Contractile proteins and hypotheses concerning their role in phloem transport

Abstract

Contractile and motile proteins have been found in plants. Cytoplasmic streaming in Nitella is not inconsistent with several aspects of an actinomyosin system. Several hypotheses have been advanced, invoking the participation of contractile or motile proteins in phloem transport. The peristalsis hypothesis lacks credibility in absence of strong evidence of existence of sufficiently large tubules. Microperistalsis could be responsible for only a small fraction of the specific mass transfer and may require a faster reaction frequency than acceptable for an ATPase system. Undulating filaments would appear to require a much higher power per unit length than could be generated by actinomyosin. Since electroosmosis and surface flow have already been discounted, the Münch hypothesis must be favoured although not all observational or experimental evidence by any means supports it.