Beyond the surface: the plant secretome as a bridge between the cell and its environment

We describe the biological importance of proteins secreted in plants under different conditions and biological processes, the secretion mechanisms, methodologies for obtaining and identifying these proteins, and future perspectives. Molecule secretion inside and outside the cell is relevant to all areas of plant biology. Protein secretion, in particular, has intriguing possibilities due to the different secretion pathways that the cell uses to send biochemical messages. The secretion of proteins-secretomes-into extracellular space in response to different stimuli or stress situations, in vitro or in planta conditions, has been studied in plants and plant tissues. Proteomics has allowed the quantitative and qualitative analysis of this process and the measurement of proteins associated with the cellular development of different tissues. This has provided the means of evaluating a more precise biochemical state of the cells and the changes that occur during their growth. With the development of new techniques in proteomics, such as mass spectrometry, sequencing, and bioinformatics, it is now possible to elucidate the main proteins secreted, with all their posttranslational modifications, in different plant species and under different specific conditions. This review presents the different pathways of protein secretion in plants, highlighting the well-known importance of signal peptides. The advances and disadvantages of in planta and in vitro systems used for proteomic purposes are discussed. The principal proteomic techniques to better understand the biological function of the secretome are summarized.