Seven Lotus japonicus Genes Required for Transcriptional Reprogramming of the Root during Fungal and Bacterial Symbiosis

Abstract

A combined genetic and transcriptome analysis was performed to study the molecular basis of the arbuscular mycorrhiza (AM) symbiosis. By testing the AM phenotype of nodulation-impaired mutants and complementation analysis, we defined seven Lotus japonicus common symbiosis genes (SYMRK, CASTOR, POLLUX, SYM3, SYM6, SYM15, and SYM24) that are required for both fungal and bacterial entry into root epidermal or cortical cells. To describe the phenotype of these mutants at the molecular level, we screened for differentiating transcriptional responses of mutant and wild-type roots by large-scale gene expression profiling using cDNA-amplified fragment length polymorphism. Two percent of root transcripts was found to increase in abundance during AM development, from which a set of AM-regulated marker genes was established. A Ser-protease (SbtS) and a Cys-protease (CysS) were also activated during root nodule development. AM-induced transcriptional activation was abolished in roots carrying mutations in common symbiosis genes, suggesting a central position of these genes in a pathway leading to the transcriptional activation of downstream genes. By contrast, AM fungus-induced gene repression appeared to be unaffected in mutant backgrounds, which indicates the presence of additional independent signaling pathways.

Rhizobial and Mycorrhizal Symbioses in Lotus japonicus Require Lectin Nucleotide Phosphohydrolase, Which Acts Upstream of Calcium Signaling

“…Unlike LNP, the SYM pathway is not necessary for the Nod factorinduced calcium influx response (Miwa et al, 2006a), although it has been reported that M. truncatula mutated in dmi1 or dmi2 cannot support the complete calcium influx response (Shaw and Long, 2003). Furthermore, analysis in L. japonicus mutants of SYM pathway components identified various steps of mycorrhizal colonization of roots (Novero et al, 2002;Demchenko et al, 2004;Kistner et al, 2005). In all the early L. japonicus mutants defective for arbuscular mycorrhization identified so far, fungal hyphae have been observed to enter between epidermal cells, but the intracellular penetration of epidermal or outer cell layers is blocked except in Ljsym15, in which mycorrhization is blocked at the separation of anticlinal walls of epidermal cells and arbuscule formation (Demchenko et al, 2004).…”

“…In all the early L. japonicus mutants defective for arbuscular mycorrhization identified so far, fungal hyphae have been observed to enter between epidermal cells, but the intracellular penetration of epidermal or outer cell layers is blocked except in Ljsym15, in which mycorrhization is blocked at the separation of anticlinal walls of epidermal cells and arbuscule formation (Demchenko et al, 2004). In the LjsymRK mutant, fungal hyphae form appressoria and balloon-like deformations, but the subsequent fungal entry in cells of epidermis and exodermis was aborted, blocking the intracellular passage (Demchenko et al, 2004;Kistner et al, 2005). Attachment of fungal hyphae and appressorium formation were observed on the root surface of three LNP antisense lines, but intercellular or intracellular entry in the epidermal cells was not observed, implying a role for LNP in events leading to the epidermal cell entry of fungal hyphae.…”

“…Nodulation-defective legume mutants have revealed a signaling pathway that is conserved between Nod factor and mycorrhizal recognition (Wais et al, 2000;Kistner et al, 2005), and considering the different evolutionary histories of these two symbioses, it is legitimate to surmise that the evolution of nodulation involved the recruitment of the preexisting mycorrhizal signaling pathway for the recognition of Nod factor. At the core of this common symbiosis signaling (SYM) pathway are oscillations in nucleusassociated cytosolic calcium levels, termed calcium spiking, acting as a secondary messenger to induce gene expression.…”

Homologs of RUNX and CBFβ/PEBP2β in C. elegans

“…It is indeed the case: sli-1 was isolated as a negative regulator of the EGFR signaling pathway in the nematode, and it was discovered that sli-1 encoded a homolog of c-CBL, a putative oncogene whose function had not been revealed at that time Yoon et al, 1995). As soon as the identity of sli-1 was published, the mammalian CBL proteins became intensively researched and soon much was known about these proteins in terms of their biochemical functions and their roles in the signaling pathway (for example, Bowtell and Langdon, 1995;Galisteo et al, 1995;Lupher et al, 1996;Hime et al, 1997;Meisner et al, 1997;Miyake et al, 1997;Smit and Borst, 1997;Lupher et al, 1998;Broome et al, 1999). Another gene, ark-1, was also identified as a component of the EGFR signaling pathway in the nematode vulval development, and it was determined to be an ACK-related kinase (Hopper et al, 2000).…”

Functional Implication of β-Carotene Hydroxylases in Soybean Nodulation

“…For example, SymRK, the receptor-like kinase gene, is required for both rhizobial and AM symbioses (Stracke et al, 2002). Similarly, the signal transduction pathways following perception are also in part the same, and the genes common to the two pathways have been referred to as the common symbiosis (SYM) genes (Kistner et al, 2005). These similarities may reflect common mechanisms for host plant cells to respond to symbionts, although the commonality is not globally defined yet.…”