Are there contractile proteins present in phloem

Actinlike structures have been found in companion cells and parenchyma of phloem and in young sieve cells, but not, according to recent work, in mature sieve tubes of angiosperms. Because of this, the question of whether P-protein is a contractile protein presents serious difficulties. Protein of low molecular weight found in phloem has not been as carefully examined as P-protein. Evidence to support contractile proteins in phloem as seen from the amino acid composition and respiratory activity of willow bark is also discussed.

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