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Brassinosteroids (BRs) are a group of polyhydroxylated plant steroid hormones that are crucial for many aspects of a plant’s life. BRs were originally characterized for their function in cell elongation, but it is becoming clear that they play major roles in plant growth, development, and responses to several stresses such as extreme temperatures and drought. A BR signaling pathway from cell surface receptors to central transcription factors has been well characterized. Here, we summarize recent progress toward understanding the BR pathway, including BR perception and the molecular mechanisms of BR signaling. Next, we discuss the roles of BRs in development and stress responses. Finally, we show how knowledge of the BR pathway is being applied to manipulate the growth and stress responses of crops. These studies highlight the complex regulation of BR signaling, multiple points of crosstalk between BRs and other hormones or stress responses, and the finely tuned spatiotemporal regulation of BR signaling.

Plants are vulnerable to a number of abiotic and biotic stresses that cause a substantial decrease in the production of plants. Plants respond to different environmental stresses by experiencing a series of molecular and physiological changes coordinated by various phytohormones. The use of phytohormones to alleviate stresses has recently achieved increasing interest. Brassinosteroids (BRs) are a group of polyhydroxylated steroidal phytohormones that are required for the development, growth, and productivity of plants. These hormones are involved in regulating the division, elongation, and differentiation of numerous cell types throughout the entire plant life cycle. BR studies have drawn the interest of plant scientists over the last few decades due to their flexible ability to mitigate different environmental stresses. BRs have been shown in numerous studies to have a positive impact on plant responses to various biotic and abiotic stresses. BR receptors detect the BR at the cell surface, triggering a series of phosphorylation events that activate the central transcription factor (TF) Brassinazole-resistant 1 (BZR1), which regulates the transcription of BR-responsive genes in the nucleus. This review discusses the discovery, occurrence, and chemical structure of BRs in plants. Furthermore, their role in the growth and development of plants, and against various stresses, is discussed. Finally, BR signaling in plants is discussed.

BRI1-EMS suppressor (BES)/brassinazole-resistant (BZR) family transcription factors are involved in a variety of physiological processes, but the biological functions of some BES/BZR transcription factors remain unknown; moreover, it is not clear if any of these proteins function in the regulation of plant stress responses. Here, wheat (Triticum aestivum) brassinazole-resistant 2 (TaBZR2)-overexpressing plants exhibited drought tolerant phenotypes, whereas down regulation of TaBZR2 in wheat by RNA interference resulted in elevated drought sensitivity. electrophoretic mobility shift assay and luciferase reporter analysis illustrate that TaBZR2 directly interacts with the gene promoter to activate the expression of T. aestivum glutathione s-transferase-1 (TaGST1), which functions positively in scavenging drought-induced superoxide anions (O2 2). Moreover, TaBZR2 acts as a positive regulator in brassinosteroid (BR) signaling. Exogenous BR treatment enhanced TaBZR2-mediated O2 2 scavenging and antioxidant enzyme gene expression. Taken together, we demonstrate that a BES/BZR family transcription factor, TaBZR2, functions positively in drought responses by activating TaGST1 and mediates the crosstalk between BR and drought signaling pathways. Our results thus provide new insights into the mechanisms underlying how BES/BZR family transcription factors contribute to drought tolerance in wheat.

Plants face a variety of abiotic stresses, which generate reactive oxygen species (ROS), and ultimately obstruct normal growth and development of plants. To prevent cellular damage caused by oxidative stress, plants accumulate certain compatible solutes known as osmolytes to safeguard the cellular machinery. The most common osmolytes that play crucial role in osmoregulation are proline, glycine-betaine, polyamines, and sugars. These compounds stabilize the osmotic differences between surroundings of cell and the cytosol. Besides, they also protect the plant cells from oxidative stress by inhibiting the production of harmful ROS like hydroxyl ions, superoxide ions, hydrogen peroxide, and other free radicals. The accumulation of osmolytes is further modulated by phytohormones like abscisic acid, brassinosteroids, cytokinins, ethylene, jasmonates, and salicylic acid. It is thus important to understand the mechanisms regulating the phytohormone-mediated accumulation of osmolytes in plants during abiotic stresses. In this review, we have discussed the underlying mechanisms of phytohormone-regulated osmolyte accumulation along with their various functions in plants under stress conditions.

