Summary: In 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 non‐colonized 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.

Summary: The 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.

The stalk, ear and root rot (SERR) of maize caused by Fusarium verticillioides (Fv) severely impacts crop production in tropical and subtropical regions. The aim of the present work was to screen bacterial isolates in order to find novel native biocontrol agents against Fv. A culturable bacterial collection consisting of 11,520 isolates enriched in Firmicutes and Proteobacteria was created from rhizosphere samples taken from SERR symptomatic or asymptomatic maize plants. The complete collection was screened for potential activity against Fv using a liquid antagonism assay followed by dual cultures in solid medium, selecting for 42 bacteria (Bacillus, Pseudomonas and Paenibacillus) that inhibit Fv growth (>45 %). In planta assays demonstrated that three Bacillus isolates: B. megaterium (B5), B. cereus sensu lato (B25) and Bacillus sp. (B35) displayed the highest antagonistic activity against Fv. Pot experiments performed in a greenhouse with Bacillus cereus sensu lato B25 confirmed these findings and showed a reduction of Fv disease severity and incidence on plants. Antagonistic activity analysis revealed that these strains produce glucanases, proteases or chitinases, as well as siderophores and auxins and suggests these as possible control mechanisms against Fv.

Camptothecin is an anticancer and antiviral alkaloid produced by the Chinese tree Camptotheca acuminata (Nyssaceae). Despite previous reports of low levels of anticancer activity in leaves of Camptotheca acuminata, we have discovered that camptothecin accumulates to approximately 0.4% of the dry weight of young leaves. This level is 1.5-fold higher than that of the seeds and 2.5-fold higher than that of the bark, the two currently used sources of the drug. As the leaves mature, the concentration and absolute amount of camptothecin decreases rapidly. The high levels of camptothecin in young leaves could provide an easily harvested, non-destructive source of this important drug.