Paenibacillus sp. strain B2, isolated from the mycorrhizosphere of sorghum colonized by Glomus mosseae, produces an antagonistic factor. This factor has a broad spectrum of activity against gram-positive and gram-negative bacteria and also against fungi. The antagonistic factor was isolated from the bacterial culture medium and purified by cation-exchange, reverse-phase, and size exclusion chromatography. The purified factor could be separated into three active compounds following characterization by amino acid analysis and by combined reverse-phase chromatography and mass spectrometry (liquid chromatography-mass spectrometry and mass spectrometry-mass spectrometry). The first compound had the same retention time as polymyxin B 1 , whereas the two other compounds were more hydrophobic. The molecular masses of the latter compounds are 1,184.7 and 1,202.7 Da, respectively, and their structure is similar to that of polymyxin B 1 , with a cyclic heptapeptide moiety attached to a tripeptide side chain and a fatty acyl residue. They both contain threonine, phenylalanine, leucine, and 2,4-diaminobutyric acid residues. The peptide with a molecular mass of 1,184.7 contains a 2,3-didehydrobutyrine residue with a molecular mass of 101 Da replacing a threonine at the A2 position of the polymyxin side chain. This modification could explain the broader range of antagonistic activity of this peptide compared to that of polymyxin B.
Septoria tritici blotch caused by the fungus Zymoseptoria tritici is one of the most devastating foliar diseases of wheat. Knowledge regarding mechanisms involved in resistance against this disease is required to breed durable resistances. This study compared the expression of defence and pathogenicity determinants in three cultivars in semicontrolled culture conditions. The most susceptible cultivar, Alixan, presented higher necrosis and pycnidia density levels than Altigo, the most resistant one. In Premio, a moderately resistant cultivar, necrosis developed as in Alixan, while pycnidia developed as in Altigo. In noninfectious conditions, genes coding for PR1 (pr1), glucanase (gluc) and allene oxide synthase (aos) were constitutively expressed at a higher level in both Altigo and Premio than in Alixan, while chitinase2 (chit2), phenylalanine ammonia-lyase (pal), peroxidase (pox2) and oxalate oxidase (oxo) were expressed at a higher level in Premio only. Except for aos, all genes were induced in Alixan during the first steps of the symptomless infection phase. Only pox2, oxo, gluc and pal genes in Altigo and pal, chs and lox genes in Premio were up-regulated at some time points. Basal cultivar-dependent resistance against Z. tritici could therefore be explained by various gene expression patterns rather than high expression levels of given genes. During the necrotrophic phase, Z. tritici cell wall-degrading enzyme activity levels were lower in Altigo and Premio than in Alixan, and were associated more with pycnidia than with necrosis. Similar tissue colonization occurred in the three cultivars, suggesting an inhibition of the switch to the necrotrophic lifestyle in Altigo.
Keywords:Wheat septoria leaf blotch Paenimyxin Paenibacillus sp. strain B2 Curtobacterium plantarum Flavonoids Salicylic acid Jasmonic acid Reactive oxygen species A B S T R A C T Many of the non-pathogenic endophytic bacteria that reside in plant roots promote plant growth as well as protection against pathogens attack. However, little is known about their mode of action in wheat. We have previously demonstrated the potential of Paenibacillus sp. strain B2 (PB2) to stimulate plant defense mechanisms via its paenimyxin lipo-polypeptide elicitor. Recently, we isolated the Curtobacterium plantarum strain EDS (EDS) from seeds of almost all wheat cultivars. In the present work, the ability of PB2 and EDS to promote wheat growth and protection against Septoria leaf blotch (SLB) was investigated. Results showed that PB2 is a general root external colonizer and cultivar-dependent endophyte. In the endophytic state and only in co-inoculation, it significantly increased the internal root colonization by EDS, resulting in an increase of root and aerial part fresh weights. qPCR analysis showed that, in the endophytic and nonendophytic states, PB2 conferred ≥59% protection against SLB by inducing systemic resistance which is characterized by the over expression of the pr1, lox, Aos, peroxidase, oxo and gst genes. Paenimyxin conferred 76% local protection characterized by the overexpression of the glu, lox, aos, pal, chs, oxo, and gst genes, and 82% systemic protection by chs. It was concluded that PB2 is potentially very interesting in the biocontrol of SLB and, in a mixture with EDS, in the wheat growth promoting. Genes involved in the flavonoid, salicylic acid, jasmonic acid, reactive oxygen species and basal defense pathways seem to play an important role in the resistance against SLB.
Plant genes exhibiting common responses to different arbuscular mycorrhizal (AM) fungi and not induced under other biological conditions have been sought for to identify specific markers for monitoring the AM symbiosis. A subset of 14 candidate Medicago truncatula genes was identified as being potentially mycorrhiza responsive in previous cDNA microarray analyses and exclusive to cDNA libraries derived from mycorrhizal root tissues. Transcriptional activity of the selected plant genes was compared during root interactions with seven AM fungi belonging to different species of Glomus, Acaulospora, Gigaspora, or Scutellospora, and under widely different biological conditions (mycorrhiza, phosphate fertilization, pathogenic/beneficial microbe interactions, incompatible plant genotype). Ten of the M. truncatula genes were commonly induced by all the tested AM fungal species, and all were activated by at least two fungi. Most of the plant genes were transcribed uniquely in mycorrhizal roots, and several were already active at the appressorium stage of fungal development. Novel data provide evidence that common recognition responses to phylogenetically different Glomeromycota exist in plants during events that are unique to mycorrhiza interactions. They indicate that plants should possess a mycorrhiza-specific genetic program which is comodulated by a broad spectrum of AM fungi.
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