Sieve-element differentiation and phloem sap contamination

•Biochemical studies suggest novel functional components in mature sieve tubes.•Functions in, for example, signaling, protein turnover and degradation have been proposed.•We find that sieve element degradation contributes some 0.1% of the phloem sap.•We conclude that the presence of a substance does not imply functional significance.•A functional cytoskeleton or protein synthesis in sieve tubes remains unlikely. Sieve elements (SEs) degrade selected organelles and cytoplasmic structures when they differentiate. According to classical investigations, only smooth ER, mitochondria, sieve element plastids, and, in most cases, P-proteins remain in mature SEs. More recent proteomics and immuno-histochemical studies, however, suggested that additional components including a protein-synthesizing machinery and a fully developed actin cytoskeleton operate in mature SEs. These interpretations are at odds with conventional imaging studies. Here we discuss potential causes for these discrepancies, concluding that differentiating SEs may play a role by ‘contaminating’ phloem exudates.