The structure of syncytia induced within galls in tomato roots by the false root-knot nematode Nacobbus aberrans has been examined by light and electron microscopy. A syncytium develops by breakdown or individual cell walls, which allows movement of cytoplasmic contents between transformed cells. The wall breakdown takes place at pit fields, where the plasmodesmata may be protected from digestion until the surrounding wall is removed. Numerous sieve elements differentiate in the cells outside the syncytium. These sieve elements, and also plasmodesmata in pit fields, are demonstrated by fluorescence microscopy. The possibility of a symplastic pathway of solute movement from the phloem to the syncytium is suggested. A massive accumulation of starch occurs in the gall cells and syncytial cells, which may be related to the proliferation of phloem. Wall ingrowths typical of transfer cells are absent, and a comparative survey of the structure and mode of solute entry into nematode-transformed cells in which ingrowths are present or absent is presented.

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