Comparative proteomics of phloem exudates reveals long-distance signals potentially involved in Litchi chinensis flowering

It is well known that phloem is essential for transporting proteins as long-distance signals. In this study, a proteomic approach was carried out to identify proteins accumulated in phloem exudates at the pre-floral induction (PFId) stage, at the floral induction (FId) stage, and at the floral initiation (FIn) stage. As a result, 45 phloem exudate proteins were detected. Six proteins were found at all three stages. Sixteen proteins were specific to the PFId stage, 14 proteins were specific to the FId and FIn stages. From the enriched gene ontology (GO)-terms related to transport, signaling, hormone, and development, 12 transport-related, 5 signaling-related, 10 hormone-related, and 9 development-related proteins were identified. It was found that arginine glycine glycine repeats nuclear RNA binding protein A (RGGA) was specific to the FId stage whereas heat shock protein 90-7, plasma membrane intrinsic protein 1-4, and the homolog protein encoded by At4g27190 were specific to the FIn stage. The relative abundances of sporulation protein F10D13.7 and ATP-dependent binding casette transporter G family member 37 were higher at the FId stage than at the PFId stage, and that of enolase 2 was higher at the FIn stage than at the PFId stage, suggesting that these phloem exudate proteins might act as long-distance signals involved in the transition from vegetative growth to reproductive growth in litchi. This study contributes to the comparative proteomic analysis of plant phloem sap, which will provide insights into proteins involved in floral induction as well as inter-organ communication during development in litchi.