A survey of FLS2 genes from multiple citrus species identifies candidates for enhancing disease resistance to Xanthomonas citri ssp. citri.

Qingchun Shi,Vicente J Febres,Jeffrey B Jones,Gloria A Moore

doi:10.1038/hortres.2016.22

Abstract

Pathogen-associated molecular patterns (PAMPs)-triggered immunity (PTI) is an important component of plant innate immunity. In a previous study, we showed that the PAMP flg22 from Xanthomonas citri ssp. citri (Xflg22), the causal agent of citrus canker, induced PTI in citrus, which correlated with the observed levels of canker resistance. Here, we identified and sequenced two bacterial flagellin/flg22 receptors (FLS2-1 and FLS2-2) from ‘Duncan’ grapefruit (Citrus paradisi, CpFLS2-1 and CpFLS2-2) and ‘Sun Chu Sha’ mandarin (C. reticulata, CrFLS2-1 and CrFLS2-2). We were able to isolate only one FLS2 from ‘Nagami’ kumquat (Fortunella margarita, FmFLS2-1) and gene flanking sequences suggest a rearrangement event that resulted in the deletion of FLS2-2 from the genome. Phylogenetic analysis, gene structure and presence of critical amino acid domains all indicate we identified the true FLS2 genes in citrus. FLS2-2 was more transcriptionally responsive to Xflg22 than FLS2-1, with induced expression levels higher in canker-resistant citrus than in susceptible ones. Interestingly, ‘Nagami’ kumquat showed the highest FLS2-1 steady-state expression levels, although it was not induced by Xflg22. We selected FmFLS2-1, CrFLS2-2 and CpFLS2-2 to further evaluate their capacity to enhance bacterial resistance using Agrobacterium-mediated transient expression assays. Both FmFLS2-1 and CrFLS2-2, the two proteins from canker-resistant species, conferred stronger Xflg22 responses and reduced canker symptoms in leaves of the susceptible grapefruit genotype. These two citrus genes will be useful resources to enhance PTI and achieve resistance against canker and possibly other bacterial pathogens in susceptible citrus types.

Highlights

Citrus is a worldwide fruit crop with high value for both fresh and processed fruit markets
The results showed that in the canker-resistant ‘Sun Chu Sha’ mandarin, CrFLS2-1 was significantly induced at 6 h and CrFLS2-2 was induced at 6 and 24 h by the Xflg[22] treatment; the induction level of
Phylogenetic analysis untreated leaves suggests this citrus species maintains high showed that, among 26 citrus proteins found through BLAST FmFLS2-1 protein abundance which allows it to achieve a quick search homologous to AtFLS2, only FLS2-1 and FLS2-2 were and strong Xflg[22] response

Summary

Introduction

Citrus is a worldwide fruit crop with high value for both fresh and processed fruit markets. Plants have two types of innate immunity inducible by pathogens.[5] One is through the perception of pathogen-associated molecular patterns (PAMPs) mediated by the host’s pattern recognition receptors (PRRs), which trigger a series of defense responses including an oxidative burst,[6] callose deposition,[7] cascade induction of mitogen-activated protein kinases (MAPK)[8] and induction of defense-associated genes.[9] The consequences of these responses are stalled pathogen multiplication and disease development (PAMP-triggered immunity or PTI).[10] The other type of immunity is achieved by recognition of specific pathogen effectors by the plant’s resistance proteins, resulting in a hypersensitive response and disease resistance to the pathogen relying on the specific effector for virulence (Effector-triggered immunity).[11]

Methods

Results

Conclusion