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.

SummaryThe plant hormone ethylene negatively regulates bacterial infection and nodule formation in legumes in response to symbiotic rhizobia, but the molecular mechanism(s) of ethylene action in symbiosis remain obscure. We have identified and characterized multiple mutant alleles of the MtSkl1 gene, which controls both ethylene sensitivity and nodule numbers. We show that this locus encodes the Medicago truncatula ortholog of the Arabidopsis ethylene signaling protein EIN2. In addition to the well-characterized role of MtSkl1 in rhizobial symbiosis, we show that MtSkl1 is involved in regulating early phases of the symbiotic interaction with mycorrhizal fungi, and in mediating root responses to cytokinin. MtSkl1 also functions in the defense against Rhizoctonia solani and Phytophthora medicaginis, with the latter interaction likely to involve positive feedback amplification of ethylene biosynthesis. Overexpression of the C-terminal domain of MtEIN2 is sufficient to block nodulation responses, consistent with previous reports in Arabidopsis on the activation of ethylene signaling. This same C-terminal region is uniquely conserved throughout the EIN2 homologs of angiosperms, which is consistent with its role as a higher plant-specific innovation essential to EIN2 function.

Camptothecin (CPT) is a valuable anti-cancer monoterpene alkaloid produced by the Chinese tree Camptotheca acuminata. Tryptophan decarboxylase (TDC) supplies tryptamine for the indole moiety of CPT and its derivatives, and is considered a key step in monoterpene indole alkaloid biosynthesis as it links primary and secondary metabolism. This report describes the isolation and characterization of tdc1 and tdc2, two autonomously regulated TDC genes from Camptotheca. When expressed in Escherichia coli, the products of each gene could decarboxylate tryptophan, but were inactive against tyrosine, phenylalanine and 3,4-dihydroxyphenylalanine (dopa), tdc1 was developmentally regulated, having its highest expression level in the apex, young stem and bark, tissues which also contain the highest levels of CPT. Expression of tdc1 also increased during seedling development and was correlated with alkaloid accumulation during germination. tdc2 expression was induced in Camptotheca leaf discs and cell suspension cultures treated with fungal elicitor or methyl jasmonate, treatments which did not affect tdc1 expression. Unlike tdc1, tdc2 expression was not detected in any unstressed Camptotheca tissues nor in developing seedlings. These data suggest that tdc1 may be part of a developmentally regulated chemical defense system in Camptotheca, while tdc2 serves as part of a defense system induced during pathogen challenge.

Arbuscular mycorrhizal symbiosis is a beneficial association between plant roots and fungi that occurs in approximately 80 % of terrestrial plants and which confers different benefits including mineral nutrient acquisition and enhanced defense capacity. Although mycorrhizal colonization takes place in roots, the symbiosis establishment has systemic effects in other parts of the plant, in processes such as nutrient translocation and systemic resistance. In order to understand the transcriptional changes that occur in leaves of mycorrhizal plants, we used RNA-seq technology to obtain the transcriptomes of leaves from mycorrhizal and nonmycorrhizal tomato plants (Solanum lycopersicum). Four weeks after inoculation with the fungus Rhizophagus irregularis, leaves from mycorrhizal and non-mycorrhizal tomato plants were used for transcriptome sequencing. Of the 21,113 genes expressed in tomato leaves, 742 genes displayed differential expression between the mycorrhizal and nonmycorrhizal conditions. Most of the transcriptional changes occurred in the Bprotein,^BRNA,^Bsignaling,^Btransport,^B biotic and abiotic stresses,^and Bhormone metabolism^categories. Some transcriptional changes also occurred in P, N, and sugar transporters, as would be expected for mycorrhizal colonization. Finally, several differentially expressed genes may be related to systemic defense priming, in agreement with our demonstration that symbiotic plants exhibited mycorrhizainduced resistance against the foliar pathogen Xanthomonas campestris pv. vesicatoria. This is the first study to take on a genome-wide analysis aimed at understanding the expression changes in leaves of mycorrhiza-colonized plants. The results will therefore be valuable to future analyses focused on specific genes, as well as detailed studies of the expression profiles of certain gene families.

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