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Subjects:
Plant Sciences
Brassinosteroids (BRs) are known as the sixth type of plant hormone participating in various physiological and biochemical activities and play an irreplaceable role in plants.
- nitric oxide
- hydrogen peroxide
- hydrogen sulfide
- Brassinosteroids
1. Introduction
Indole-acetic acid (IAA) and gibberellin have been recognized as the known plant hormones found in plants many decades ago. Some studies have recently demonstrated that various phytohormones such as cytokinins (CTK), abscisic acid (ABA), ethylene, strigolactone, and melatonin are involved in plant growth and development and in responses to stress [1,2,3,4]. Brassinosteroids (BRs), a new type of plant hormone, have drawn an increased amount of attention. BRs, as a steroidal phytohormone, have been found to be involved in a wide range of physiological processes in plants, including cell elongation, cell division, seed development, flowering, and senescence, as well as both abiotic and biotic stress responses [5,6,7,8]. In addition, BRs have also been found to interact with other plant hormones to regulate plant growth and development as well as stress resistance. For example, co-treatment of melatonin and BRs significantly improved the resistance of Festuca arundinacea Schreb. to heat stress by decreasing the reactive oxygen species (ROS) level and malondialdehyde (MDA) content and increasing chlorophyll content and antioxidant enzyme activities [9]. In addition, studies involving BR-insensitive and BR-deficient mutants in the model plant Arabidopsis thaliana increasingly indicate that BRs might be vital endogenous growth modulators in plants. Meanwhile, BR loss-of-function mutants have also shown similar phenotypes, such as a dark-green color, obvious dwarfism, and a de-etiolation phenotype when grown in the dark [10]. She et al. elucidated the BR structure and found that kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1) is the receptor of BRs [11]. They also further provided detailed molecular insights into BR recognition [11].
Different kinds of molecules play an essential role in transmitting information between cells of multicellular organisms, including small-molecule compounds (SMCs). The SMCs are produced and induced by signals in cells and then covalently bind to target cell receptors to cause multiple biological processes and stimulate responses both in animals and plants [12]. In the past, SMCs, such as nitric oxide (NO), hydrogen sulfide (H2S), and carbon monoxide (CO), were widely known for their toxicity. Their function in numerous plant growth and development processes is an inspiringly new development. Various studies have demonstrated the function of SMCs on a wide range of developmental and physiological processes, from root formation to postharvest senescence. Niu et al. suggested that NO promoted adventitious rooting in cucumber by protein post-translational modification (S-nitrosylation) [13]. Further, H2S at proper doses also improved the longevity and quality of cut roses and chrysanthemums by maintaining water balance, reducing the degradation of pigments and nutrients and enhancing antioxidant capacity [14]. As a class of abundant membrane components and signaling molecules, sphingosines increased the embryo biomass in Gossypium hirsutum Linn [15]. Additionally, SMCs have been proven to resist abiotic stresses in plants [16,17].
2. Brassinosteroids and Nitric Oxide
The interaction between BRs and the small gas molecule NO has an essential role in the growth, development, and stress response of plants. However, the specific mechanism of their interaction is still not clear and needs further study. Further, S-nitrosylation, a redox-based posttranslational modification, is an NO-dependent regulatory mechanism. Thus, whether BRs interact with NO through protein S-nitrosylation in the BR signaling pathway might warrant further attention.
3. Brassinosteroids and Ethylene
Ethylene is a simple gaseous plant hormone that consists of two carbon and four hydrogen atoms. It is synthesized in almost all plant tissues and organs. It affects key physiological processes and stress responses in plants. Ethylene biosynthesis begins with methionine and forms the end product through three main steps.
BRs can participate in ethylene biosynthetic genes, signal transduction, and related enzymes. Ethylene can be involved in the growth, development, and stress responses in a BR-dependent way. Given the importance of ethylene for the postharvest of crop products, the interactions between BRs and ethylene have great prospects for the future.
4. Brassinosteroids and Hydrogen Peroxide
H2O2, a crucial small signaling molecule, affects the physiologic and biochemical processes in plants. As an ROS, H2O2 is generated at the cell surface, which may regulate plant growth and stress response at low concentrations. Salama et al. showed that the application of 600 ppm H2O2 increased growth and yield in Solanum tuberosum by enhancing root respiration and the content of chlorophyll and soluble carbohydrates under drought stress [83]. At elevated levels, H2O2 can cause oxidative burst to destroy the structure of some proteins and further interfere with the signal transmission process of cells [40].
