Xylella fastidiosa

Xylella fastidiosa is an aerobic, Gram-negative bacterium of the monotypic genus Xylella. It is a plant pathogen, and is transmitted exclusively by xylem fluid feeding sap insects. Many plant diseases are due to symptomatic infections of X. fastidiosa, including bacterial leaf scorch, oleander leaf scorch, coffee leaf scorch (CLS), alfalfa dwarf, phony peach disease, and the economically important Pierce's disease of grapes (PD) and citrus variegated chlorosis (CVC). In Europe it has attacked olive trees in the Salento area of Southern Italy causing the olive quick decline syndrome (OQDS). While distributions of X. fastidiosa–related diseases are mostly limited to the Americas, outbreaks have occurred in Taiwan, Slovakia, and other countries worldwide. Many plants are asymptomatic carriers of the bacteria, which can contribute to its spread. Pathogenicity of the bacterium occurs only when a large proportion of xylem vessels in a plant are colonized; often, the colonies of bacteria themselves are not large enough to completely block the vessels, and the mechanism of pathogenesis is largely unknown. A subspecies of X. fastidiosa responsible for citrus variegated chlorosis was the first non-viral plant pathogen to have its genome sequenced, in part because of its potential to devastate affected crops. Xylella fastidiosa is rod-shaped, and at least one subspecies has two types of pili on only one pole; longer type IV pili are used for locomotion while shorter type I pili assists in biofilm formation inside its hosts. As demonstrated using a PD-related strain, the bacterium has a characteristic twitching motion that enables groups of bacteria to travel upstream against heavy flow, such as that found in xylem vessels. It is obligately insect-vector transmitted from xylem-feeding insects directly into xylem, but infected plant material for vegetative propagation (e.g. grafting) can produce mature plants that also have a X. fastidiosa disease. In the wild, infections tend to occur during warmer seasons, when insect vector populations peak. The bacterium is not seed transmitted and has been historically difficult to culture. Xylella fastidiosa can be divided into four subspecies that affect different plants and have separate origins. X. fastidiosa subsp. fastidiosa is the most studied subspecies, as it is the causal agent of PD; it is thought to have originated in southern Central America and also affects other kinds of plants. X. fastidiosa subsp. multiplex affects many trees, including stone-fruit ones such as peaches and plums and is thought to originate in temperate and southern North America. Xylella fastidiosa subsp. pauca is believed to have originated in South America. It is the causal agent of CVC in Brazil and also affects South American coffee crops, causing coffee leaf scorch. Xylella fastidiosa subsp. sandyi is thought to have originated in the southern part of the United States of America, and is notable for causing oleander leaf scorch. X. fastidiosa has a two-part life cycle which occurs inside an insect vector and inside a susceptible plant. While the bacterium has been found across the globe, only once the bacterium reaches systemic levels do symptoms present themselves. Within susceptible plant hosts, X. fastidiosa forms a biofilm-like layer within xylem cells and tracheary elements that can completely block the water transport in affected vessels. There is significant variation in symptoms between diseases, though some symptoms are expressed across species. On a macroscopic scale, plants infected with a X. fastidiosa-related disease exhibit symptoms of water deficiencies, manifesting as leaf scorching and stunting in leaves, fruit, and overall plant height. As the bacterium progressively colonizes xylem tissues, affected plants often block off their xylem which can limit the spread of this pathogen; blocking can occur in the form of polysaccharide rich gels, tyloses, or both. These plant defenses do not seem to hinder the movement of X. fastidiosa. Occlusion of vascular tissue, while a normal plant response to infection, makes symptoms significantly worse: as the bacterium itself also reduces vascular function, a 90% reduction of vascular hydraulic function was seen in susceptible Vitis vinifera. This bacterium rarely completely blocks vascular tissue. Smaller colonies usually occur throughout a high proportion of xylem vessels of a symptomatic plant. Severe PD symptoms include shriveled fruit, leaf scorching, and premature abscission of leaves, with bare petioles remaining on stems.