br Conclusions br Acknowledgements This

Conclusions
Acknowledgements This work was supported by grants from the Polish National Science Center: 2014/14/E/NZ1/00139 (to A. Jazwa-Kusior) and 2015/17/N/NZ1/00041 (to M. Tomczyk) and National Centre for Research and Development (STRATEGMED2/269415/11/NCBR/2015) (to J. Dulak). Faculty of Biochemistry, Biophysics and Biotechnology of Jagiellonian University is a partner of the Leading National Research Center (KNOW) supported by the Ministry of Science and Higher Education. Figures and graphical abstract were partially designed using Servier Medical Art according to a Creative Commons Attribution 3.0 Unported License guidelines 3.0 (https://creativecommons.org/licenses/by/3.0/pl). The authors declared no conflict of interest.
Introduction Arbuscular mycorrhizal (AM) symbiosis is the most widespread symbiotic association formed between plants and fungi (Smith and Read, 2008). AM formation improves plant growth and Arecaidine but-2-ynyl ester tosylate by activating additional mechanisms for water and mineral nutrient uptake in the host plant in exchange for carbon compounds to the fungus. AM symbiosis confers other benefits on the plants such as improved resistance to abiotic and biotic stress (Bhandari and Garg, 2017; Lenoir et al., 2016). Extensive signalling events, cellular remodelling and metabolic alterations occur in plant roots to accommodate the fungal endosymbiont (MacLean et al., 2017). Numerous microarray analyses have demonstrated that root colonization by AM fungi regulates transcriptional changes in the host plant, and mycorrhizal-induced plant genes at prior, early and late stages of mycorrhization have been identified (Mohanta and Bae, 2015). Defence genes, involved in the control and regulation of AM symbiosis, are particularly important (García-Garrido et al., 2010; López-Ráez et al., 2010). In this regard, the induction and suppression of host defence mechanisms during AM formation have been described (Liu et al., 2007; Pozo et al., 2002), and a certain degree of defence gene modulation during root colonization by AM fungi has been described (Kloppholz et al., 2011). The changes in plant defence gene expression, which is induced and downregulated during the initial stages of AM symbiosis, have been reported to be essential for establishing AM associations (García-Garrido and Ocampo, 2002). However, at the later stages of mycorrhization when arbuscule formation begins, a mechanism of plant defence response activation appears to strengthen, although only in those plant cells containing fungal arbuscules (García-Garrido and Ocampo, 2002; Brechenmacher et al., 2004). In this respect, some defence features, such as jasmonic acid (JA) metabolism activation (Hause et al., 2007) and oxilipin pathway up-regulation (León-Morcillo et al., 2012), have been coupled with the mycorrhization process. Defence genes expressed in cells containing arbuscules could play a role in the control of hyphal spread and arbuscule formation in the root, and defence gene expression may be regulated by arbuscule morphogenesis. Transcriptional studies have also highlighted analogies between plant responses to AM fungi and plant responses to biotrophic pathogens (Güimil et al., 2005). Furthermore, the biological control exerted by AM against pathogens is probably related to these changes in plant defence, in conjunction with other mechanisms such as nutritional improvements in AM plants (Pozo and Azcón-Aguilar, 2007). A comparative analysis of transcriptomic data obtained from mycorrhizal wild-type tomatoes and ABA-deficient sitiens mutant plants revealed that the impairment in AM formation linked with incomplete arbuscule development in the sitiens mutants is associated with an upregulation of genes related to defence and cell wall modification (García-Garrido et al., 2010). This suggests that the initial activation of defence responses during arbuscule formation might be followed by a downregulation of defence and cell wall-modifying genes in the later stages, in which arbuscule maturation could play a crucial role.