Abstract
Low temperature is a major environmental factor that severely impairs plant growth and productivity. Watermelon (Citrullus lanatus) is a chilling-sensitive crop. Grafting of watermelon onto pumpkin rootstock is an effective technique to increase the chilling tolerance of watermelon when exposure to short-time chilling stress. However, the mechanism by which pumpkin rootstock increases chilling tolerance remains poorly understood. Under 10°C/5°C (day/night) chilling stress treatment, pumpkin-grafted watermelon seedlings showed higher chilling tolerance than self-grafted watermelon plants with significantly reduced lipid peroxidation and chilling injury (CI) index. Physiological analysis revealed that pumpkin rootstock grafting led to the notable accumulation of putrescine in watermelon seedlings under chilling conditions. Pre-treat foliar with 1 mM D-arginine (inhibitor of arginine decarboxylase, ADC) increased the electrolyte leakage (EL) of pumpkin-grafted watermelon leaves under chilling stress. This result can be ascribed to the decrease in transcript levels of ADC, ornithine decarboxylase, spermidine synthase, and polyamine oxidase genes involved in the synthesis and metabolism of polyamines. Transcriptome analysis showed that pumpkin rootstock improved chilling tolerance in watermelon seedlings by regulating differential gene expression under chilling stress. Pumpkin-grafted seedling reduced the number and expression level of differential genes in watermelon scion under chilling stress. It specifically increased the up-regulated expression of ADC (Cla97C11G210580), a key gene in the polyamine metabolism pathway, and ultimately promoted the accumulation of putrescine. In conclusion, pumpkin rootstock grafting increased the chilling tolerance of watermelon through transcription adjustments, up regulating the expression level of ADC, and promoting the synthesis of putrescine, which ultimately improved the chilling tolerance of pumpkin-grafted watermelon plants.
Highlights
Low temperature is a key factor limiting plant growth and development and crop quality and productivity (Thomashow, 1999; Zhu et al, 2007; Liu et al, 2018)
The MDA contents of the Cl/Cl and Cl/Cm seedlings were significantly higher than those of their control seedlings, but pumpkin grafted alleviated the increase trend of MDA content. These results indicate that Cl/Cl and Cl/Cm watermelon seedlings respond to chilling stress differently, and rootstock grafting apparently enhances the chilling tolerance of watermelon
Spermine content showed no definite change characteristics in the Cl/Cl and Cl/Cm seedlings under chilling stress (Figure 2C). These results suggest that putrescine may be involved in the chilling tolerance of watermelon induced by pumpkin rootstock grafting
Summary
Low temperature is a key factor limiting plant growth and development and crop quality and productivity (Thomashow, 1999; Zhu et al, 2007; Liu et al, 2018). Accumulation of polyamines by modulating metabolism-related genes is a useful strategy for plants to withstand low temperature-induced adverse damages, such as cell membrane permeability, osmotic potential increase, ROS accumulation and others (Alet et al, 2011; Kou et al, 2018; Chen et al, 2019). Polyamines synthesis and metabolization-related genes and enzymes are responsive to abiotic stress, generally showing a trend of polyamine accumulation and playing an important role in abiotic tolerance (Shi and Chan, 2014; Liu et al, 2015). Whether grafting watermelon onto chilling-tolerant rootstock can regulate ADC gene expression to modulate putrescine biosynthesis in response to chilling stress remains to be investigated
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