Lost in Transit: Long-Distance Trafficking and Phloem Unloading of Protein Signals in Arabidopsis Homografts.
In addition to moving sugars and nutrients, the phloem transports many macromolecules. While grafting and aphid stylectomy experiments have identified many macromolecules that move in the phloem, the functional significance of phloem transport of these remains unclear. To gain insight into protein trafficking, we micrograftedArabidopsis thalianascions expressing GFP-tagged chloroplast transit peptides under the 35S promoter onto nontransgenic rootstocks. We found that plastids in the root tip became fluorescent 10 d after grafting. We obtained identical results with the companion cell-specific promoterSUC2and with signals that target proteins to peroxisomes, actin, and the nucleus. We were unable to detect the respective mRNAs in the rootstock, indicating extensive movement of proteins in the phloem. Outward movement from the root protophloem was restricted to the pericycle-endodermis boundary, identifying plasmodesmata at this interface as control points in the exchange of macromolecules between stele and cortex. Intriguingly, signals directing proteins to the endoplasmic reticulum and Golgi apparatus from membrane-bound ribosomes were not translocated to the root. It appears that many organelle-targeting sequences are insufficient to prevent the loss of their proteins into the translocation stream. Thus, nonspecific loss of proteins from companion cells to sieve elements may explain the plethora of macromolecules identified in phloem sap.
Institute of Molecular Plant Sciences, University of Edinburgh, Edinburgh EH9 3JH, United Kingdom.
Danae Simone Genevieve
http://orcid.org/0000-0001-6299-7596
DSG
Paultre
Laboratoire des Pathogènes Emergents-Fondation Mérieux, Centre International de Recherche en Infectiologie, Inserm U1111, CNRS UMR5308, ENS de Lyon, UCBL1, 69008 Lyon, France.
Marie-Paule
MP
Gustin
Institute of Molecular Plant Sciences, University of Edinburgh, Edinburgh EH9 3JH, United Kingdom.
Attila
A
Molnar
Institute of Molecular Plant Sciences, University of Edinburgh, Edinburgh EH9 3JH, United Kingdom karl.oparka@ed.ac.uk.
Karl J
KJ
Oparka