The enemy within: phloem‐limited pathogens

Abstract

SUMMARYThe growing impact of phloem-limited pathogens on high-value crops has led to a renewed interest in understanding how they cause disease. Although these pathogens cause substantial crop losses, many are poorly characterized. In this review, we present examples of phloem-limited pathogens that include intracellular bacteria with and without cell walls, and viruses. Phloem-limited pathogens have small genomes and lack many genes required for core metabolic processes, which is, in part, an adaptation to the unique phloem environment. For each pathogen class, we present multiple case studies to highlight aspects of disease caused by phloem-limited pathogens. The pathogens presented include Candidatus Liberibacter asiaticus (citrus greening), Arsenophonus bacteria, Serratia marcescens (cucurbit yellow vine disease), Candidatus Phytoplasma asteris (Aster Yellows Witches' Broom), Spiroplasma kunkelii, Potato leafroll virus and Citrus tristeza virus. We focus on commonalities in the virulence strategies of these pathogens, and aim to stimulate new discussions in the hope that widely applicable disease management strategies can be found.

Antimicrobial Activity of Metabolites Secreted by the Endophytic Bacterium Frateuria defendens

“…Current management approaches of phytoplasma diseases rely mainly on limiting the population of the insect vector by chemical pesticide application. However, in many cases such as Bois Noir disease, the polyphagous nature of the vector dramatically reduces the efficiency of this approach [5,7]. Another effective treatment is the liquid trunk injection of antibiotics (mainly oxytetracycline), but the usage of antibiotics as a pest management strategy is expensive and prohibited in most countries [8].…”

“…Since phytoplasma cannot be grown in an axenic culture, a nurse culture system infected with this bacterium is essential for assessing various means of control strategies in planta [7]. Periwinkle (Catharanthus roseus; Gentianales, Apocynaceae) can serve as a model nurse culture system for a wide variety of phytoplasma types that cause disease in multiple host plants.…”

Rapid Somatic Embryogenesis of Citrus reticulata Blanco cv. Madu in an Air-Lift Bioreactor Culture

“…Madu is one of the most popular citrus mandarin species in South-East Asia. However, the production is limited due to the common citrus disease, greening, caused by Liberobacter (Bendix & Lewis, 2018). Hence, vegetative propagation of infected plant using traditional method is strongly avoided to reduce the spread of the disease (Siverio et al, 2017).…”

Identification of sieve elements and companion cell protoplasts by a combination of brightfield and fluorescence microscopy

“…Phloem‐limited diseases are becoming increasingly pervasive due to the rapid globalization of agricultural systems and to global warming (Bendix and Lewis, ). These diseases have resulted in massive crop losses and economic hardships, as evidenced by the virtual collapse of the citrus industry in the state of Florida (Albrecht and Bowman, ; Folimonova et al., ; Gottwald, ) and in Brazil (Coletta‐Filho et al., ; Texeira et al., ).…”

“…Studies of phloem diseases (e.g., cucurbit yellow vine disease, zebra chip disease in potato, corn stunt disease, grapevine yellows disease, and onion yellow dwarf disease; Bendix and Lewis, ) are hindered by the almost inaccessible location of the phloem tissue. Phloem cells are commonly found buried inside the plant body and interspersed with storage parenchyma cells (Knoblauch and Oparka, ).…”