Invasion of plant vascular systems by bacteria
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Overview
abstract
Bacteria that cause vascular diseases in plants, including grapes, were discovered to migrate against the transpiration-induced flow of xylem sap using pilus-mediated twitching movements. Colonization of the vascular system by these bacteria was examined in vitro using nanofabricated microfluidic chambers in which the bacteria were discovered to aggregate into cell masses that plugged the channels.
A number of plant pathogenic bacterial species inhabit the water-conducting xylem vessels of plants where they develop biofilms, large aggregates of cells that block xylem sap flow, or produce plant cell degrading enzymes. These actions result in wilting of the plant, decreased yield, and in many instances death of the plant-all of which impact the grower and consumer. Plants affected range from woody plants like grapevine, to vegetables such as tomato and cabbage. How the bacteria spread in and colonize the vascular system is poorly understood; in particular, how they migrate against the direction of sap flow has long been a puzzle and an important problem in explaining spread of the bacteria within many plant hosts. One such bacterium, Xylella fastidiosa, causes economically important plant diseases including Pierce's disease of grapevine. Researchers of Pierce's disease in grapes are keenly interested in how pathogenic bacteria in this plant are spread. Ultimately, growers will benefit from these discoveries that may lead to better control measures in disease regulation.
response
The presence of bacteria in plant vascular systems can be detected and enumerated at most any time, but because the procedures used are destructive the bacterial population can not be examined over time in the same plant. In addition, because they live within the plant vessels they are not easily viewed by microscopy. These shortcomings have been overcome by using nanofabricated microfluidic devices that mimic the xylem vascular system and allows both temporal and spatial analysis of the bacterial population. Using this approach, it was discovered that Xylella fastidiosa bacteria are directed to migrate against a fluid flow using hair-like type IV pili. Such movement in bacteria is known as twitching motility. We further demonstrate that type IV pili-deficient strains of X. fastidiosa do not twitch and are significantly inhibited from colonizing upstream vascular regions in plants. Aggregation of Xylella fastidiosa cells into large masses capable of blocking fluidic passages was shown to be dependent upon the presence of type IV pili and that shorter type I pili yielded only looser lace-like cell aggregates.
impact assessment
The discovery that a plant pathogenic bacterium can migrate against a flow (xylem sap in this case) has lead researchers in this area of research to rethink how such xylem inhabiting bacteria are spread within the host plants. This explains, in part, how such bacteria are capable of moving from the site of entry into the plant e.g., leaves, to distant locations down the stem of trunk. The results of our investigations will impact how investigators study xylem inhabiting bacteria, including Xylella fastidiosa, Clavibacter michiganesis, Agrobacterium vitis and Xanthomonas campestris.
