The rendez-vous of mobile sieve-element and abundant companion-cell proteins

•Thousands of proteins collected from sieve tube exudates have been described.•Sieve tube exudate proteins (STEP) represent putative sieve element (SE) proteins.•SE proteins originate from both SE and companion cells (CC).•Advanced methods are critical for demonstrating a role of STEPs in the SE.•From the identified bonafide SE proteins, unexpected new roles have been revealed. Thousands of sieve tube exudate proteins (STEP) have now been identified and predicted to fulfill a diversity of functions. However, most STEPs should be considered putative, since methods to collect sieve tube exudates have many technical drawbacks, and advanced functional characterization will be required to distinguish contaminant from bonafide proteins, and determine the latter's location and activity in sieve elements (SE). One major challenge is to develop new approaches to elucidate the function of these SE proteins, which in turn, is expected to shed light on intriguing aspects of SE cell biology.