Cell Wall Ingrowths in Nematode Induced Syncytia Require <em>UGD2</em> and <em>UGD3</em>

<div><p>The cyst nematode <em>Heterodera schachtii</em> infects roots of Arabidopsis plants and establishes feeding sites called syncytia, which are the only nutrient source for nematodes. Development of syncytia is accompanied by changes in cell wall structures including the development of cell wall ingrowths. UDP-glucuronic acid is a precursor of several cell wall polysaccharides and can be produced by UDP-glucose dehydrogenase through oxidation of UDP-glucose. Four genes in Arabidopsis encode this enzyme. Promoter::GUS analysis revealed that <em>UGD2</em> and <em>UGD3</em> were expressed in syncytia as early as 1 dpi while expression of <em>UGD1</em> and <em>UGD4</em> could only be detected starting at 2 dpi. Infection assays showed no differences between Δ<em>ugd1</em> and Δ<em>ugd4</em> single mutants and wild type plants concerning numbers of males and females and the size of syncytia and cysts. On single mutants of Δ<em>ugd2</em> and Δ<em>ugd3</em>, however, less and smaller females, and smaller syncytia formed compared to wild type plants. The double mutant ΔΔ<em>ugd23</em> had a stronger effect than the single mutants. These data indicate that <em>UGD2</em> and <em>UGD3</em> but not <em>UGD1</em> and <em>UGD4</em> are important for syncytium development. We therefore studied the ultrastructure of syncytia in the ΔΔ<em>ugd23</em> double mutant. Syncytia contained an electron translucent cytoplasm with degenerated cellular organelles and numerous small vacuoles instead of the dense cytoplasm as in syncytia developing in wild type roots. Typical cell wall ingrowths were missing in the ΔΔ<em>ugd23</em> double mutant. Therefore we conclude that <em>UGD2</em> and <em>UGD3</em> are needed for the production of cell wall ingrowths in syncytia and that their lack leads to a reduced host suitability for <em>H. schachtii</em> resulting in smaller syncytia, lower number of developing nematodes, and smaller females.</p>