Citrus sinensis leaf petiole and blade colonization by Xylella fastidiosa: details of xylem vessel occlusion

Abstract

Citrus variegated chlorosis (CVC), caused by Xylella fastidiosa, is an important disease of citrus in Brazil. X. fastidiosa is restricted to xylem vessels of plants and knowledge regarding xylem colonization is still limited. Our goal was to verify how this bacterium colonizes and spreads within xylem vessels of sweet orange Citrus sinensis cv. Pêra. Petioles and pieces of leaf blades from naturally infected plant exhibiting characteristic symptoms were prepared for light microscopy (LM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and immunogold labeling (IGL). Petioles from healthy plants were used as control. IGL results, using an antibody against wall hemicelluloses, revealed that the pit membrane of vessels was altered. Bacterial cells were observed in the pit between adjacent vessels. Results support the contention that X. fastidiosa produces cellulases to reach adjacent vessels. SEM revealed that colonization of sweet orange started with X. fastidiosa cells attaching to the xylem wall, followed by an increase in the number of bacterial cells, the production of fibrous material, and finally vessel occlusion by biofilm composed of copious amounts of amorphous material, strands and cells. Phenolic materials, hyperplasia and hypertrophy were noticed in leaves with gummy material. Xylem vessels frequently contained an unknown needle-like, crystallized matter blocking the vessel. Key words: ultrastructure, scanning electron microscopy, transmission electron microscopy, immunolabel COLONIZAÇÃO DE PECÍOLO E FOLHA DE Citrus sinensis POR Xylella fastidiosa: DETALHES DA OBSTRUÇÃO DE VASOS DO XILEMARESUMO: A clorose variegada dos citrus (CVC), causada por uma bactéria restrita ao xilema (Xylella fastidiosa), é uma importante doença de citros no Brasil, entretanto, pouco se sabe sobre a colonização dos vasos do xilema pela bactéria. O objetivo deste trabalho foi estudar como X. fastidiosa invade os vasos adjacentes do xilema e algumas das alterações expressas por plantas de laranja Pêra. Foram coletadas 15 amostras de pecíolos e áreas das folhas de plantas com sintomas característicos da doença, as quais foram preparadas para microscopia de luz, microscopia eletrônica de varredura (MEV) e transmissão (MET) e imunomarcação de polissacarídeos da parede primária. Pecíolos de folhas sadias foram utilizados como controle. Os resultados da imunomarcação para hemicelulose mostraram alterações na integridade da membrana da pontuação. A bactéria foi localizada nas imediações e no interior das pontuações entre dois vasos. Estas observações suportam a hipótese de que X. fastidiosa produz celulases para alcançar os vasos adjacentes do xilema. Imagens de MEV revelaram que a colonização dos vasos do xilema de laranja doce inicia-se com as células da bactéria aderidas às paredes do xilema, seguida pelo aumento no número de células bacterianas, produção de material extracelular e finalmente a obstrução do vaso. Também, nos tecidos das folhas, verificou-se a deposição de goma, compostos fenólic...

Differential colonization patterns of Xylella fastidiosa infecting citrus genotypes

“…Alves et al . () also verified that in citrus petioles, the predominant secondary cell wall deposition is the pitted type instead of the helical type predominant in grape (Fritschi et al ., ), which reinforces the hypothesis that the slower X. fastidiosa colonization in citrus compared to grape might be related to morphological differences in the xylem between these species.…”

Assessment of the genetic diversity of Xylella fastidiosa in imported ornamental Coffea arabica plants

“…Two petioles from a healthy C. arabica plant were used as control. Transmission electron micrographs of cross‐sectioned leaf petioles indicated the presence of abundant rod‐shaped bacterial cells exhibiting the characteristics of X. fastidiosa cells in the xylem vessel under different magnifications, as described previously (Alves et al ., ; Fig. ).…”

In Situ Detection and Identification of Hesperidin Crystals in Satsuma Mandarin (Citrus unshiu) Peel Cells

“…() and Alves et al . () suggested that the needle‐shaped crystals could respond to infections by Diaporthe citri and Xylella fastidiosa in C. unshiu and C. sinensis , respectively. The crystals, which were identified as hesperidin by Homma et al .…”