The ability of woody perennial trees to survive relies heavily on their adaptation to environmental stresses, particularly salinity, which disrupts vital biological processes and reduces growth and productivity. The impact of salt stress can vary across different parts of plants and tissue types. Due to scarce research on the spatial molecular mechanisms underlying the regulation of plant responses to salinity stress in perennial trees, we utilized laser capture microdissection on Populus tremula × alba clones. This enabled us to isolate specific cell types, Palisade (PAL) and Vascular (VAS) cells, from mature leaves of trees subjected to prolonged salt stress (PSS) followed by a recovery (REC) period. Through this method, we could discern how different tissues and cell types respond uniquely to salinity.
Under salt stress, vascular cells exhibited an increase in proteins associated with photorespiration. This was corroborated by the elevation of serine, 3-phosphoglycerate, and NH4+ levels, suggesting the synthesis of glutamine from NH4+ released during photorespiration. The enhancement in NH4+ contents, GS1.1 gene expression, and chloroplastic GLUTAMINE SYNTHETASE 2 protein abundance indicate the synthesis of glutamine from the NH4+ released from photorespiration. Moreover, the N remobilization efficiency experiment revealed N was allocated in intermediate leaves, stems, and roots during PSS periods before returning to similar levels as control during REC periods. This aligns with the accumulation of bark storage proteins in stems and roots salinity stress.
In PAL tissues, salt stress triggered an accumulation of proteins related to one-carbon (C1) metabolism and methylation-associated pathways. This increase likely supports the synthesis of lignin and compatible solutes (glycinebetaine and polyamines), improving cell wall strength and osmoregulation under salinity stress. Pectin methylesterase activity in these cells led to modifications in primary cell wall structure, resulting in thicker leaves with improved water retention capacity. Additionally, methanol produced during these reactions may contribute to alternative carbon sources for C1 metabolism and methyl group availability.
Overall, our study uncovered the tissue-specific responses of Populus leaves to salt stress. In vascular tissues, photorespiration is the main contributor that provides NH4+ for glutamine synthesis and nitrogen reallocation. On the other hand, palisade cells engage in osmoregulation, cell wall modification, and leaf anatomical adaptations via C1 metabolism.
