Sieve-Pore Plugging Mechanisms
Aart J.E. van Bel
Sieve tubes seem to dispose of diverse mechanisms which are able to seal or plug sieve pores -
very specialized and evolutionarily transformed cell-cell channels - in response to injury. Two
major devices for their closure are distinguished: a putative rapid (several seconds) plugging by phloem-specific proteins and a slower (several minutes) sealing by callose. Sieve-pore plugging is probably executed by several classes of phloem-specific proteins such as water-soluble proteins, proteins which occur as a fine meshwork in the sieve-element lumen, parietal proteins attached to the sieve-element plasma membrane and proteins ordered in so-called forisomes. A striking feature of the forisomes is the calcium-mediated reversibility of dispersion and contraction, even in vitro. The calcium-dependence of forisome conformation and callose production suggests a general calcium regulation of sieve-pore closure.