Trans-Zeatin induce regulation the biosynthesis of 2-acetyl-1-pyrroline in fragrant rice (Oryza sativa L.) seedlings

Adaptive roles of cytokinins in enhancing plant resilience and yield against environmental stressors

Innovative agricultural strategies are essential for addressing the urgent challenge of food security in light of climate change, population growth, and various environmental stressors. Cytokinins (CKs) play a pivotal role in enhancing plant resilience and productivity. These compounds, which include isoprenoid and aromatic types, are synthesized through pathways involving key enzymes such as isopentenyl transferase and cytokinin oxidase. Under abiotic stress conditions, CKs regulate critical physiological processes by improving photosynthetic efficiency, enhancing antioxidant enzyme activity, and optimizing root architecture. They also reduce the levels of reactive oxygen species and malondialdehyde, resulting in improved plant performance and yield. CKs interact intricately with other phytohormones, including abscisic acid, ethylene, salicylic acid, and jasmonic acid, to modulate stress-responsive pathways. This hormonal cross-talk is vital for finely tuning plant responses to stress. Additionally, CKs influence nutrient uptake and enhance responses to heavy metal stress, thereby bolstering overall plant resilience. The application of CKs helps plants maintain higher chlorophyll levels, boost antioxidant systems, and promote root and shoot growth. The strategic utilization of CKs presents an adaptive approach for developing robust crops capable of withstanding diverse environmental stressors, thus contributing to sustainable agricultural practices and global food security. Ongoing research into the mechanisms of CK action and their interactions with other hormones is essential for maximizing their agricultural potential. This underscores the necessity for continued innovation and research in agricultural practices, in alignment with global goals of sustainable productivity and food security.

Aromatic compound 2-acetyl-1-pyrroline coordinates nitrogen assimilation and methane mitigation in fragrant rice

Rice paddy has been the main source of anthropogenic methane (CH4) emissions, with significant variations among rice varieties. 2-Acetyl-1-pyrroline (2-AP) is the key component of the pleasant aroma in fragrant rice. Here, we show that fragrant rice is metabolically active in nitrogen assimilation and exhibits high levels of 2-AP and that CH4 fluxes at the booting stage and cumulative emissions are 25.5% and 14.8% lower, respectively, in fragrant rice paddies compared with nonfragrant rice paddies. Three precursors involved in 2-AP synthesis-proline, glutamic acid, and ornithine-are identified as crucial nitrogen compounds that significantly promote CH4 oxidation in the rhizosphere. Augmenting 2-AP synthesis, either through foliar spraying or by utilizing CRISPR-Cas9 technology to generate knockout lines of BETAINE ALDEHYDE DEHYDROGENASE 2 gene, effectively enhances CH4 oxidation and reduces CH4 fluxes. Our findings reveal that the 2-AP metabolic pathway coordinates the carbon/nitrogen cycle to improve nitrogen assimilation along with high 2-AP levels and mitigate CH4 emissions in paddy ecosystems.

Regulation of 2-Acetyl-1-pyrroline Content in Fragrant Rice under Different Temperatures at the Grain-Filling Stage

2-Acetyl-1-pyrroline (2-AP) is a key volatile organic compound in fragrant rice aroma. However, the effects of temperature on 2-AP biosynthesis in fragrant rice and its regulation mechanism have been rarely reported. In the present study, three fragrant rice varieties were used as plant materials, and four temperature treatments during the grain-filling stage, i.e., (T1) 22/17 °C, (T2) 27/22 °C, (T3) 32/27 °C, and (T4) 37/32 °C, were adopted. The results showed that grain contents of 2-AP, proline, and γ-aminobutyric acid (GABA) significantly (P < 0.05) increased with decreased temperature, while the lowest and highest 2-AP contents were recorded in the T4 and T1 treatments, respectively. Higher pyrroline-5-carboxylic acid (P5C) content was recorded in low-temperature treatments (T1 and T2) than in high-temperature treatments (T3 and T4). The transcript levels of genes BADH2, PRODH, and OAT significantly (P < 0.05) decreased with decreased temperature. Lower transcript levels of genes P5CR, P5CS2, DAO2, DAO4, and DAO5 were recorded in low-temperature treatments (T1 and T2) than in high-temperature treatments (T3 and T4). In conclusion, low temperature increased 2-AP content and high temperature decreased 2-AP content in fragrant rice. We deduced that temperature regulated 2-AP biosynthesis through the metabolism of proline and GABA.

Effects of paclobutrazol seed priming on seedling quality, photosynthesis, and physiological characteristics of fragrant rice

BACKGROUND

Paclobutrazol is widely used in the agricultural field. This study investigated the effects of seed priming with different concentrations of paclobutrazol on seedling quality, 2-acetyl-1-pyrroline (2-AP, a key aroma component of fragrant rice) biosynthesis, and related physiological and biochemical indicators in fragrant rice seedlings.

