Membrane infection thread

The process of infection of lupine nodule cells by Rhizobia was examined by the thin-section electron microscopic technique, as well as the freeze-fracture technique. Different membranes such as infection thread membranes, peribacterioid membranes, plasma membranes, membranes of cytoplasmic vesicles, and membranes of the Golgi bodies and ER were stained with uranium-lead, silver, phosphotungstic acid, and ZIO (31). ZIO stained the membranes of the proximal face of the Golgi bodies and endoplasmic reticulum. ZIO staining has given good contrast in thick sections such as a cotyledon cell, a root cell, and an aleurone layer for ER, dictyosomes cisternae, mitochondria, and nuclear envelopes (17,32-37). 

Robertson JG, Lyttleton P, Bullivant S, Grayston GF. Membranes in lupine root nodules. I. The role of Golgi bodies in the biogenesis of infection threads and peri-bacteroid membranes. J Cell Sci 1978 30 129-149. 

Successful infection proceeds simultaneously with nodule morphogenesis triggered by the signal compounds produced by Rhizobium in response to its host, and leads to the release of bacteria from the infection threads into the cortical cells. Concomitantly, the bacteria are enveloped in a host-derived plasma membrane called the peribacteroid membrane (PBM) (Verma et al., 1978 Robertson et al., 1978). Both host- and bacterium-derived proteins are specifically targeted to this membrane (Fortin et al., 1985, 1987 Katinakis Verma, 1985 Katinakis et al., 1988), making it a unique subcellular compartment. 

Fig. 1. Proposed process by which an infection thread passes through a cell wall between two plant cells, in this case between the root hair cell and an adjacent cortical cell. (1) The membrane at the tip of the infection thread has fused with the root hair cell plasma membrane to form a pore. Tonoplast membrane, TM infection thread membrane ITM rhizobia, R infection thread wall, ITW root hair cell cytoplasm, RHCC root hair cell plasma membrane, RHCP plant cell wall, PCW cortex cell plasma membrane, CCP cortex cell cytoplasm, CCC. The arrows and vesicles represent intense activity of the endomembrane system involved in the synthesis of membranes and wall material. (2) The rhizobia have forced their way, by dividing, into the region between the two cells although they are still enclosed by wall material. (3) An intercellular infection thread is formed by degradation and resyntheses of cell wall material in the vicinity of the dividing rhizobia. (4) The infection thread has entered the cortical cell by promoting invagination and growth of the plasma membrane and wall of that cell, in front of the dividing bacteria.

Fig. 1. Infection and early development of root nodules. (A) Infection thread (IT) and curled root hair (CRH) 48 hr after inoculation. (B) Nodule meristem differentiation in root cortical parenchyma prior to infection thread proliferation. Arrowhead points to mitotic figure (M). (C) Bacterial (Ba) release from infection thread into host cell cytoplasm. Bacteria are surrounded by a polysaccharide matrix (P) within the infection thread and immediately upon release. (D) After release from the infection thread bacteria (Ba) are enclosed within the peribacteroid membrane (PBM) and the polysaccharide matrix (P) formerly surrounding the bacteria is degraded. Bars indicate approximate sizes. 

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