BR and H2O2 co-treatment could improve plant resistance to abiotic stresses. In Lycopersicon esculentum, the application of EBR and H2O2 significantly increased SPAD chlorophyll, the net photosynthetic rate, and the activity of carbonic anhydrase and different antioxidant enzymes (CAT and SOD) under cold stress [76]. Heavy metals at high concentrations are harmful to plant tissues and organs. Nazir et al. investigated whether the combination of BRs and H2O2 can reduce the toxicity of Cu in Solanum lycopersicum [40]. They found that the co-treatment of EBR and H2O2 had significantly increased chlorophyll content and Fv/Fm compared with EBR or H2O2 alone. EBR and H2O2 increased the net photosynthetic rate and related traits (the internal carbon dioxide concentration, stomatal conductance, and the transpiration rate) and reduced the electrolyte leakage. Cu treatment decreased the leaf area and dry mass of shoots and roots in tomato seedlings, while the combined application of EBR and H2O2 significantly increased these parameters. Similarly, EBR and H2O2 also modified the chloroplast ultrastructure and stomatal behavior and increased the total protein content and the activities of antioxidant enzymes and carbonic anhydrase in Cu-treated tomato seedlings under Cu stress [40]. Thus, the interaction between BRs and H2O2 might enhance photosynthetic capacity and total protein content and might maintain the antioxidant system and plasma membrane, thereby increasing plant resistance to abiotic stress. In Nicotiana benthamiana, Deng et al. indicated that BRs increased the resistance of TMV [77]. However, pretreatment with dimethylthiourea (DMTU), a scavenger of H2O2, decreased the tolerance of TMV in Nicotiana benthamiana. Therefore, BR-mediated virus resistance requires H2O2, which participates in the regulation of virus resistance. Overall, H2O2 plays an important role in BR-induced growth, development, and stress responses. Additionally, H2O2 might regulate the complex signaling network mechanism as a downstream signaling messenger in BR signaling in the growth and stress responses of plants. However, many theoretical mechanisms of the interaction between BRs and H2O2 are still unclear, so further research and discoveries are needed.
5. Brassinosteroids and Hydrogen Sulfide
H2S is an endogenous biological signal molecule with a unique odor of rotten eggs. H2S is known to be a poisonous gas, and its toxicity has always been a focus of research. In recent years, research on H2S has been increasingly concerned with its roles in plant growth, development, and stress response [84,85]. As a second signaling messenger, the interaction between H2S and BRs might play a crucial role in plants.
H2S might play an irreplaceable role in BR-mediated stomatal movement and the photosynthetic system. Overall, H2S, as a signaling molecule downstream of the BR signaling transduction pathway, participates in plant growth and development, and H2S as a downstream signal molecule in other plant hormones may enhance abiotic stress tolerance, which may be important to provide new insights into how the combined effect of H2S and BRs is involved in abiotic and biotic stress responses in plants.
6. Brassinosteroids and Sphingolipids
Sphingolipids are an essential component of plant biomembranes. Sphingolipids have been extensively studied in animals and yeast and have been proved to be a class of active molecules. Sphingolipids are involved in cell growth, differentiation, senescence, and programmed cell death [88,89]. The roles of sphingolipids in plants have been studied in recent years.
Corbacho et al. observed the interaction between sphingolipids and BRs during the early fleshy-fruit growth in Olea europaea L. The application of exogenous EBR significantly reduced the total content of sphingolipid long-chain base (LCB) and the transcript levels of sphingolipid-related genes {the serine palmitoyltransferase I (OeSPT); sphingosine kinase (OeSPHK); glucosylceramidase (OeGlcCerase)}. However, BRZ application improved the sphingolipid LCB content and the gene expression [80]. Thus, BRs might negatively regulate the content of sphingolipids during fruit development. Sphingolipids could inhibit fruit growth, while BRs can alleviate the negative effects of sphingolipids. The crosstalk between BRs and sphingolipids might be extremely complicated.
This entry is adapted from the peer-reviewed paper 10.3390/biom11121800
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