Anisotropic contraction in forisomes: simple models won't fit.

Area of Science:

• Plant biology
• Biophysics
• Materials science

Background:

• Forisomes are ATP-independent, Ca(2+)-driven contractile protein bodies.
• They function as reversible valves in the phloem of legume plants.
• Forisome contraction is anisotropic, involving length shrinkage and radial expansion.

Purpose of the Study:

• To test the hypothesis that changes in forisome length and width are causally related.
• To investigate Ca(2+) and pH-dependent deformations in forisomes.
• To identify size effects on forisome contractility.

Main Methods:

• High-speed photography was used to monitor Ca(2+)- and pH-dependent deformations.
• Geometric parameters, including volume and surface area, were computed.
• Forisomes from Canavalia gladiata (large) and soybean (small) were studied.

Main Results:

• Calcium induced sixfold volume increases in forisomes of both species.
• Soybean forisomes responded faster (0.15 s) than Canavalia forisomes (0.5 s).
• Time-courses of length and diameter changes lacked correlation, and Ca(2+) vs. pH responses differed qualitatively.

Conclusions:

• Forisome contractility may involve mechanisms similar to polyelectrolyte gels.
• There is no simple causal relationship between forisome length and width changes.
• Divalent cations and protons likely target different sites on forisome proteins.