Department of Botany, Warsaw Agricultural University, Rakovviecka 26130, 02528 Warsaw, Poland

Background and objectives
Predominance of males is a well known effect of plant resistance against infection by plant parasitic nematodes. There are many indications available that sex in the amphimictic species Heterodera schachtii is epigenetically regulated by the amount and composition of nutrients that are provided to the nematode by a specific plant organ called the syncytium. The aim of this study was to determine differences in structure of syncytia induced by male and female juveniles in roots of susceptible and resistant plants.

Materials and methods
Experiments were conducted in vitro on susceptible (Raphanus sativus cv. Siletinaand Sinapis alba cv. Trico) and resistant (R. sativus cvs Pegietta and Nemex, S. alba cv. Maxi) nematodes. Additionally Arabidopsis thaliana was used as a model plant with the possibility of creating culture conditions favouring the development of either females or males [1, 2].

Results and conclusions
Under conditions favouring development of females, the initial syncytial cell (ISC) was always selected among procambial cells, while under conditions favouring development of males the ISCs were selected among procambial and pericyclic cells. However, only syncytia induced in pericycle were able to support the development of male juveniles, while syncytia induced in the procambium necrotized during the next 2 days and the attached juveniles starved. The differences in the type of selected ISC subsequently led to differences in the structure of syncytia of sexually differentiated juveniles. Syncytia associated with males (SAMs) and females (SAFs) were similar at the ultrastructural level [1, 2]. The most obvious changes concerned the size and number of syncytial elements, and cell wall ingrowths and openings [2]. In comparison to SAMs the SAFs were composed of fewer but much stronger enlarged elements. Additionally, syncytial elements were uniformly enlarged over the entire length of syncytia and were connected by wide cell-wall openings. The SAMs close to the nematode head were composed of several strongly enlarged elements derived from pericycle and interconnected by wide cell-wall openings. In the distal parts, the syncytia were created by many weakly hypertrophied elements interconnected by few openings [2]. In the SAF cell, wall ingrowths were formed exclusively at walls abutting xylem vessels [1], while in SAMs they were less developed and formed on different parts of syncytial cell walls [2].

The structure of syncytia induced in other susceptible crucifers was similar to syncytia induced in A. thaliana. In the susceptible cultivars the syncytia usually appeared in the elongation zones of lateral roots. They were initiated in the procambium and pericycle but also included the parenchymatic cells of vascular cylinder. In the susceptible cultivars the cells forming the SAFs were subject to hypertrophy. Their cytoplasmic density increased. The cytoplasm contained numerous organelles, proliferation of the smooth endoplasmic reticulum took place and branched cell-wall ingrowths were formed next to the vessels. In the SAMs the cells were only slightly increased. Their protoplasts contained few organelles and cell-wall ingrowths were poorly developed. In the syncytia induced in resistant cultivars there was only a slight increase of the cell volume. A well developed system of rough endoplasmic reticulum was observed in the protoplast. Distended ER cisterns contained fine fibrillar material and similar material also appeared in numerous small vacuoles. A few syncytia spreading in procambium developed fully and functioned long enough for the parasite females to mature. At an advanced stage of infection a well developed system of a rough ER was observed in those syncytia, and numerous vacuoles appeared. A different mechanism of resistance was observed in cv. Maxi that revealed the hypersensitive reaction.

1. Golinowski W, Grundier FMW, Sobczak M, 1996. Protoplasma 194,103-116.
2. Sobczak M, Golinowski W, Grundier FMW, 1997. European Journal of Plant Pathology 103, 113-124.