Modified Monosaccharides Content of Xanthan Gum Impairs Citrus Canker Disease by Affecting the Epiphytic Lifestyle of Xanthomonas citri subsp. citri.

Simone Cristina Picchi,Maria Júlia Festa Franzini,Laís Moreira Granato,Maxuel Oliveira Andrade,Marcos Antonio Machado,Alessandra Alves De Souza,Marco Aurélio Takita

doi:10.3390/microorganisms9061176

Abstract

Xanthomonas citri subsp. citri (X. citri) is a plant pathogenic bacterium causing citrus canker disease. The xanA gene encodes a phosphoglucomutase/phosphomannomutase protein that is a key enzyme required for the synthesis of lipopolysaccharides and exopolysaccharides in Xanthomonads. In this work, firstly we isolated a xanA transposon mutant (xanA::Tn5) and analyzed its phenotypes as biofilm formation, xanthan gum production, and pathogenesis on the sweet orange host. Moreover, to confirm the xanA role in the impaired phenotypes we further produced a non-polar deletion mutant (ΔxanA) and performed the complementation of both xanA mutants. In addition, we analyzed the percentages of the xanthan gum monosaccharides produced by X. citri wild-type and xanA mutant. The mutant strain had higher ratios of mannose, galactose, and xylose and lower ratios of rhamnose, glucuronic acid, and glucose than the wild-type strain. Such changes in the saccharide composition led to the reduction of xanthan yield in the xanA deficient strain, affecting also other important features in X. citri, such as biofilm formation and sliding motility. Moreover, we showed that xanA::Tn5 caused no symptoms on host leaves after spraying, a method that mimetics the natural infection condition. These results suggest that xanA plays an important role in the epiphytical stage on the leaves that is essential for the successful interaction with the host, including adaptive advantage for bacterial X. citri survival and host invasion, which culminates in pathogenicity.

Highlights

Xanthomonas citri subsp. citri (X. citri) is a Gram-negative plant pathogenic bacterium causing citrus canker, one of the most economically damaging diseases that affects almost all commercial citrus varieties worldwide [1,2]
We showed changes in the xanthan gum monosaccharide content that were associated with the xanA function in Xanthomonas
Disruption of xanA in X. citri genome, which codes for enzyme XanA that is necessary for isomerization of glucose-6-phosphate to glucose-1-phosphate and conversion of mannose-6- phosphate to mannose-1-phosphate [12,26,27], led to impairment of xanthan gum production

Summary

Introduction

Xanthomonas citri subsp. citri (X. citri) is a Gram-negative plant pathogenic bacterium causing citrus canker, one of the most economically damaging diseases that affects almost all commercial citrus varieties worldwide [1,2]. X. citri invades the host through natural openings, such as stomata and wounds [2], and many studies have shown that biofilms formed on the leaf surface and bacterial motility are important features in the early stages of infection [3,4]. Mutants of X. citri impaired in biofilm formation, and/ or motility results in the decrease of citrus canker symptoms [3,4,5,6,7,8,9]. Either biofilm or sliding motility is affected by X. citri ability to produce exopolysaccharides (EPS) [4]. EPS production in X. citri interferes directly with citrus canker disease development. The main EPS produced by Xanthomonads is the xanthan, which is a polymer of repeating pentasaccharide units with the mannose-(β-1,4)- glucuronic acid-(β-1,2)-mannose(α-1,3)-cellobiose structure [10,11].

Methods

Results

Conclusion