Abstract
BackgroundAlbumin 1b peptides (A1b) are small disulfide-knotted insecticidal peptides produced by Fabaceae (also called Leguminosae). To date, their diversity among this plant family has been essentially investigated through biochemical and PCR-based approaches. The availability of high-quality genomic resources for several fabaceae species, among which the model species Medicago truncatula (Mtr), allowed for a genomic analysis of this protein family aimed at i) deciphering the evolutionary history of A1b proteins and their links with A1b-nodulins that are short non-insecticidal disulfide-bonded peptides involved in root nodule signaling and ii) exploring the functional diversity of A1b for novel bioactive molecules.ResultsInvestigating the Mtr genome revealed a remarkable expansion, mainly through tandem duplications, of albumin1 (A1) genes, retaining nearly all of the same canonical structure at both gene and protein levels. Phylogenetic analysis revealed that the ancestral molecule was most probably insecticidal giving rise to, among others, A1b-nodulins. Expression meta-analysis revealed that many A1b coding genes are silent and a wide tissue distribution of the A1 transcripts/peptides within plant organs. Evolutionary rate analyses highlighted branches and sites with positive selection signatures, including two sites shown to be critical for insecticidal activity. Seven peptides were chemically synthesized and folded in vitro, then assayed for their biological activity. Among these, AG41 (aka MtrA1013 isoform, encoded by the orphan TA24778 contig.), showed an unexpectedly high insecticidal activity. The study highlights the unique burst of diversity of A1 peptides within the Medicago genus compared to the other taxa for which full-genomes are available: no A1 member in Lotus, or in red clover to date, while only a few are present in chick pea, soybean or pigeon pea genomes.ConclusionThe expansion of the A1 family in the Medicago genus is reminiscent of the situation described for another disulfide-rich peptide family, the “Nodule-specific Cysteine-Rich” (NCR), discovered within the same species. The oldest insecticidal A1b toxin was described from the Sophorae, dating the birth of this seed-defense function to more than 58 million years, and making this model of plant/insect toxin/receptor (A1b/insect v-ATPase) one of the oldest known.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0745-0) contains supplementary material, which is available to authorized users.
Highlights
Albumin 1b peptides (A1b) are small disulfide-knotted insecticidal peptides produced by Fabaceae
Specific expansion of the albumin 1b peptides (A1b) family The survey of the Medicago truncatula genome led to the identification of 52 A1 gene homologues. 44 genes were located on a M. truncatula chromosome (1 to 8), labeled Medtrng, while eight genes were unassembled (four from the new V4.0 version: Medtr0093s0090, Medtr0112s0040, Medtr0112s0050 and Medtr 0416 s0030, and three were from the older V3.5 version: AC146565_12.1; AC146565_18.1; AC146565_34.1, plus the single, AJ574790.1 gene [18]; these were transiently located on a fictitious chromosome zero (Fig. 2, Additional file 1: Table S2)
AG41 acts as a potent blocker of insect cell membrane current We further investigated the AG41’s biological activity by performing an electrophysiological experiment on cultured Spodoptera frugiperda 9 (Sf9) cells to check whether its cell-membrane ion transport alteration differed from that of PA1b, the model pea peptide [13]
Summary
Albumin 1b peptides (A1b) are small disulfide-knotted insecticidal peptides produced by Fabaceae ( called Leguminosae). Many legumes have been used in folk medicine, indicative of their bioactive chemical diversity [3, 4] They play a critical role in natural agricultural and forest ecosystems because of their position in the nitrogen cycle [5]. Due to this nodal ecological position, pests, being nitrogen-limited feeders, are a major constraint to legume production. They have been involved in an evolutionary armsrace with legumes that defend themselves and their seeds through a wide array of chemical defenses and, remarkably, N-containing alkaloids, non-protein aminoacids and anti-nutritive peptides [6]. The isolation of legume peptides found to be acutely toxic for insect pests in stored vegetables and crops, and non-toxic to other taxa [7], has enlarged this defense arsenal, and, as a result, our possibilities for cereal grain protection [8, 9]
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