Fastidious Prokaryotes and Plant Health

Abstract

The prokaryotes are almost everywhere or we can phrase like this “prokaryotes are wherever there is life”. They were the earliest organisms on earth. Today, they still dominant the biosphere for the following two facts: 1) their collective biomass outweighs all eukaryotes combined at least tenfold, and 2) more prokaryotes inhabit a handful of fertile soil or the mouth or skin of a human than the total number of people who have ever lived. They thrive in habitats that are too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote because they display diverse adaptations that allow them to inhabit many environments and they have great genetic diversity. Phytopathogenic fastidious prokaryotes are plant pathogens that either resist to grow in any available bacterial culture media or require specific or enriched media to grow. They include Xylella fastidiosa, Leifsonia xyli subsp. xyli, L. xyli subsp. cynodontis and Clavibacter michiganensis subsp. sepedonicus and C. michiganensis subsp. michiganensis that reside in xylem and spiroplasmas, phytoplasmas and Candidatus Liberibacter spp. that reside in phloem. X. fastidiosa is the causal agent of more than 19 diseases; among them Pierce’s disease of grape and citrus variegated chlorosis are two major maladies that cause serious economic loss on wine and citrus juice industry. L. xyli subsp. xyli, and L. xyli subsp. cynodontis are associated with ratoon stunting disease of sugarcane and Bermuda grass stunting respectively and C. michiganensis subsp. sepedonicus with bacterial ring rot in potato and C. michiganensis subsp. michiganensis with bacterial tomato canker. Spiroplasmas are the causal agents of citrus stubborn, corn stunt and periwinkle diseases. Phytoplasmas are associated with more than 500 diseases worldwide. Ca. Liberibacter spp., are the causal agents of citrus Huanglongbing or citrus greening, zebra chip disease of potato and others. Pierce’s disease is the limiting factor for the establishment of wine industry for the entire southeastern United States from Texas to the Carolinas along the gulf coast of Mexico. Recent introduction of the glassy-winged sharpshooter leafhoppers in California has threatened the winery industry of California. The successful isolation of X. fastidiosa from the tissues with citrus variegated chlorosis (CVC) symptoms followed by the identification of the major insect vectors provided crucial information for citrus growers and citrus juice industry to deal with the CVC crisis in Brazil. The successful isolation of X. fastidiosa from blueberry tissues with leaf scorch symptoms followed by the identification of the susceptibility/resistance of various blueberry cultivars provided significant information for the blueberry industry which has recently become the number one fruit commodity in Georgia. The biological characteristics of the three phloem-limited prokaryotes, namely spiroplasmas, phytoplasmas and Ca. Liberibacter spp., and the diseases they induce and their vectors will be discussed. Most plant pathogenic prokaryotes do not require an active insect vector to spread them from plants to plants, while X. fastidiosa, Ralstonia syzygii, Ca. Liberibacter spp., phytoplasmas, and spiroplasmas do. To date among all known vectors, the single most successful insects vectoring the diseases belong to the Order of Hemiptera.