The Phloem

4. Forisomes 

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Forisomes belong to the group of non-dispersive P protein-bodies (see P-proteins for definition). They occur in faboid legumes (bean plants) only. Forisomes undergo a rapid, reversible switch from an elongated low-volume conformation into a spherical high-volume conformation. The high-volume state is induced when Ca2+ enters the cytoplasm of disturbed or severed sieve elements. High volume-state forisomes impede flow in the sieve tube.

Figure legend: Forisomes (asterisk) in sieve elements (SE) of broad bean (A-E) and green bean (F). Forisomes are spindle shaped in the low volume state (A,C,D). After injury they contract longitudinally and swell radially, which leads to a significant volume increase and occlusion of the sieve tube (B,E,F). In green been forisomes have long terminal extrusions called tails. Forisome morphology is of taxonomic significance (see Peters et al. 2010 in the literature section.) Figure from Knoblauch et al. 2001. Calcium regulated stopcocks in legume sieve tubes. Plant Cell 13 1221-1230. Copyright: American Society of Plant Biologists

In vitro, forisome volume increases up to 9-fold in response to divalent cations without requirement for chemical energy sources such as ATP. Cycling between low- and high-volume states can be induced in isolated forisomes by repetitive exchange of bathing media containing either Ca2+ or chelators. Diffusional electrotitration allows electric control of forisome expansion. Forisomes can be isolated in large numbers and remain functional after prolonged periods of storage. This unique combination of useful properties renders forisomes a paradigmatic model for proteinaceous smart materials (see Knoblauch et al. 2003 in the literature list).


 Figure legend: A forisome attached to two glass capillaries (upper left). Application of calcium medium leads to longitudinal contraction. The forisome is capable of generating a significant force by pulling the glass capillary (upper right). In contrast to other biological motor proteins, forisomes excert force during contraction and expansion. No ATP is necessary to drive the reaction. From Knoblauch et al. 2003 ATP independent contractile proteins from Plants. Nature Materials 2, 600-603. Copyright: Nature Publishing Group




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