and induced during the early stages of symbiosis. It is significantly upregulated in mycorrhizal roots under severe drought conditions. The nucleus‐localized RiMsn2 regulates osmotic homeostasis and trehalose contents of yeasts. Importantly, gene silencing analyses indicate that RiMsn2 is essential for arbuscule formation and enhances plant tolerance to drought stress. Results from yeasts and biochemical experiments suggest that the RiHog1‐RiMsn2‐STREs module controls the drought stress‐responsive genes in AM fungal symbiont. In conclusion, our findings reveal that a module centered on the transcriptional activator RiMsn2 from AM fungus regulates drought stress tolerance in host plant.

Neolamarckia macrophylla (Rubiaceae) is a fast-growing tree with high economic value that is endemic to Sulawesi and Moluccas, Indonesia. Here, we presented the complete chloroplast (cp) genome sequence of N. macrophylla. The complete chloroplast genome is 155,406 bp in size and includes a large single-copy region (86,013 bp), a small single-copy region (18,063 bp), and a pair of IR regions (25,665 bp each). A total of 128 genes were predicted, including 8 rRNA, 36 tRNA, and 84 protein-coding genes. Phylogenetic analysis based on 11 plant species chloroplast genomes indicated that the genus Neolamarckia was monophyletic which clustered as a sister group to Antirhea chinensis. These complete chloroplast genomes can be subsequently used for valuable species research of Rubiaceae.

Arbuscular mycorrhizal (AM) fungi play an important role in improving plant growth and development under drought stress. The MAPK cascade may regulate many physiological and biochemical processes in plants in response to drought stress.