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Plasma membrane-localized pattern recognition receptors (PRRs) such as FLAGELLIN SENSING2 (FLS2), EF-TU RECEPTOR (EFR) and CHITIN ELICITOR RECEPTOR KINASE 1 (CERK1) recognize microbe-associated molecular patterns (MAMPs) to activate pattern-triggered immunity (PTI). A reverse genetics approach on genes responsive to the priming agent beta-aminobutyric acid (BABA) revealed IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as a critical PTI player. Arabidopsis thaliana ios1 mutants were hyper-susceptible to Pseudomonas syringae bacteria. Accordingly, ios1 mutants showed defective PTI responses, notably delayed up-regulation of the PTI-marker gene FLG22-INDUCED RECEPTOR-LIKE KINASE1 (FRK1), reduced callose deposition and mitogen-activated protein kinase activation upon MAMP treatment. Moreover, Arabidopsis lines over-expressing IOS1 were more resistant to bacteria and showed a primed PTI response. In vitro pull-down, bimolecular fluorescence complementation, co-immunoprecipitation, and mass spectrometry analyses supported the existence of complexes between the membrane-localized IOS1 and BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1)-dependent PRRs FLS2 and EFR, as well as with the BAK1-independent PRR CERK1. IOS1 also associated with BAK1 in a ligand-independent manner, and positively regulated FLS2-BAK1 complex formation upon MAMP treatment. In addition, IOS1 was critical for chitin-mediated PTI. Finally, ios1 mutants were defective in BABA-induced resistance and priming. This work reveals IOS1 as a novel regulatory protein of FLS2-, EFR- and CERK1-mediated signaling pathways that primes PTI activation.

By screening an EMS mutagenised common bean population, we had previously identified the lpa (low phytic acid) 280-10 line that carries a monogenic recessive mutation conferring a 90% reduction of InsP6 content and, differently from most other lpa mutants, does not display negative pleiotropic effects associated to the mutation. In the present work we mapped the lpa-280-10 mutation on bean chromosome 1, in a region syntenic to soybean (Glycine max) chromosomes 19 and 3, where the lpa mutant CX1834 was previously mapped and correlated to multi-drug resistance protein (MRP) type ATP-binding cassette (ABC) transporter gene orthologous to Arabidopsis AtMRP5/AtABCC5 and maize ZmMRP4 (lpa1). Fine mapping of bean lpa-280-10 and DNA sequencing data showed the occurrence of a single amino acid substitution (E to K) in the orthologous PvMRP-1 gene. We also provide data on a second MRP gene (PvMRP-2) present both in soybean and bean genomes, which, despite the high sequence identity (90%) and a very similar pattern of expression during seed development to PvMRP-1, is not able to complement the lpa mutation, suggesting that it may have a different role in the plant. Transcriptional and metabolic analysis of phytic acid pathway demonstrated the existence of a negative feedback mechanism of InsP6, or its degradation products, acting on genes coding for key enzymes of phytic acid pathway and leading to a 30% reduction of myo-inositol. Our results show that the reduced content of myo-inositol is responsible of the ABA hypersensitive response found in lpa-280-10 seed germination, indicating a key role of this metabolite in the multiple pleiotropic effects often reported for lpa mutants.

Several environmental stresses generate high amounts of reactive oxygen species (ROS) in plant cells, resulting in oxidative stress. Symbiotic nitrogen fixation (SNF) in the legume-rhizobia symbiosis is sensitive to damage from oxidative stress. Active nodules of the common bean (Phaseolus vulgaris) exposed to the herbicide paraquat (1,1'-dimethyl-4,4'-bipyridinium dichloride hydrate), which stimulates ROS accumulation, exhibited reduced nitrogenase activity and ureide content. We analyzed the global gene response of nodules subjected to oxidative stress using the Bean Custom Array 90K, which includes probes from 30,000 expressed sequence tags (ESTs). A total of 4280 ESTs were differentially expressed in stressed bean nodules; of these, 2218 were repressed. Based on Gene Ontology analysis, these genes were grouped into 42 different biological process categories. Analysis with the PathExpress bioinformatic tool, adapted for bean, identified five significantly repressed metabolic pathways related to carbon/nitrogen metabolism, which is crucial for nodule function. Quantitative reverse transcription (qRT)-PCR analysis of transcription factor (TF) gene expression showed that 67 TF genes were differentially expressed in nodules exposed to oxidative stress. Putative cis-elements recognized by highly responsive TF were detected in promoter regions of oxidative stress regulated genes. The expression of oxidative stress responsive genes and of genes important for SNF in bacteroids analyzed in stressed nodules revealed that these conditions elicited a transcriptional response.

Phytic acid, myo-inositol 1,2,3,4,5,6-hexakisphosphate, is the major storage compound of phosphorous (P) in plants, predominantly accumulating in seeds (up to 4-5% of dry weight) and pollen. In cereals, phytic acid is deposited in embryo and aleurone grain tissues as a mixed "phytate" salt of potassium and magnesium, although phytates contain other mineral cations such as iron and zinc. During germination, phytates are broken down by the action of phytases, releasing their P, minerals and myo-inositol which become available to the growing seedling. Phytic acid represents an anti-nutritional factor for animals, and isolation of maize low phytic acid ( lpa) mutants provides a novel approach to study its biochemical pathway and to tackle the nutritional problems associated with it. Following chemical mutagenesis of pollen, we have isolated a viable recessive mutant named lpa 241 showing about 90% reduction of phytic acid and about a tenfold increase in seed-free phosphate content. Although germination rate was decreased by about 30% compared to wild-type, developement of mutant plants was apparentely unaffected. The results of the genetic, biochemical and molecular characterization experiments carried out by SSR mapping, MDD-HPLC and RT-PCR are consistent with a mutation affecting the MIPS1S gene, coding for the first enzyme of the phytic acid biosynthetic pathway.