Abstract
The physiological mechanisms driving treeline formation succession captured the attention of ecologists many years ago, yet they are still not fully understood. In this study, physiological parameters (soluble sugars, starch, and nitrogen) were investigated in combination with transcriptomic analysis in the treeline tree species Picea crassifolia. The study was conducted in the middle of Qilian Mountain Reserves, Gansu Province, China, within the elevation range of 2500–3300 m. The results showed that the concentrations of non-structural carbohydrates decreased with increasing elevation in the current-year needles and current-year branches, as well as in the coarse and fine roots. RNA-Seq demonstrated that 483 genes were upregulated and 681 were downregulated in the comparison of 2900 and 2500 m (2900 vs. 2500), 770 were upregulated and 1006 were downregulated in 3300 vs. 2500, and 282 were upregulated and 295 were downregulated in 3300 vs. 2900. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that the differentially expressed genes were highly enriched in photosynthesis-related processes, carbon fixation and metabolism, and nitrogen metabolism. Furthermore, almost all photosynthesis-related genes were downregulated, whereas many genes involved in cuticle lipids and flavonoid biosynthesis were upregulated, contributing to the survival of P. crassifolia under the treeline condition. Thus, our study provided not only molecular evidence for carbon limitation hypothesis in treeline formation, but also a better understanding of the molecular mechanisms of treeline tree survival under adverse conditions.
Highlights
The treeline, defined here as the highest altitude where groups of upright trees reach at least 3 m of height, is one of the most recognizable bioclimatic boundaries and it has been suggested as a critical indicator of trees’ response to climate warming [1,2]
This study showed that the expression of six chlorophyll a-b binding protein genes were significantly downregulated with the increasing elevations, which helped to explain the decreased C fixation, suggesting that cold soil and ultraviolet-B radiation (UV-B ) radiation leads to the downregulation of several transcripts encoding key photosynthetic proteins
The expression of GOLS1, which is the first committed enzyme in the raffinose family oligosaccharides (RFOs) synthesis pathway [46], decreased with the increasing elevations. These results indicated that the RFO pathway might play a crucial role in the response of P. crassifolia to elevation changes
Summary
The treeline, defined here as the highest altitude where groups of upright trees reach at least 3 m of height, is one of the most recognizable bioclimatic boundaries and it has been suggested as a critical indicator of trees’ response to climate warming [1,2]. The decrease in temperature with increasing elevations has been recognized as a major reason for treeline formation, and the mechanism by which low temperature affects the growth of treeline trees is unclear. The environmental hypothesis stated that the formation of treeline was due to the following factors: climatic stress, mechanical damage, short growing season, and temperature-limited reproduction [7,8,9,10]. This hypothesis can only be used to interpret local treeline phenomena, which are related to various local site conditions [11]. The growth limitation hypothesis (GLH) considers that colder temperatures at higher elevations limit C sink activity [5], supported by the results of trees in 13 alpine tree line regions [13] and Larix potaninii in the eastern
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