Title : Is autophagy involved in nitrogen remobilization during plant organs senescence in Populus trichocarpa?
S enescence of plant organs, despite its destructive character, belongs to processes which are well-regulated. This regulation is necessary to achieve the main goal of senescence – remobilization, which allows to transport valuable nutrients from senescent organs to another parts of plants. Recent studies have shown that one of the main mechanisms that is needed to properly course the remobilization in leaves is autophagy. However, little is known about root senescence. These organs, especially fine ones, are responsible for water and nutrients absorption and at the end of growing season when demand for water decreases most of them undergoes senescence and die. Given that the biomass of fine roots in forest ecosystems is often equal or greater than leaf biomass, remobilization process is important also in the case of nutrients cycling in nature. The aim of our research was to check whether processes related to nitrogen remobilization are switched on and are crucial during senescence of fine roots. We compared fine roots and leaves in order to check if common mechanisms can be identified during senescence of such diverse organs, which play different function and are characterized by dissimilar structure. GCMS analyses were performed to analyze nitrogen content and qRT-PCR were conducted for analyses of the expression of genes encoding enzymes related to possible nitrogen translocation. Additionally, to confirm that in studied organs autophagy process are also active several studies (qRT-PCR, Western blot, Immunolocalization and TEM) were carried out. Our results suggest that as well in leaves as in fine roots autophagy plays a critical role during senescence. In both organs, we observed increased expression of autophagy-related genes (atg) and amount of ATG8 protein. Moreover, in senescent cells, we observed plenty of autophagic structures such as autophagic bodies or autophagosomes. These alterations are correlated with a decrease of nitrogen amount and changing the expression of mRNA encoding proteins, necessary for nitrogen translocation e.g. glutamine synthetase (GS1) and glutamate dehydrogenase (GDH). It has been shown that different ephemeral organs undergone senescence in a similar way and involved the same mechanisms. One of them is a dual role of autophagy, which apart from degradation and executive role during senescence, is important also for remobilization of nitrogen. This work was supported by grants no. 2012/07/E/NZ9/00194 and 2016/23/N/NZ3/00073 from the National Science Centre.