Pit connection

In Plant anatomy, a pit connection is a hole in the septum between two algal cells, and is found only in the red algae^[1] - specifically, all orders except the Porphyridiales and haploid Bangiales.^[2] They are often stoppered with "pit plugs". They are in fact neither pits, nor connections, yet the term remains in use due to its longevity.^[2] By contrast, many fungi (only ascomycetes and basidomycetes, as other groups lack septa) contain septal pores - an unrelated phenomenon.^[3]

Characteristics

A sieve-like membrane may cover the pit in living algae,^[4] but in the majority of algae a plug forms, limiting the transfer of metabolites between neighbouring cells.^[5][2]

Pit connections come in many varieties: the micropores of the zygomycetes; the simple, Woronin body-bearing pores of the ascomycetes, and the dolipores of the holobasidiomycetes and phragmobasidiomycetes. Simple, unornamented, pores are also associated with the teliomycetes, where they are accompanied by "pulley wheel occlusions". "Munk pores" are found in the walls of some Sordariales^[6] and are surrounded by a raised rim.^[6] Dolipores too have a thick wall surrounding their opening,^[7] which prevents the movement of cell organelles between adjoining cells, while permitting the sharing of cell fluids. These may be further covered by septal pore caps.^[8]

The dolipores of the Agaricomycotina (basidomycetes) are often surrounded by bracket-shaped structures called parenthesomes - but other basidomycetes have differently structured pores.^[9]

Formation

Primary pit connections are formed between cells in the same filament, derived from the same parent cell by its division.^[4] Such connections are always single, and usually circular;^[4] this is a result of their method of formation. The septa is formed as the walls of a filament grow inwards, dividing the cell; this results in a hole in the middle of the tube where the walls don't quite merge.^[2] Thus connections are visible in the youngest of septa, widening as the septum thickens, until in some cases they may ultimately occupy the entire septum.^[4] Secondary connections, by contrast, occur between unrelated cells, and serve a role in transferring cell contents and nutrients.^[2] They may even form between cells of different species, as in the parasite Holmsella.^[10]

External links

• Transverse SEM images of pit connections in the coralline Synarthrophyton can be seen in Figs. 54 & 60 in Keats, D. W; Chamberlain, Y. M (1997), "The non-geniculate coralline algae Synarthrophyton eckloniae (Foslie) comb. nov. and S. magellanicum (Foslie) comb. nov. (Rhodophyta) in South Africa including comparison with relevant types", European Journal of Phycology 32 (01): 55–79, doi:10.1080/09541449710001719375 
• (Lower quality) transverse SEM images of pit connections in the coralline Phymatolithon are available on page 138 at Johnson, C.; Mann, K. (1986). "The crustose coralline alga, Phymatolithon Foslie, inhibits the overgrowth of seaweeds without relying on herbivores". Journal of Experimental Marine Biology and Ecology 96: 127. doi:10.1016/0022-0981(86)90238-8.  edit.

References