The ER Lumen

The desmotubule is now accepted to be a tightly furled membrane, running through the plasmodesma, in many cases appearing as an electron-dense rod (Lopez-Saez et al. 1966; Gunning and Hughes 1976; Overall et al. 1982; Olesen 1979; Gunning and Overall 1983; Robinson-Beers and Evert 1991; Ding et al. 1992b; Waigmann et al. 1997). There is a layer of proteinaceous material either integral to, or closely associated with, the desmotubule. Ding and co-workers (1992b) have shown that the wall of the desmotubule is com posed of up to ten electron-dense particles about 3 nm in diameter, embedded in the desmotubule membrane, possibly in a helical arrangement with a pitch of 20-30°. In contrast, Overall and Blackman (1996) suggest that the proteinaceous material appears as negatively stained, electron-lucent particles, these being interpreted by Ding and co-workers (1992b) as spaces between the electron-dense particles. In transverse views, at the centre of the desmotubule is an electron-dense rod of about 3 nm in diameter (Ding et al. 1992b) (Fig. 2a). Fine filamentous structures 1-1.5 nm in diameter may extend between this central rod and the desmotubule wall. In material treated without tannic acid or uranyl acetate/osmium tetroxide, most of the desmotubule particles, including the central rod particles, are absent (Ding et al. 1992b). These central rod particles, along with the proteins embedded in the outer membrane of the desmotubule cylinder, are believed to occlude the desmotubule lumen (Overall et al. 1982). However, this may be due to the fixation protocol employed, since other workers have estimated that the space between the central particles is between 2.5 and 3 nm (Botha et al. 1993).

The presence or absence of a desmotubule lumen may be tissue specific. In certain trichomes, instead of the desmotubule appearing as an electron-dense solid rod, the desmotubule may have an "open" electron-lucent lumen (Eleftheriou and Hall 1983; Waigmann et al. 1997).

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