Low pyrrolizidine alkaloid levels in perennial ryegrass is associated with the absence of a homospermidine synthase gene

Geoffrey P Gill,Mingshu Cao,Jan Huege,Catherine J Bryant,Karl Fraser,Mikhail Fokin,Chris Jones,Marty J Faville

doi:10.1186/s12870-018-1269-6

Abstract

BackgroundPyrrolizidine alkaloids (PAs) are a class of secondary metabolites that function as feeding deterrents in a range of different plant species. In perennial ryegrass (Lolium perenne L.) the only PAs that have been identified are the thesinine-rhamnoside group, which displays significant genetic variation. Homospermidine synthase (HSS) has evolved from deoxyhypusine synthase (DHS) and catalyses the first step in the PA pathway, making it a key candidate for the investigation of genes influencing observed PA trait variation.ResultsDuring PCR amplification and sequence analysis of DHS we identified two putative HSS genes in perennial ryegrass. One of the genes (LpHSS1) was absent in some perennial ryegrass plants. Thesinine-rhamnoside levels were measured using liquid chromatography coupled with mass spectrometry in a diverse association mapping population, consisting of 693 plants free of fungal endophytic symbionts. Association tests that accounted for population structure identified a significant association of absence of the LpHSS1 gene with lower levels of thesinine-rhamnoside PAs. HSS-like gene sequences were identified for other grass species of the Poaceae, including tall fescue, wheat, maize and sorghum.ConclusionHSS is situated at the crucial first step in the PA pathway making it an important candidate gene for investigation of involvement in PA phenotypic variation. In this study, PA level in perennial ryegrass was strongly associated with the presence or absence of the LpHSS1 gene. A genetic marker, developed for the presence/absence of LpHSS1, may be used for marker-assisted breeding to either lower or increase PAs in breeding populations of perennial or Italian ryegrass to investigate a potential role in the deterrence of herbivore pests. The presence of HSS-like genes in several other Poaceae species suggests that PA biosynthesis may occur in plant family members beyond perennial ryegrass and tall fescue and identifies a potential route for manipulating PA levels.

Highlights

Pyrrolizidine alkaloids (PAs) are a class of secondary metabolites that function as feeding deterrents in a range of different plant species
In this study we have identified a Homospermidine synthase (HSS) gene that is absent in a diverse range of perennial and Italian ryegrass populations
The absence of the gene is strongly associated with reduced levels of the thesinine-rhamnoside group of PAs in perennial ryegrass

Summary

Introduction

Pyrrolizidine alkaloids (PAs) are a class of secondary metabolites that function as feeding deterrents in a range of different plant species. During the analysis of perennial ryegrass (Lolium perenne L.) metabolites, several abundant novel alkaloids were identified as ions using direct infusion mass spectrometry [13] and later characterised as the PAs E-thesinine-O-4′-αrhamnoside, Z-thesinine-O-4′-α-rhamnoside and their glycoside derivatives [14]. This was the first time plantderived PAs were identified in a grass from the Poaceae family. Whether these thesinine-rhamnosides play a role in deterring herbivores or have an alternative function is currently unknown

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