Abstract
Potassium (K) deficiency is a common abiotic stress that can inhibit the growth of fruit and thus reduce crop yields. Little research has been conducted on pear transcriptional changes under low and high K conditions. Here, we performed an experiment with 7-year-old pot-grown “Huangguan” pear trees treated with low, Control or high K levels (0, 0.4, or 0.8 g·K2O/kg soil, respectively) during fruit enlargement and mature stages. We identified 36,444 transcripts from leaves and fruit using transcriptome sequencing technology. From 105 days after full blooming (DAB) to 129 DAB, the number of differentially expressed genes (DEGs) in leaves and fruit in response to low K increased, while in response to high K, the number of DEGs in leaves and fruit decreased. We selected 17 of these DEGs for qRT-PCR analysis to confirm the RNA sequencing results. Based on GO enrichment and KEGG pathway analysis, we found that low-K treatment significantly reduced K nutrient and carbohydrate metabolism of the leaves and fruit compared with the Control treatment. During the fruit development stages, AKT1 (gene39320) played an important role on K+ transport of the leaves and fruit response to K stress. At maturity, sucrose and acid metabolic pathways were inhibited by low K. The up-regulation of the expression of three SDH and two S6PDH genes involved in sorbitol metabolism was induced by low K, promoting the fructose accumulation. Simultaneously, higher expression was found for genes encoding amylase under low K, promoting the decomposition of the starch and leading the glucose accumulation. High K could enhance leaf photosynthesis, and improve the distribution of the nutrient and carbohydrate from leaf to fruit. Sugar components of the leaves and fruit under low K were regulated by the expression of genes encoding 8 types of hormone signals and reactive oxygen species (ROS). Our data revealed the gene expression patterns of leaves and fruit in response to different K levels during the middle and late stages of fruit development as well as the molecular mechanism of improvement of fruit sugar levels by K and provided a scientific basis for improving fruit quality with supplemental K fertilizers.
Highlights
Pear (Pyrus L.) is cultivated world-wide, with China ranking as the top producer of Asian pear (Yang et al, 2015)
Each tree around the drip line in the orchard was supplied with urea (46% N), calcium superphosphate (12% P2O5) at the base fertilizer period, and potassium sulfate (51% K2O)
To determine which stages of fruit development could be used for profiling transcriptomes related to sugar metabolism in pear leaves and fruit, we tracked and analyzed the six processes of fruit development from flowering (0 days after full blooming (DAB)) to maturity (129 DAB), and determined single-fruit weight (Figure S1B) and fruit diameter (Figure S1C) under the different K treatments
Summary
Pear (Pyrus L.) is cultivated world-wide, with China ranking as the top producer of Asian pear (Yang et al, 2015). In Pyrus species, sucrose and fructose are the major soluble sugars, and overall sugar content varies greatly between different cultivars. The low sugar content in fresh pear directly influences the sweetness of fruit, leading to great economic losses in postharvest storage periods (Hudina and Stampar, 2005). Potassium K+ is an essential plant nutrient and plays a major role in different physiological processes such as turgor regulation, osmotic adjustment, stomatal movement, cell elongation, and signal transduction (Clarkson and Hanson, 1980; Armengaud et al, 2004). K+ contributes to balancing the electrical charge of membranes, movement of ions, enzyme activation, stabilization of protein synthesis, and metabolism of sugars (Maathuis and Sanders, 1996). K+ deficiency directly affects plant growth, leading to decreased crop yield and production, so supplemental K fertilizers are required for sustainable agricultural practices
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