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INTRODUCTION Cucumber (Cucumis sativus) is one of the foremost vegetable crops globally. Photosynthesis intricately influences the fruit yield of cucumber, and leaf color determines the photosynthetic efficiency to a large extent. Therefore, Leaf color mutants serve as ideal materials for scrutinizing diverse physiological processes, including photomorphogenesis, chloroplast development, chlorophyll metabolism, and photosynthetic mechanisms. Currently, the molecular mechanisms underlying the yellowing lethal phenotype remain unclear.
RATIONALE In this study, a stable cucumber yellowing lethal mutant, ycl(yellow cotyledon lethal), was isolated from the near-isogenic line XYYH-2-1-1. The phenotype, leaf microstructure and chloroplast ultrastructure, as well as physiological and biochemical analyses, were conducted on the mutant ycl and the wild-type XYYH-3-1 to explore the physiological mechanisms underlying the yellowing lethal phenotype. Preliminary localisation of yellowing lethal mutation genes was performed by whole genome resequencing using BSA. The integration of transcriptome sequencing allowed us to analyze the expression of genes related to yellowing death and the main pathways. This approach laid a solid foundation for further investigation into the molecular mechanisms responsible for the lethal phenotype associated with yclyellowing.
RESULTS The ycl mutant exhibited yellow cotyledons, which ultimately withered and perished within approximately two weeks. Notably, its growth-inhibiting phenotype appeared to be light-independent. Compared to the wild type, ycl accumulated extremely low Chl a and Chl b contents, which was consistent with the blockade in the magnesium ion chelation process within the chlorophyll biosynthesis pathway. Microscopic and ultrastructural analyses revealed disordered ycl leaf structure and inhibited chloroplast development. Additionally, the ycl mutant displayed significantly increased antioxidant enzyme activities and malondialdehyde contents, suggesting elevated oxidative stress levels and robust antioxidant capacities. The substantial decrease in net photosynthetic rate and rise in intercellular CO2 concentration in ycl were hypothesized to stem from reduced stomatal conductance, diminished chlorophyll content, and impaired chloroplast development in the mutant. Transcriptomic analyses suggested that key pathways including photosynthesis, flavonoid biosynthesis, chlorophyll metabolism, and reactive oxygen species metabolism were affected in ycl. The ycl mutant gene was preliminarily mapped to a region between 1.48 to 1.9 Mb on chromosome 3 through BSA-seq analysis, encompassing 41 candidate genes.
CONCLUSION The study investigated the physiological mechanisms underlying the yellowing lethal phenotype of the yclmutant, preliminarily mapped the mutant gene to chromosome 3, and identified differentially expressed genes (DEGs) and key pathways associated with the lethal phenotype. These findings provide valuable insights into the molecular mechanisms of chloroplast development in cucumber.
Phenotypic changes of WT and the ycl mutant at the cotyledon stage under natural light conditions, and preliminary mapping of the mutant gene.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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