The evolutionary appearance of non‐cyanogenic hydroxynitrile glucosides in the Lotus genus is accompanied by the substrate specialization of paralogous β–glucosidases resulting from a crucial amino acid substitution. (23rd June 2014)

• Record Type: Journal Article
• Authors: Lai, Daniela; Abou Hachem, Maher; Robson, Fran; Olsen, Carl Erik; Wang, Trevor L.; Møller, Birger L.; Takos, Adam M.; Rook, Fred
• Is Part Of: Plant journal. Volume 79:Number 2(2014:Jul.)
• Page Start: 299
• Page End: 311
• Publication Date: 2014-06-23
• Subjects: Plant molecular biology -- Periodicals; Plant cells and tissues -- Periodicals; Botany -- Periodicals; 580
• DOI: 10.1111/tpj.12561
• Languages: English
• ISSNs: 0960-7412

Abstract

<abstract abstract-type="main" id="tpj12561-abs-0001"> <title>Summary</title> <p> <italic>Lotus japonicus</italic>, like several other legumes, biosynthesizes the cyanogenic α–hydroxynitrile glucosides lotaustralin and linamarin. Upon tissue disruption these compounds are hydrolysed by a specific β–glucosidase, resulting in the release of hydrogen cyanide. <italic>Lotus japonicus</italic> also produces the non‐cyanogenic γ‐ and β–hydroxynitrile glucosides rhodiocyanoside A and D using a biosynthetic pathway that branches off from lotaustralin biosynthesis. We previously established that BGD2 is the only β–glucosidase responsible for cyanogenesis in leaves. Here we show that the paralogous BGD4 has the dominant physiological role in rhodiocyanoside degradation. Structural modelling, site‐directed mutagenesis and activity assays establish that a glycine residue (G211) in the aglycone binding site of BGD2 is essential for its ability to hydrolyse the endogenous cyanogenic glucosides. The corresponding valine (V211) in BGD4 narrows the active site pocket, resulting in the exclusion of non‐flat substrates such as lotaustralin and linamarin, but not of the more planar rhodiocyanosides. Rhodiocyanosides and the <italic>BGD4</italic> gene only occur in <italic>L. japonicus</italic> and a few closely related species associated with the <italic>Lotus corniculatus</italic> clade within the <italic>Lotus</italic> genus. This suggests the evolutionary scenario that substrate specialization for rhodiocyanosides evolved from a promiscuous activity of a progenitor cyanogenic β–glucosidase, resembling BGD2, and required no more than a single amino acid substitution.</p> </abstract>