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.

The majority of vascular flowering plants are able to form symbiotic associations with arbuscular mycorrhizal fungi. These symbioses, termed arbuscular mycorrhizas, are mutually beneficial, and the fungus delivers phosphate to the plant while receiving carbon. In these symbioses, phosphate uptake by the arbuscular mycorrhizal fungus is the first step in the process of phosphate transport to the plant. Previously, we cloned a phosphate transporter gene involved in this process. Here, we analyze the expression and regulation of a phosphate transporter gene (GiPT) in the extra-radical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices during mycorrhizal association with carrot or Medicago truncatula roots. These analyses reveal that GiPT expression is regulated in response to phosphate concentrations in the environment surrounding the extra-radical hyphae and modulated by the overall phosphate status of the mycorrhiza. Phosphate concentrations, typical of those found in the soil solution, result in expression of GiPT. These data imply that G. intraradices can perceive phosphate levels in the external environment but also suggest the presence of an internal phosphate sensing mechanism.

¶ First co-authors. SummaryTwo rapid and simple in planta transformation methods have been developed for the model legume Medicago truncatula. The ®rst approach is based on a method developed for transformation of Arabidopsis thaliana and involves in®ltration of¯owering plants with a suspension of Agrobacterium. The second method involves in®ltration of young seedlings with Agrobacterium. In both cases a proportion of the progeny of the in®ltrated plants is transformed. The transformation frequency ranges from 4.7 to 76% for the¯ower in®ltration method, and from 2.9 to 27.6% for the seedling in®ltration method. Both procedures resulted in a mixture of independent transformants and sibling transformants. The transformants were genetically stable, and analysis of the T 2 generation indicates that the transgenes are inherited in a Mendelian fashion. These transformation systems will increase the utility of M. truncatula as a model system and enable large-scale insertional mutagenesis. T-DNA tagging and the many adaptations of this approach provide a wide range of opportunities for the analysis of the unique aspects of legumes.

During the endosymbiosis formed between plants and arbuscular mycorrhizal (AM) fungi, the root cortical cells are colonized by branched hyphae called arbuscules, which function in nutrient exchange with the plant [1]. Despite their positive function, arbuscules are ephemeral structures, and their development is followed by a degeneration phase, in which the arbuscule and surrounding periarbuscular membrane and matrix gradually disappear from the root cell [2, 3]. Currently, the root cell's role in this process and the underlying regulatory mechanisms are unknown. Here, by using a Medicago truncatula pt4 mutant in which arbuscules degenerate prematurely [4], we identified arbuscule degeneration-associated genes, of which 38% are predicted to encode secreted hydrolases, suggesting a role in disassembly of the arbuscule and interface. Through RNAi and analysis of an insertion mutant, we identified a symbiosis-specific MYB-like transcription factor (MYB1) that suppresses arbuscule degeneration in mtpt4. In myb1, expression of several degeneration-associated genes is reduced. Conversely, in roots constitutively overexpressing MYB1, expression of degeneration-associated genes is increased and subsequent development of symbiosis is impaired. MYB1-regulated gene expression is enhanced by DELLA proteins and is dependent on NSP1 [5], but not NSP2 [6]. Furthermore, MYB1 interacts with DELLA and NSP1. Our data identify a transcriptional program for arbuscule degeneration and reveal that its regulators include MYB1 in association with two transcriptional regulators, NSP1 and DELLA, both of which function in preceding phases of the symbiosis. We propose that the combinatorial use of transcription factors enables the sequential expression of transcriptional programs for arbuscule development and degeneration.