Plant steroid hormones, brassinosteroids (BRs), play important roles in growth and development. BR signaling controls the activities of BRASSINOSTERIOD INSENSITIVE1-EMS-SUPPRESSOR1/BRASSINAZOLE-RESISTANT1 (BES1/BZR1) family transcription factors. Besides the role in promoting growth, BRs are also implicated in plant responses to drought stress. However, the molecular mechanisms by which BRs regulate drought response have just begun to be revealed. The functions of WRKY transcription factors in BR-regulated plant growth have not been established, although their roles in stress responses are well documented. Here, we found that three Arabidopsis thaliana group III WRKY transcription factors, WRKY46, WRKY54, and WRKY70, are involved in both BR-regulated plant growth and drought response as the wrky46 wrky54 wrky70 triple mutant has defects in BR-regulated growth and is more tolerant to drought stress. RNA-sequencing analysis revealed global roles of WRKY46, WRKY54, and WRKY70 in promoting BR-mediated gene expression and inhibiting drought responsive genes. WRKY54 directly interacts with BES1 to cooperatively regulate the expression of target genes. In addition, WRKY54 is phosphorylated and destabilized by GSK3-like kinase BR-INSENSITIVE2, a negative regulator in the BR pathway. Our results therefore establish WRKY46/54/70 as important signaling components that are positively involved in BR-regulated growth and negatively involved in drought responses.

Rice (Oryza sativa), the staple food for almost half of the world's population, prefers ammonium (NH4 +) as the major nitrogen resource, and while NH4 + has profound effects on rice growth and yields, the underlying regulatory mechanisms remain largely unknown. Brassinosteroids (BRs) are a class of steroidal hormones playing key roles in plant growth and development. In this study, we show that NH4 + promotes BR biosynthesis through miR444 to regulate rice root growth. miR444 targeted five homologous MADS-box transcription repressors potentially forming homologous or heterogeneous complexes in rice. miR444 positively regulated BR biosynthesis through its MADS-box targets, which directly repress the transcription of BR-deficient dwarf 1 (OsBRD1), a key BR biosynthetic gene. NH4 + induced the miR444-OsBRD1 signaling cascade in roots, thereby increasing the amount of BRs, whose biosynthesis and signaling were required for NH4 +-dependent root elongation inhibition. Consistently, miR444-overexpressing rice roots were hypersensitive to NH4 + depending on BR biosynthesis, and overexpression of miR444's target, OsMADS57, resulted in rice hyposensitivity to NH4 + in root elongation, which was associated with a reduction of BR content. In summary, our findings reveal a cross talk mechanism between NH4 + and BR in which NH4 + activates miR444-OsBRD1, an undescribed BR biosynthesis-promoting signaling cascade, to increase BR content, inhibiting root elongation in rice.

Autophagy, an innate cellular destructive mechanism, plays crucial roles in plant development and responses to stress. Autophagy is known to be stimulated or suppressed by multiple molecular processes, but the role of phytohormone signaling in autophagy is unclear. Here, we demonstrate that the transcripts of autophagy-related genes (ATGs) and the formation of autophagosomes are triggered by enhanced levels of brassinosteroid (BR). Furthermore, the BR-activated transcription factor brassinazole-resistant1 (BZR1), a positive regulator of the BR signaling pathway, is involved in BR-induced autophagy. Treatment with BR enhanced the formation of autophagosomes and the transcripts of ATGs in BZR1-overexpressing plants, while the effects of BR were compromised in BZR1-silenced plants. Yeast one-hybrid analysis and chromatin immunoprecipitation coupled with quantitative polymerase chain reaction revealed that BZR1 bound to the promoters of ATG2 and ATG6. The BR-induced formation of autophagosomes decreased in ATG2- and ATG6-silenced plants. Moreover, exogenous application of BR enhanced chlorophyll content and autophagosome formation and decreased the accumulation of ubiquitinated proteins under nitrogen starvation. Leaf chlorosis and chlorophyll degradation were exacerbated in BZR1-silenced plants and the BR biosynthetic mutant d^ im but were alleviated in BZR1- and BZR1-1D-overexpressing plants under nitrogen starvation. Meanwhile, nitrogen starvation-induced expression of ATGs and autophagosome formation were compromised in both BZR1-silenced and d^ im plants but were increased in BZR1- and BZR1-1D-overexpressing plants. Taken together, our results suggest that BZR1-dependent BR signaling up-regulates the expression of ATGs and autophagosome formation, which plays a critical role in the plant response to nitrogen starvation in tomato (Solanum lycopersicum).

