Annals of Botany

Tails – you win: getting a handle on forisome function

Over a period of several decades, many observations of phloem sieve tubes appeared to show protein bodies (P-protein), blocking the sieve-plate pores; how then did translocation occur? It was suggested that this blockage was a fixation artefact but that still did not tell us anything about P-protein function. Then came the discovery that plants in the sub-family Faboideae contain P-protein bodies that, in response to Ca²⁺, but independently of ATP, expand to plug or partially plug the sieve pores. These were named forisomes, or gate bodies (from the Latin foris, the wing of a gate). Now a joint German–American group (Peters et al., pp. 101–109) has used forisomes from the climbing faboid, Canavalia gladiata (sword bean) to study reactions to Ca²⁺ in vitro. Forisomes from this species have two main advantages. First they are very large (up to 55 μm long) and secondly they possess tails that allow them to be handled without touching the main contractile part. The authors were thus able to study individual isolated forisomes under the microscope. In the absence of Ca²⁺ (the ‘resting’ state), forisomes had a square cross-section and thus resembled a long box. When forisomes were irrigated with Ca²⁺ the rectangular cross-section was retained but the length decreased by about 30 % and the cross-sectional dimensions increased by up to 4-fold. Overall, this process took between 10 and 15 seconds and, strangely, final length took longer to achieve than final thickness. Especially important for the plugging function, the mean changes in dimension were calculated to increase forisome volume 9-fold. Evidence that the trigger for these changes was indeed Ca²⁺ was obtained by flooding the forisomes with a chelating agent: they reverted to the resting state and, again strangely, this reversion occurred ten times faster than the response to Ca²⁺. Canavalia forisomes thus have great potential for study of calcium-regulated protein contraction.