Auxins are a class of phytohormones with roles involved in the establishment and maintenance of the arbuscular mycorrhizal symbiosis (AMS). Auxin response factors (ARFs) and Auxin/Indole-acetic acids (AUX/IAAs), as two transcription factors of the auxin signaling pathway, coregulate the transcription of auxin response genes. However, the interrelation and regulatory mechanism of ARFs and AUX/IAAs in regulating AMS are still unclear. In this study, we found that the content of auxin in tomato roots increased sharply and revealed the importance of the auxin signaling pathway in the early stage of AMS. Notably, SlARF6 was found to play a negative role in AMF colonization. Silencing SlARF6 significantly increased the expression of AM-marker genes, as well as AMF-induced phosphorus uptake. SlIAA23 could interact with SlARF6 in vivo and in vitro, and promoted the AMS and phosphorus uptake. Interestingly, SlARF6 and SlIAA23 played a contrary role in strigolactone (SL) synthesis and accumulation in AMF-colonized roots of tomato plants. SlARF6 could directly bind to the AuxRE motif of the SlCCD8 promoter and inhibited its transcription, however, this effect was attenuated by SlIAA23 through interaction with SlARF6. Our results suggest that SlIAA23-SlARF6 coregulated tomato-AMS via an SL-dependent pathway, thus affecting phosphorus uptake in tomato plants.

Endophytic fungi play an important role in the induction of plant tolerance to abiotic and biotic stresses. However, the role of endophytic fungi in the response of horticultural plants to plant stress remains largely unknown. Here, we addressed the role of the endophytic fungus Falciphora oryzae in enhancing salt tolerance in pepper (Capsicum annuum L.) by inoculation with the endophyte in the rhizosphere. F. oryzae could indeed colonize the roots of pepper and significantly improved the tolerance of pepper to salt stress. This resulted in increased plant growth and photosynthetic performance compared with control plants, which was accompanied by increases in indole acetic acid and abscisic acid biosynthesis and signaling. Furthermore, inoculation with F. oryzae significantly upregulated a subset of transcripts involved in Na+ homeostasis (NHX3, NHX6, NHX8, HKT2-1, and SOS1) and the high-affinity K+ transporter protein-related gene HAK1 in the leaves to maintain Na+/K+ homeostasis. Moreover, the activity of antioxidant enzymes (catalase, peroxidase, glutathione, and ascorbate peroxidase), the content of glutathione, the transcript level of genes related to antioxidants (catalase, ascorbate peroxidase, glutathione reductase, peroxidase, glutamate-cysteine ligase, and glutamine synthetase) in the leaves were significantly upregulated after inoculation with F. oryzae, which led to decreased levels of lipid peroxidation (malondialdehyde) and reactive oxygen species. These results indicate that inoculation with F. oryzae can enhance the salt tolerance of pepper by promoting ion homeostasis and upregulating antioxidant defense systems.