A roadmap of cell-type specific gene expression during sequential stages of the arbuscular mycorrhiza symbiosis

Abstract

BackgroundAbout 80% of today’s land plants are able to establish an arbuscular mycorrhizal (AM) symbiosis with Glomeromycota fungi to improve their access to nutrients and water in the soil. On the molecular level, the development of AM symbioses is only partly understood, due to the asynchronous development of the microsymbionts in the host roots. Although many genes specifically activated during fungal colonization have been identified, genome-wide information on the exact place and time point of their activation remains limited.ResultsIn this study, we relied on a combination of laser-microdissection and the use of Medicago GeneChips to perform a genome-wide analysis of transcription patterns in defined cell-types of Medicago truncatula roots mycorrhized with Glomus intraradices. To cover major stages of AM development, we harvested cells at 5-6 and at 21 days post inoculation (dpi). Early developmental stages of the AM symbiosis were analysed by monitoring gene expression in appressorial and non-appressorial areas from roots harbouring infection units at 5-6 dpi. Here, the use of laser-microdissection for the first time enabled the targeted harvest of those sites, where fungal hyphae first penetrate the root. Circumventing contamination with developing arbuscules, we were able to specifically detect gene expression related to early infection events. To cover the late stages of AM formation, we studied arbusculated cells, cortical cells colonized by intraradical hyphae, and epidermal cells from mature mycorrhizal roots at 21 dpi. Taken together, the cell-specific expression patterns of 18014 genes were revealed, including 1392 genes whose transcription was influenced by mycorrhizal colonization at different stages, namely the pre-contact phase, the infection of roots via fungal appressoria, the subsequent colonization of the cortex by fungal hyphae, and finally the formation of arbuscules. Our cellular expression patterns identified distinct groups of AM-activated genes governing the sequential reprogramming of host roots towards an accommodation of microsymbionts, including 42 AM-activated transcription factor genes.ConclusionsOur genome-wide analysis provides novel information on the cell-specific activity of AM-activated genes during both early and late stages of AM development, together revealing the road map of fine-tuned adjustments of transcript accumulation within root tissues during AM fungal colonization.

The plasma membrane proteome of Medicago truncatula roots as modified by arbuscular mycorrhizal symbiosis

“…In agreement with previous studies underlining an active role of plant cells in accommodating AM fungi (Hogekamp and Küster 2013;Rich et al 2014), a large number of proteins that display an increased abundance in the PM fraction of mycorrhizal roots were related to protein turnover and cell wall construction (Table 1). Among the candidate proteins mediating cell wall loosening in the cells accommodating fungal structures (Balestrini and Bonfante 2014), the secreted enzymes glucosidase II and xylose isomerase were only detected in symbiotic roots (Table 1).…”

Hyphal branching during arbuscule development requires RAM1

“…However, neither the exo70i nor the str phenotype is as severe as that of ram1-3, which suggests that expression of other genes involved in early arbuscule branching is also regulated by RAM1. Large numbers of mycorrhiza-induced TFs were identified in transcriptome analyses (Hogekamp and Küster, 2013;Xue et al, 2015), and therefore, TFs other than RAM1 must also regulate gene expression during symbiosis. Expression of some of these TFs may be regulated by RAM1, and we provide initial evidence that RAM1 can induce expression of RAD1.…”

“…Among these shared components is a transcription factor (TF) called CYCLOPs (Yano et al, 2008;Singh et al, 2014), which is required for arbuscule development. There are extensive transcriptional changes in the root cortex associated with arbuscule development (Gaude et al, 2012;Hogekamp and Küster, 2013), but currently, genes regulated by CYCLOPS that enable arbuscule development are unknown.…”

The Symbiosis-Related ERN Transcription Factors Act in Concert to Coordinate Rhizobial Host Root Infection

“…The data represent mean values of three to five independent biological experiments, with at least two technical replicates after normalization with the three reference genes, Ubiquitin, Mt0089_00067, and Mt0085_00020, shown before to exhibit invariable expression levels (Cerri et al, 2012;Roux et al, 2014). Primer pairs to amplify PT4 and Mtr3213.1S1_at (Hogekamp and Küster, 2013) are listed in Supplemental Table S1. PT4 transcript levels represent mean values obtained after normalization with the three reference genes and Mtr.3213.1.S1_at levels.…”