Arbuscular mycorrhizal symbiosis is accompanied by local and systemic alterations in gene expression and an increase in disease resistance in the shoots

Abstract

SummaryIn natural ecosystems, the roots of many plants exist in association with arbuscular mycorrhizal (AM) fungi, and the resulting symbiosis has profound effects on the plant. The most frequently documented response is an increase in phosphorus nutrition; however, other effects have been noted, including increased resistance to abiotic and biotic stresses. Here we used a 16 000-feature oligonucleotide array and real-time quantitative RT-PCR to explore transcriptional changes triggered in Medicago truncatula roots and shoots as a result of AM symbiosis. By controlling the experimental conditions, phosphorus-related effects were minimized, and both local and systemic transcriptional responses to the AM fungus were revealed. The transcriptional response of the roots and shoots differed in both the magnitude of gene induction and the predicted functional categories of the mycorrhiza-regulated genes. In the roots, genes regulated in response to three different AM fungi were identified, and, through split-root experiments, an additional layer of regulation, in the colonized or noncolonized sections of the mycorrhizal root system, was uncovered. Transcript profiles of the shoots of mycorrhizal plants indicated the systemic induction of many genes predicted to be involved in stress or defense responses, and suggested that mycorrhizal plants might display enhanced disease resistance. Experimental evidence supports this prediction, and mycorrhizal M. truncatula plants showed increased resistance to a virulent bacterial pathogen, Xanthomonas campestris. Thus, the symbiosis is accompanied by a complex pattern of local and systemic changes in gene expression, including the induction of a functional defense response.

Plant immune responses triggered by beneficial microbes

“…In most cases only weak, transient, or strictly localized defense-associated responses were elicited, which differs greatly from the massive induction of defense responses triggered during plant-pathogen interactions [39][40][41]42 ]. Transcriptome analysis of Arabidopsis expressing WCS417-ISR revealed a set of 94 genes that were differentially expressed locally in the roots [39].…”

“…The endophytic fungus Piriformospora indica induced systemic resistance in barley without priming for JA-mediated, ET-mediated, or SA-mediated defenses, but was associated with the activation of the glutathione-ascorbate cycle, indicating an increased antioxidative capacity [5]. In some cases, shoots of mycorrhizal plants showed changes in defense-related gene expression in the absence of a pathogen [42 ], but in other cases priming seems to be the dominant mechanism involved in mycorrhiza-induced systemic resistance [9]. For instance, colonization of tomato roots by mycorrhizal fungi systemically provided protection against Phytophthora parasitica infection without direct accumulation of PR proteins.…”

Diversidade de fungos micorrízicos arbusculares em área de Caatinga, PE, Brasil

“…Os FMA promovem maior absorção de nutrientes, beneficiando o hospedeiro principalmente com a translocação do fósforo (He et al 2002),e também outros íons, proporcionam aumento da resistência vegetal contra patógenos (Liu et al 2007) e maior tolerância a estresses hídrico (Beltrano & Ronco 2008) e salino (Maia & Yano-Melo 2005), contribuindo ainda para a agregação do solo (Mergulhão et al 2010). Além disso, atuam na definição de nichos ecológicos ocupados pelos vegetais, determinando a composição das comunidades de plantas (Francis & Read 1995).…”

Community shifts of soybean stem-associated bacteria responding to different nodulation phenotypes and N levels

“…Recent studies have shown that the wild-type and symbiosisdefective mutants of legumes such as Medicago truncatula and Glycine max have different microbial community structures, and certain microbes associate preferentially with nodulated or mycorrhizal root systems (Offre et al, 2007;Ikeda et al, 2008a). Furthermore, transcriptional analyses revealed that both nodulation and mycorrhization systemically induced a series of defense genes in a host plant and consequently increased its disease resistance (Liu et al, 2007;Kinkema and Gresshoff, 2008). These examples indicate that genetic alteration in the nodulation or mycorrhization signaling pathways can in turn alter the accompanying plant microflora, aside from rhizobia and mycorrhizae.…”