Key messagePlant bioregulators play an important role in managing oxidative stress tolerance in plants. Utilizing their ability in stress sensitive crops through genetic engineering will be a meaningful approach to manage food production under the threat of climate change.Exploitation of the plant defense system against oxidative stress to engineer tolerant plants in the climate change scenario is a sustainable and meaningful strategy. Plant bioregulators (PBRs), which are important biotic factors, are known to play a vital role not only in the development of plants, but also in inducing tolerance in plants against various environmental extremes. These bioregulators include auxins, gibberellins, cytokinins, abscisic acid, brassinosteroids, polyamines, strigolactones, and ascorbic acid and provide protection against the oxidative stress-associated reactive oxygen species through modulation or activation of a plant’s antioxidant system. Therefore, exploitation of their functioning and accumulation is of considerable significance for the development of plants more tolerant of harsh environmental conditions in order to tackle the issue of food security under the threat of climate change. Therefore, this review summarizes a new line of evidence that how PBRs act as inducers of oxidative stress resistance in plants and how they could be modulated in transgenic crops via introgression of genes. Reactive oxygen species production during oxidative stress events and their neutralization through an efficient antioxidants system is comprehensively detailed. Further, the use of exogenously applied PBRs in the induction of oxidative stress resistance is discussed. Recent advances in engineering transgenic plants with modified PBR gene expression to exploit the plant defense system against oxidative stress are discussed from an agricultural perspective.

The brassinosteroids (BRs) represent a class of phytohormones, which regulate numerous aspects of growth and development. Here, a det2-9 mutant defective in BR synthesis was identified from an EMS mutant screening for defects in root length, and was used to investigate the role of BR in root development in Arabidopsis. The det2-9 mutant displays a short-root phenotype, which is result from the reduced cell number in root meristem and decreased cell size in root maturation zone. Ethylene synthesis is highly increased in the det2-9 mutant compared with the wild type, resulting in the hyper-accumulation of ethylene and the consequent inhibition of root growth. The short-root phenotype of det2-9 was partially recovered in the det2-9/acs9 double mutant and det2-9/ein3/eil1-1 triple mutant which have defects either in ethylene synthesis or ethylene signaling, respectively. Exogenous application of BR showed that BRs either positively or negatively regulate ethylene biosynthesis in a concentration-dependent manner. Different from the BR induced ethylene biosynthesis through stabilizing ACSs stability, we found that the BR signaling transcription factors BES1 and BZR1 directly interacted with the promoters of ACS7, ACS9 and ACS11 to repress their expression, indicating a native regulation mechanism under physiological levels of BR. In addition, the det2-9 mutant displayed over accumulated superoxide anions (O2-) compared with the wild-type control, and the increased O2- level was shown to contribute to the inhibition of root growth. The BR-modulated control over the accumulation of O2- acted via the peroxidase pathway rather than via the NADPH oxidase pathway. This study reveals an important mechanism by which the hormone cross-regulation between BRs and ethylene or/and ROS is involved in controlling root growth and development in Arabidopsis.

Autophagy is a highly conserved and regulated catabolic process involved in the degradation of protein aggregates, which plays critical roles in eukaryotes. In plants, multiple molecular processes can induce or suppress autophagy but the mechanism of its regulation by phytohormones is poorly understood. Brassinosteroids (BRs) are steroid phytohormones that play crucial roles in plant response to stresses. Here, we investigate the role of BRs in NBR1-dependent selective autophagy in response to chilling stress in tomato. BRs and their signaling element BZR1 can induce autophagy and accumulation of the selective autophagy receptor NBR1 in tomato under chilling stress. Cold increased the stability of BZR1, which was promoted by BRs. Cold- and BR-induced increased BZR1 stability activated the transcription of several autophagy-related genes (ATGs) and NBR1 genes by directly binding to their promoters, which resulted in selective autophagy. Furthermore, silencing of these ATGs or NBR1 genes resulted in a decreased accumulation of several functional proteins and an increased accumulation of ubiquitinated proteins, subsequently compromising BR-induced cold tolerance. These results strongly suggest that BRs regulate NBR1-dependent selective autophagy in a BZR1-dependent manner in response to chilling stress in tomato.