**4. Nutrient uptake/exchange during mutualistic plant-fungus interactions**

One of the characteristics of the beneficial mycorrhizal interactions is the bidirectional nutrient exchange between both the partners and to support the growth of plant host [48, 49]. In these relationships, the fungus provides nitrogen, phosphate and sulfate nutrients to the host, whereas, in return, the host plant transfers photosynthetically fixed carbon (4–20%) to the mycorrhizal fungus [50]. In AM roots, the fungus proliferates into the root cortex intercellularly as well as intracellularly, whereas in case of ECM roots, it only covers intercellular regions, indicating the differences in the mechanism of colonization and nutrient uptake/exchange. The uptake of nutrients by plants from soil is limited by the repressed mobility of nutrients. Importantly, during AM symbiosis, the plant phosphate (P) transporters are down-regulated [51, 52]. Under these conditions, the nutrient such as P uptake is predominantly achieved through mycorrhizal pathway [1, 53, 54]. It has been observed that contribution of the mycorrhizal pathway in nutrient uptake varies with the plant and fungal partners that are involved in the interaction and also on the nutrient type [48, 52, 55–57]. To facilitate the nutrient uptake via mycorrhizal interface, the peri-arbuscular membrane (PAM) harbors high affinity transporters that are specifically induced in mycorrhizal roots. For instance, Pt4 and AMT2 are the high affinity transporters for P and ammonium (NH4+ ) that mediate transfer of respective nutrients from fungus to plant host [58–60]. Moreover, a few mycorrhiza-inducible sulfate transporters have also been reported in AM roots [61, 62]. Recently, a sulfate transporter (SiSulT) and iron transporter (PiFtr) from *Serendipita indica* (previously known as *Piriformospora indica*) has been characterized, which transfers sulfur to the maize and iron to the rice plants, respectively and improves its growth [63, 64]. These studies highlight the importance of sulfur transport via mycorrhizal associations.

On the other hand, the plant transfers photosynthates as sucrose from source to ECM roots that serve as a carbon sink, which is then converted to simpler sugars such as glucose or fructose by invertase enzyme of host. The glucose and fructose are taken up by fungal counterparts through mycorrhizal interface. For instance, an arbuscular membrane localized monosaccharide transporter (MST2) is involved in the uptake of glucose and xylose molecules by AM fungi [65]. Intriguingly, host

carbon supply has been found to trigger the fungal gene expression, and P and N uptake during AM and ECM symbiosis [66–68]. This also leads to increased hydrolysis of polyP (an important source of P and N) and release of Pi and Arg in the fungal cytoplasm. The Arg is further broken down to NH4<sup>+</sup> and is transferred to host plant via mycorrhizal interface. Importantly, these transport processes, from host to fungus and from fungus to the host, involve diverse molecular players that mediate membrane transport for the nutrition exchange between mycorrhizal plants and fungus at the interaction interface. The membrane transporters and channels that mediate the transport of molecules such as P, N, S, K, sugar and water are collectively referred as the 'transportome' [69].

Moreover, several robust and tightly regulated signaling processes are involved in establishing the successful mycorrhizal colonization for the efficient exchange of nutrients between plant and fungus. Although, the regulatory processes of plant and fungus both are important during symbiosis, the major proportion of studies has focused on the regulation from plant's perspective. These regulators include a variety of non-coding RNAs, phytohormones, peptide signals, transcription factors such as *CYCLOPS* and *NODULATION SIGNALING PATHWAY* (NSP1 and NSP2) [70–72]. The *CYCLOPS*, NSP1 and NSP2 are conserved members of rhizobial and mycorrhizal symbiosis phenomenon. The detailed overview of transcriptional regulation of AM development has been provided by Pimprikar and Gutjahr [73]. The non-coding RNA mediated-regulation of mycorrhizal symbiosis is now gaining the scientist's attention and emerging as new area of research.
