Root-knot nematodes induce plants to form new vascular structures

Bartlem, et al., recently published "Vascularization and nutrient delivery at root-knot nematode feeding sites in host roots," which reviews the state of knowledge of an important plant-animal interaction and how the animal penetrates the root, attaches itself to vasculature precursor cells, and then feeds off the plant's photosynthates (sugar) until completing its life cycle by forming eggs. The plant vascular system is composed of xylem and phloem vessels, or tubes. Xylem can be thought of as tubes taking soil resources from roots to the shoot of the plant, whereas phloem takes sugar produced in the shoot down to the roots. Study of the changes induced to the vasculature itself by nematodes has lagged behind other aspects of this cycle.

The process of root-knot nematode parasitism, from Bartlem, et al., 2013.

Nematodes look like worms, though they form their own phylum at the same levels as arthropods, chordates, and mollusks, and separate from the phylum of annelids that contains the common earthworm. They may be more closely related to insects than earthworms due to the fact that they moult as they grow. Nematodes include hookworms than can parasitize mammals, like people. Root-knot nematodes live in the soil and hatch from eggs, then navigate to roots. They attach themselves at the base of the plants vascular system near the root tip and give up their mobile lifestyle as they feed off the plant and swell. The cells that the nematode (N) attaches to become giant cells (GC) through a complex process.
The changes to xylum (Xy) and phloem (Ph) are the focus of this article. The vessels form cages around the giant cells to which the nematode is attached. Image from Bartlem, et al., 2013.

The phloem (Ph) and xylem (Xy) form cages around the giant cells with many connections that allow the transfer of nutrients and water into the giant cells. The nematodes, in turn, are connected via their mouth parts to the giant cells so have a ready supply of food and water. Eventually these structures swell and are visible by the naked eye as galls on the roots. The lucky nematode feeds till laying eggs which will continue the life cycle of the respective species.

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