Physiological and Transcriptomic Analyses Uncover the Reason for the Inhibition of Photosynthesis by Phosphate Deficiency in Cucumis melo L.

Phosphate (Pi) deficiency is a common phenomenon in agricultural production and limits plant growth. Recent work showed that long-term Pi deficiency caused the inhibition of photosynthesis and inefficient electron transport. However, the underlying mechanisms are still unknown. In this study, we used the physiological, histochemical, and transcriptomic methods to investigate the effect of low-Pi stress on photosynthetic gas exchange parameters, cell membrane lipid, chloroplast ultrastructure, and transcriptional regulation of key genes in melon seedlings. The results showed that Pi deficiency significantly downregulated the expression of aquaporin genes, induced an increase in ABA levels, and reduced the water content and free water content of melon leaves, which caused physiological drought in melon leaves. Therefore, gas exchange was disturbed. Pi deficiency also reduced the phospholipid contents in leaf cell membranes, caused the peroxidation of membrane lipids, and destroyed the ultrastructure of chloroplasts. The transcriptomic analysis showed that 822 differentially expressed genes (DEGs) were upregulated and 1254 downregulated by Pi deficiency in leaves. GO and KEGG enrichment analysis showed that DEGs significantly enriched in chloroplast thylakoid membrane composition (GO:0009535), photosynthesis-antenna proteins (map00196), and photosynthesis pathways (map00195) were downregulated by Pi deficiency. It indicated that Pi deficiency regulated photosynthesis-related genes at the transcriptional level, thereby affecting the histochemical properties and physiological functions, and consequently causing the reduced light assimilation ability and photosynthesis efficiency. It enriches the mechanism of photosynthesis inhibition by Pi deficiency.

Physiological and Transcriptomic Analysis Reveals the Responses and Difference to High Temperature and Humidity Stress in Two Melon Genotypes.

Due to the frequent occurrence of continuous high temperatures and heavy rain in summer, extremely high-temperature and high-humidity environments occur, which seriously harms crop growth. High temperature and humidity (HTH) stress have become the main environmental factors of combined stress in summer. The responses of morphological indexes, physiological and biochemical indexes, gas exchange parameters, and chlorophyll fluorescence parameters were measured and combined with chloroplast ultrastructure and transcriptome sequencing to analyze the reasons for the difference in tolerance to HTH stress in HTH-sensitive 'JIN TAI LANG' and HTH-tolerant 'JIN DI' varieties. The results showed that with the extension of stress time, the superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) activities of the two melon varieties increased rapidly, the leaf water content increased, and the tolerant varieties showed stronger antioxidant capacity. Among the sensitive cultivars, Pn, Fv/Fm, photosystem II, and photosystem I chlorophyll fluorescence parameters were severely inhibited and decreased rapidly with the extension of stress time, while the HTH-tolerant cultivars slightly decreased. The cell membrane and chloroplast damage in sensitive cultivars were more severe, and Lhca1, Lhca3, and Lhca4 proteins in photosystem II and Lhcb1-Lhcb6 proteins in photosystem I were inhibited compared with those in the tolerant cultivar. These conclusions may be the main reason for the different tolerances of the two cultivars. These findings will provide new insights into the response of other crops to HTH stress and also provide a basis for future research on the mechanism of HTH resistance in melon.

Physiological and Biochemical Responses of Cucumis melo L. Chloroplasts to Low-Phosphate Stress.

Phosphorus (P) is a limiting plant soil nutrient. Long-term low inorganic phosphate (Pi) irreversibly damages plant cells and hinders plant growth. Plants have evolved several adaptive biochemical, physiological, and developmental responses to low-Pi stress. However, little is known about chloroplast responses to low-Pi stress. In this study, we used physiological and biochemical analyses to investigate melon chloroplast responses to low-Pi stress. The results indicated that low-Pi stress impeded melon seedling growth and reduced its dry matter content by inhibiting the photosynthesis. Low-Pi stress reduced the P content in shoots, which inhibited ATP synthase (ATP-ase) activity, and disturbed the proton and electron transport efficiency on chloroplast photosynthetic electron transport chain. In addition, low-Pi stress induced reactive oxygen species (ROS) production in the leaves, which caused membrane peroxidation. Therefore, redox homeostasis was not maintained, and the melon leaves presented with symptoms of photooxidative stress. To mitigate photoinhibition, the melon plants initiated non-photochemical chlorophyll fluorescence quenching (NPQ) initiated by acidification of the thylakoid lumen to dissipate excess excitation energy, significantly improved ROS-scavenging enzyme activity. Based on these experimental results, we concluded that low Pi inhibited photosystem activity and caused photooxidative stress and photoinhibition. To alleviate these negative effects, the plant activated its NPQ mechanism, alternative electron transport pathways, and antioxidant system to protect its chloroplasts.

Transcriptome analysis of Chinese bayberry (Myrica rubra Sieb. et Zucc.) fruit treated with heat and 1-MCP.

Chinese bayberry (Myrica rubra Sieb. et Zucc.) is a typical fruit tree grown in the hilly region of Southern China. The fruit is sensitive to storage and transportation conditions and presents a major problem in its commercialization. The present study was conducted to investigate the regulation of gene expression involved in plant hormone signaling pathway in the Chinese bayberry with different treatments of heat and 1-methylcyclopene (1-MCP) during postharvest storage. In one treatment group (HM group), we exposed Chinese bayberry fruit to 48 °C for 10 min and then sealed them in a desiccator with 5 µl·L-1 of 1-MCP for 24 h at 20 °C, followed by storage at 10 °C. Another group (CK group) was directly stored at 10 °C without any prior treatment. Samples of fruit were collected every three days, at 3, 6, 9, 12 and 15 d (CK3, CK6, CK9, CK12 and CK15; and HM3, HM6, HM9, HM12, and HM15, respectively). The decay index of fruits in the CK group increased after six days of storage but did not increase until nine days of storage in the HM group. Superoxide dismutase (SOD) activity in the CK group was shown a downtrend during storage, and almost no fluctuation from six days. In the HM group, SOD activity increased after three days, but decreased sharply after six days storage. Besides, peroxidase (POD) and catalase (CAT) activities were shown the similar trend during the storage, both of them first increased and then decreased form the six days of storage. These physiological data indicated that the sixth day is a crucial time during the storage of Chinese bayberry treated with heat and 1-MCP. Therefore, the transcriptome libraries were constructed from CK0, CK6, HM6 group, respectively. The analysis of top 20 KEGG pathways showed that most differentially expressed genes were involved in the biosynthesis of secondary metabolites, particularly flavonoids and flavanols biosynthesis, in CK0 vs. CK6 and CK0 vs. HM6. However, the top three KEGG pathways in CK6 vs. HM6 were the ribosome, RNA transport and endocytosis during the storage. Expression of six ethylene receptor (ETR) genes and four ethylene-responsive transcription factor (ERF) genes were activated at transcriptional level during the postharvest stage and were decreased by heat and 1-MCP treatment, and serine/threonine-protein kinase 1 (CTR1) was also repressed by treatment. Abscisic acid (ABA) -responsive element binding factor (ABF) gene, auxin-responsive GH3 gene and transcription factor MYC2 gene also showed similar expression pattern with ethylene pathway genes. These results might improve our understanding of the mechanisms of heat and 1-MCP inhibition of fruit postharvest physiology and prolongation of fruit shelf life.