RESULTS

The experiment is being conducted at the College of Agriculture, South China Agricultural University. In the experiment, three concentrations of paclobutrazol (Pac 1: 20 mg·L-1; Pac 2: 40 mg·L-1; Pac 3: 80 mg·L-1) were used to initiate the treatment of fragrant rice seeds, while water treatment was used as a control (CK). The results showed that compared with CK, paclobutrazol treatment reduced plant height, increased stem diameter, and increased fresh and dry weight of aromatic rice seedlings. Moreover, paclobutrazol treatment also increased the seedlings' photosynthetic pigment content and net photosynthetic rate.

CONCLUSIONS

This study demonstrates that paclobutrazol primarily increases the content of proline by reducing the content of glutamate and down-regulating the expression of P5CS2, thereby promoting the conversion of proline to the aromatic substance 2-AP. Under the appropriate concentration of paclobutrazol (40 mg·L-1~80 mg·L-1), the seedling quality, stress resistance, and aroma of fragrant rice can be improved.

l-Glutamate Seed Priming Enhances 2-Acetyl-1-pyrroline Formation in Fragrant Rice Seedlings in Response to Arsenite Stress

2-Acetyl-1-pyrroline (2-AP) is a fragrance compound and flavor in fragrant rice whose precursors are generally glutamate (Glu) and proline (Pro). Our previous study revealed that exogenous Glu enhanced the arsenic (As) tolerance in fragrant rice by improving the ascorbic acid-glutathione cycle and the Pro content in roots. However, less is known about how Glu is involved in 2-AP biosynthesis in fragrant rice under As stress. Herein, a hydroponic experiment of L-Glu seed priming with 0, 100, and 500 μM l-glutamic acid solutions was conducted with two fragrant rice varieties. After that, the 10-day-old seedlings were cultured under 0 and 100 μM arsenite stress for 10 d. Results showed that the 2-AP and Pro contents were increased by 18-30% and 21-78% under As100 μM-Glu100 μM treatment in comparison to the control As100 μM to Glu0 μM, while the activities of pyrroline-5-carboxylate synthetase (P5CS) and proline dehydrogenase (ProDH) were increased by 19-46% and 3-19%, respectively. Furthermore, the 2-AP, Pro contents, and P5CS activity were correlated positively. Correspondingly, a significant abundance of differential expressed metabolites (18) and differential expressed genes (26) was observed in amino acid metabolism and glutathione metabolism pathways. In addition, several essential genes were verified and grouped into the pathways of glutathione metabolism, proline, and arginine metabolism with antioxidant defense system to comodulate 2-AP biosynthesis and stress detoxification. Therefore, the Glu seed priming treatment had a positive impact on the 2-AP biosynthesis of fragrant rice under 100 μM arsenite toxicity.

Novel Insights into Exogenous Phytohormones: Central Regulators in the Modulation of Physiological, Biochemical, and Molecular Responses in Rice under Metal(loid) Stress

Rice (Oryza sativa) is a research model for monocotyledonous plants. Rice is also one of the major staple foods and the primary crop for more than half of the world's population. Increasing industrial activities and the use of different fertilizers and pesticides containing heavy metals (HMs) contribute to the contamination of agriculture fields. HM contamination is among the leading causes that affect the health of rice plants by limiting their growth and causing plant death. Phytohormones have a crucial role in stress-coping mechanisms and in determining a range of plant development and growth aspects during heavy metal stress. This review summarizes the role of different exogenous applications of phytohormones including auxin, cytokinin, gibberellins, ethylene, abscisic acid, strigolactones, jasmonates, brassinosteroids, and salicylic acids in rice plants for mitigating heavy metal stress via manipulation of their stress-related physiological and biochemical processes, and alterations of signaling and biosynthesis of genes. Exogenous administration of phytohormones and regulation of endogenous levels by targeting their biosynthesis/signaling machineries is a potential strategy for protecting rice from HM stress. The current review primarily emphasizes the key mechanistic phytohormonal-mediated strategies for reducing the adverse effects of HM toxicity in rice. Herein, we have provided comprehensive evidence for the effective role of exogenous phytohormones in employing defense responses and tolerance in rice to the phytotoxic effects of HM toxicity along with endogenous hormonal crosstalk for modulation of subcellular mechanisms and modification of stress-related signaling pathways, and uptake and translocation of metals. Altogether, this information offers a systematic understanding of how phytohormones modulate a plant's tolerance to heavy metals and may assist in directing the development of new approaches to strengthen rice plant resistance to HM toxicity.