Abstract
Sedentary plant parasitic nematodes such as root-knot nematodes and cyst nematodes induce giant cells or syncytia, respectively, in their host plant's roots. These highly specialized structures serve as feeding sites from which exclusively the nematodes withdraw nutrients. While giant cells are symplastically isolated and obtain assimilates by transporter-mediated processes syncytia are massively connected to the phloem by plasmodesmata. To support the feeding sites and the nematode during their development, phloem is induced around syncytia and giant cells. In the case of syncytia the unloading phloem consists of sieve elements and companion cells and in the case of root knots it consists exclusively of sieve elements. We applied immunohistochemistry to identify the cells within the developing phloem that responded to auxin and cytokinin. Both feeding sites themselves did not respond to either hormone. We were able to show that in root knots an auxin response precedes the differentiation of these auxin responsive cells into phloem elements. This process appears to be independent of B-type Arabidopsis response regulators. Using additional markers for tissue identity we provide evidence that around giant cells protophloem is formed and proliferates dramatically. In contrast, the phloem around syncytia responded to both hormones. The presence of companion cells as well as hormone-responsive sieve elements suggests that metaphloem development occurs. The implication of auxin and cytokinin in the further development of the metaphloem is discussed.
Highlights
Nematodes are destructive pathogens that inflict considerable damage to a wide variety of plants (Sasser and Freckman, 1987)
We wanted to investigate the effect of auxin and cytokinin on phloem development in the two different feeding sites, root knots and syncytia induced by root knot nematodes (RKN) or cyst nematodes (CN), respectively
Based on the characteristic position of the auxin responsive cells with respect to the sieve elements and the xylem, cells with wide lumens and thick walls, we identified these cells as companion cells
Summary
Nematodes are destructive pathogens that inflict considerable damage to a wide variety of plants (Sasser and Freckman, 1987). Among the plant-parasitic nematodes are migratory species and sedentary nematodes. The latter invade a host plant’s root and induce specialized feeding sites, from which exclusively nutrients are withdrawn. While the initial events have received much attention, the later stages of a successful interaction between sedentary nematodes and plants are only poorly understood. The male nematodes leave the feeding sites and CN males will proceed to fertilize females whereas the offspring of RKN is formed by parthenogenesis (Liu et al, 2007). The eggs of RKN are disposed in a gelatinous egg-mass and in case of cyst nematodes the eggs are embedded in the eponymous cyst, which is formed by the dead female and represents a durable stage that can survive in the soil for years
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