Anatomical and biochemical studies of Spartium junceum infected by Xylella fastidiosa subsp. multiplex ST 87

S Falsini,A Gnerucci,P Faraoni,F Peduto Hand,A Papini,S Campigli,I Scarpelli,G Sambuco,M Ricciolini,L Ghelardini,G Bleve,S Schiff,C Tani,G Marchi,L Drosera,D Rizzo

doi:10.1007/s00709-021-01640-2

S Falsini, A Gnerucci + Show 14 more

Open Access

https://doi.org/10.1007/s00709-021-01640-2

Copy DOI

Abstract

Spartium junceum L. is a typical species of Mediterranean shrubland areas, also grown in gardens and parks as an ornamental. In recent years in Europe, S. junceum has been recurrently found to be infected by different subspecies and genotypes of the quarantine regulated bacterium Xylella fastidiosa (Xf). This work presents for the first time the anatomy of S. junceum plants that we found, by means of genetic and immunochemistry analysis, to be naturally infected by Xf subsp. multiplex ST87 (XfmST87) in Monte Argentario (Grosseto, Tuscany, Italy), a new outbreak area within the EU. Our anatomical observations showed that bacteria colonized exclusively the xylem conductive elements and moved horizontally to adjacent vessels through pits. Interestingly, a pink/violet matrix was observed with Toluidine blue staining in infected conduits indicating a high content of acidic polysaccharides. In particular, when this pink-staining matrix was observed, bacterial cells were either absent or degenerated, suggesting that the matrix was produced by the host plant as a defense response against bacterial spread. In addition, a blue-staining phenolic material was found in the vessels and, at high concentration, in the pits and inter-vessels. SEM micrographs confirmed that polysaccharide and phenolic components showed different structures, which appear to be related to two different morphologies: fibrillary and granular, respectively. Moreover, our LM observations revealed bacterial infection in xylem conductive elements of green shoots and leaves only, and not in those of other plant organs such as roots and flowers.

Highlights

Xylella fastidiosa (Wells et al 1987) (Xf) is a Gram-negative bacterium, which presents six subspecies: fastidiosa, multiplex, sandyi, morus, tashke, and pauca (EFSA 2020)
We investigated the interaction of X. fastidiosa subsp. multiplex (Xfm) ST87 with S. junceum tissues and the mechanism of bacterial spread through the plant xylem vessels until the formation of vascular plugs
The results presented in this paper indicate that a single phylotype of Xf subsp. multiplex (Xfm), i.e., ST87, is infecting S. junceum plants growing in the demarcated area of Monte Argentario (Grosseto, Tuscany, Italy)

Summary

Introduction

Xylella fastidiosa (Wells et al 1987) (Xf) is a Gram-negative bacterium, which presents six subspecies: fastidiosa, multiplex, sandyi, morus, tashke, and pauca (EFSA 2020) It is the causal agent of Pierce’s disease in grapevine, the olive quick decline syndrome in olive, the citrus variegated chlorosis in citrus plants, and leaf scorch disease in almond, coffee, oleander, elm, sycamore, pecan, pear, mulberry, maple, and oak (Hopkins and Purcell 2002; Janse and Obradovic 2010; Purcell 2013). The similarity between the two exclusive niches of Xf (i.e., host plant and insect vector) is that in both cases, the colonized tissues are composed of non-living cells with which the bacterium mainly interacts during its life cycle. This phenomenon is probably due to the Xf genome adaptation. T3SS are complex bacterial structures that are delivered in living tissues to suppress the immune response of the host; the lack of this system may explain why Xf survives only when it is surrounded by dead cells (Roper et al 2019)

Methods

Results

Conclusion