Plant-parasitic nematodes are major economic pests worldwide. Among them, the endoparasitic sedentary phytonematodes of genera Heterodera or Globodera (known as cyst nematodes, CN) and Meloidogyne (the root-knot nematodes, RKN) cause the most important crop losses. CN and RKN larvae (or juveniles) are soil dwelling worms who need to complete their life cycle on plant roots.
These biotrophic parasites interact with their hosts in a remarkable manner. They infect and colonize plant root systems using a sophisticated stylet structure, a hollow retractable needle connected to the esophagus and three unicellular esophageal glands (Fig1). This structure is used to pierce and penetrate plant cell walls, to release esophageal secretions, into the host tissue (in the inter-cellular space but also directly in the plant cells) and to take up nutrients from the plant cells. Remarkably, they induce the redifferentiation of roots cells into specialized hypertrophied and multinucleate feeding cells essential for nematode growth and reproduction. Depending on the nematode species, the initial feeding cell develops into either a syncytium (for CN) or a system of giant cells (for RKN). Syncytia result from cell fusions after cell wall dissolutions between the initial cell on which the nematode starts feeding and an increasing number of neighboring cells. Up to 200 cells can be incorporated in a large syncytium. Conversely, giant cell formation is the result of repeated nuclear divisions of the initial feeding cell without complete cytokinesis (Fig 1). Concomitantly to giant cell ontogenesis, hyperplasia and hypertrophy of the surrounding cells lead to the formation of the typical root gall, the primary visible symptom of infection (Fig1). These feeding cells function as specialized sinks, supplying nutrients to the nematode until reproduction.
Figure 1. Plant-RKN interaction.
a. Gall symptom on tomato root; b. Proteins secreted via the stylet by second stage juveniles visualized with Coomassie staining; c. Arabidopsis hypertrophied giant cells induced by M. incognita
It is not yet understood how feeding cells are induced, but the effector proteins secreted by the nematode play key roles during invasion and migration and have effects on recipient host cells. Secretions containing these proteins could potentially originate from different organs, but most work has focused on stylet secretions coming from oesophageal glands. A set of genes encoding secreted enzymes, in particular pectocellulolytic enzymes, has been first identified in the salivary secretions of these nematode. Differential gene expression and cDNA library analysis on oesophageal glands have been used to identify potential parasitism genes (Huang et al., 2003) and direct analysis of M. incognita secretions via a proteomic approach has allowed the identification of nearly 500 secreted proteins (Bellafiore et al., 2008). Knowledge of M. hapla and Meloidogyne incognita genome sequences (Abad et al., 2008; Opperman et al., 2008) provide new insights into nematode adaptation to parasitize plants, and open the way for understanding parasitism genes and discovering new antiparasitic strategies.
You can find here a non-exhaustive list of plant parasitic nematode effectors related publications.