Transcriptional snapshots provide insights into the molecular basis of arbuscular mycorrhiza in the model legume Medicago truncatula

Abstract

Abstract. The arbuscular mycorrhizal (AM) association between terrestrial plants and soil fungi of the phylum Glomeromycota is the most widespread beneficial plant-microbe interaction on earth. In the course of the symbiosis, fungal hyphae colonise plant roots and supply limiting nutrients, in particular phosphorus, in exchange for carbon compounds. Owing to the obligate biotrophy of mycorrhizal fungi and the lack of genetic systems to study them, targeted molecular studies on AM symbioses proved to be difficult. With the emergence of plant genomics and the selection of suitable models, an application of untargeted expression profiling experiments became possible. In the model legume Medicago truncatula, high-throughput expressed sequence tag (EST)-sequencing in conjunction with in silico and experimental transcriptome profiling provided transcriptional snapshots that together defined the global genetic program activated during AM. Owing to an asynchronous development of the symbiosis, several hundred genes found to be activated during the symbiosis cannot be easily correlated with symbiotic structures, but the expression of selected genes has been extended to the cellular level to correlate gene expression with specific stages of AM development. These approaches identified marker genes for the AM symbiosis and provided the first insights into the molecular basis of gene expression regulation during AM.

Laser Microdissection Unravels Cell-Type-Specific Transcription in Arbuscular Mycorrhizal Roots, Including CAAT-Box Transcription Factor Gene Expression Correlating with Fungal Contact and Spread

“…Among the core set of AM-related genes, 15 wellknown AM expression markers (Hohnjec et al, 2006;Kü ster et al, 2007b) were identified as strongly induced in our conditions ( Table I). As an example, the MtPt4 gene encoding a phosphate transporter known to be required for an efficient AM symbiosis (Javot et al, 2007) was activated more than 1,000 (log 2 = 10.02)-and 400 (log 2 = 8.65)-fold in G. intraradices-and G. mosseae-colonized roots, respectively.…”

“…FDR-corrected P values (FDR-P) are indicated. References for the AM-induced genes identified are as follows: MtPt4 (Javot et al, 2007), MtMyb1 (Liu et al, 2003), MtTi1 (Grunwald et al, 2004), MtLec7 (Frenzel et al, 2005), MtGst1 (Wulf et al, 2003), MtBcp1 (Hohnjec et al, 2005), MtLec5 (Frenzel et al, 2005), MtGlp1 , MtHa1 (Krajinski et al, 2002), MtScp1 (Liu et al, 2003), MtVapyrin (Pumplin et al, 2010), MtSbtM1 (Takeda et al, 2011), MtZip7 (Burleigh et al, 2003), MtAnn2 , and MtDxs2 (Floss et al, 2008 roots (Liu et al, 2003;Wulf et al, 2003;Hohnjec et al, 2006;Kü ster et al, 2007b;Guether et al, 2009). Interestingly, the categories membrane transporters, signaling, and TFs were not only prominent but their members were also expressed most significantly different from others in AM roots (data not shown), based on statistical analyses implemented in MapMan (Usadel et al, 2005).…”

“…In the last couple of years, untargeted highthroughput expression profiling was pursued to generate an inventory of AM-induced genes (Liu et al, 2003;Wulf et al, 2003;Frenzel et al, 2005;Hohnjec et al, 2005Hohnjec et al, , 2006Kü ster et al, 2007b). These approaches benefited from the identification of two legumes that proved to be excellent AM models: Medicago truncatula (Barker et al, 1990;Rose, 2008) and Lotus japonicus (Handberg and Stougaard, 1992).…”

“…Although the transcriptome studies performed so far resulted in an identification of hundreds of AMrelated genes (Balestrini and Lanfranco, 2006;Hohnjec et al, 2006), there is limited information on the signaling components activated during the formation of arbuscules and in particular on the transcriptional regulators involved in the reprogramming of root cortical cells toward an accommodation of symbiotic fungi. One obvious reason for this can be seen in the fact that most AM-related expression profiles were based on pooled tissue samples containing a mixture of different cell types and stages of arbuscule development.…”

Medicago truncatula as a model for understanding plant interactions with other organisms, plant development and stress biology: past, present and future

“…(Liu et al 2003;Hohnjec et al 2006). The AM symbiosis association enables a carbon supply for the fungus and an enhanced supply of mineral nutrients, notably phosphorus, to the plant (Liu et al 2003;Hohnjec et al 2006).…”