Genome-wide analysis identifies gain and loss/change of function within the small multigenic insecticidal Albumin 1 family of Medicago truncatula

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.

Genome-Wide Identification and Comparative Analysis of Albumin Family in Vertebrates

“…As mentioned previously, plants also contain albumins; however, those are not phylogenetically related to the animal albumins. 39 This fact is consistent with the concept of albumin, which are water soluble, moderately soluble in concentrated salt solutions, not evolutionarily related, and experience heat denaturation. 1 Therefore, just to avoid possible confusion, for the following sections of this article, albumins solely refer to those from animals if not otherwise specified.…”

“…The detail distribution of albumins is presented in Table S1 (see supporting information). As mentioned previously, plants also contain albumins; however, those are not phylogenetically related to the animal albumins 39. This fact is consistent with the concept of albumin, which are water soluble, moderately soluble in concentrated salt solutions, not evolutionarily related, and experience heat denaturation 1.…”

Clitoria ternatea (butterfly pea) cyclotides: insights on functional diversity, regulation and biotechnological applications

“…Overall, these studies suggest that both the PA1b peptides and cyclotides have evolved for plant defence. A phylogenetic analysis of the albumin-1 genes showed that the ancestral bchain gene encoded for a seed-protecting toxin [59]. The study also showed that the plant tribe Sophorae under the Fabaceae plant family contains the oldest A1b toxin to date, suggesting that the seed-protecting function of albumin-1 b-chain may have already evolved 58 million years ago [59].…”

“…Why the albumin-1 multigene family radiated extensively in C. ternatea is not known. However, the expansion of albumin-1 genes amongst Fabaceae species is not unprecedented as a study in 2016 showed that, like C. ternatea, the albumin-1 genes in Medicago truncatula also extensively diversified with 52 predicted albumin-1b peptides encoded in its genome [59]. That study showed that, in addition to having a wide tissue distribution of the A1 transcript/peptide, some of its A1b genes did not exhibit insecticidal activity, suggesting functional diversification [59].…”

“…In contrast to the cyclotide domain and or the PA1b domain of albumin-1 precursors, the role of the a-chain remains uncertain. Multiple sequence alignments have demonstrated that the a-chain is much more conserved than the b-chain, suggesting an important conserved role [59]. In 2012, a plant expression study of albumin-1 precursor genes demonstrated that the linked a-chain is required for the production of b-chain peptides [56].…”