Plant hormones (Literature sources on phytohormones and plant signalling)

Authors: Alok Ranjan, Ragini Sinha, Shambhu Krishan Lal, Sujit Kumar Bishi and Anil Kumar Singh.

Plant Cell Reports (2021)

Abstract: "Aluminium (Al) is one of the most abundant metals in earth crust, which becomes toxic to the plants growing in acidic soil. Phytohormones like ethylene, auxin, cytokinin, abscisic acid, jasmonic acid and gibberellic acid are known to play important role in regulating Al toxicity tolerance in plants. Exogenous applications of auxin, cytokinin and abscisic acid have shown significant effect on Al-induced root growth inhibition. Moreover, ethylene and cytokinin act synergistically with auxin in responding against Al toxicity. A number of studies showed that phytohormones play vital roles in controlling root responses to Al toxicity by modulating reactive oxygen species (ROS) signalling, cell wall modifications, organic acid exudation from roots and expression of Al responsive genes and transcription factors. This review provides a summary of recent studies related to involvement of phytohormone signalling and cross-talk with other pathways in regulating response against Al toxicity in plants."

Authors: Abid Ali, Guy Kateta Malangisha, Haiyang Yang, Chen Li, Chi Wang, Yubin Yang, Ahmed Mahmoud, Jehanzeb Khan, Jinghua Yang, Zhongyuan Hu and Mingfang Zhang.

Agriculture (2021)

Abstract: "Strigolactone (SL) plays essential roles in regulating plant growth, development, and stress response. This study was conducted to evaluate the effect of exogenous SL on watermelon resistance against herbicides penoxsulam (PXL) and bensulfuron-methyl (BSM). These herbicides were found to negatively impact watermelon root growth and photosynthetic pigments, probably due to the ultrastructural damage and cell death in leaf and root tissues under herbicide stresses. The activation of SL-related gene expression suggested that the SL pathway may mitigate herbicide toxicity in watermelon. The exogenous SL dose-dependently reversed the PXL- or BSM-induced antioxidant activity, suggesting that SL may participate in maintaining antioxidant enzyme homeostasis under herbicide stresses. The up-regulation of herbicide metabolization and detoxification-related genes (Cytochrome P450 and Acetolactate Synthase) by exogenous SL also in part explained how this phytohormone alleviates herbicide toxicity in watermelon. Our findings will provide valuable information underlying the regulatory effects of SL on herbicide tolerance in Cucurbitaceae crops."

Authors: Kosuke Mase and Hironaka Tsukagoshi.

Frontiers in Plant Science (2021)

Abstract: "Plant development under altered nutritional status and environmental conditions and during attack from invaders is highly regulated by plant hormones at the molecular level by various signaling pathways. Previously, reactive oxygen species (ROS) were believed to be harmful as they cause oxidative damage to cells; however, in the last decade, the essential role of ROS as signaling molecules regulating plant growth has been revealed. Plant roots accumulate relatively high levels of ROS, and thus, maintaining ROS homeostasis, which has been shown to regulate the balance between cell proliferation and differentiation at the root tip, is important for proper root growth. However, when the balance is disturbed, plants are unable to respond to the changes in the surrounding conditions and cannot grow and survive. Moreover, ROS control cell expansion and cell differentiation processes such as root hair formation and lateral root development. In these processes, the transcription factor-mediated gene expression network is important downstream of ROS. Although ROS can independently regulate root growth to some extent, a complex crosstalk occurs between ROS and other signaling molecules. Hormone signals are known to regulate root growth, and ROS are thought to merge with these signals. In fact, the crosstalk between ROS and these hormones has been elucidated, and the central transcription factors that act as a hub between these signals have been identified. In addition, ROS are known to act as important signaling factors in plant immune responses; however, how they also regulate plant growth is not clear. Recent studies have strongly indicated that ROS link these two events. In this review, we describe and discuss the role of ROS signaling in root development, with a particular focus on transcriptional regulation. We also summarize the crosstalk with other signals and discuss the importance of ROS as signaling molecules for plant root development."

Authors: Pingping Fang, Yu Wang, Mengqi Wang, Feng Wang, Cheng Chi, Yanhong Zhou, Jie Zhou, Kai Shi, Xiaojian Xia, Christine Helen Foyer and Jingquan Yu.

Antioxidants (2021)

Abstract: "Brassinosteroids (BRs) play a critical role in plant responses to stress. However, the interplay of BRs and reactive oxygen species signaling in cold stress responses remains unclear. Here, we demonstrate that a partial loss of function in the BR biosynthesis gene DWARF resulted in lower whilst overexpression of DWARF led to increased levels of C-REPEAT BINDING FACTOR (CBF) transcripts. Exposure to cold stress increased BR synthesis and led to an accumulation of brassinazole-resistant 1 (BZR1), a central component of BR signaling. Mutation of BZR1 compromised the cold- and BR-dependent increases in CBFs and RESPIRATORY BURST OXIDASE HOMOLOG 1(RBOH1) transcripts, as well as preventing hydrogen peroxide (H2O2) accumulation in the apoplast. Cold- and BR-induced BZR1 bound to the promoters of CBF1, CBF3 and RBOH1 and promoted their expression. Significantly, suppression of RBOH1 expression compromised cold- and BR-induced accumulation of BZR1 and related increases in CBF transcripts. Moreover, RBOH1-dependent H2O2 production regulated BZR1 accumulation and the levels of CBF transcripts by influencing glutathione homeostasis. Taken together, these results demonstrate that crosstalk between BZR1 and reactive oxygen species mediates cold- and BR-activated CBF expression, leading to cold tolerance in tomato (Solanum lycopersicum)."

Authors: Hao Li, Yanliang Guo, Zhixiang Lan, Kai Xu, Jingjing Chang, Golam Jalal Ahammed, Jianxiang Ma, Chunhua Wei and Xian Zhang.

Horticultural Research (2021)

Abstract: "Root–shoot communication has a critical role in plant adaptation to environmental stress. Grafting is widely applied to enhance the abiotic stress tolerance of many horticultural crop species; however, the signal transduction mechanism involved in this tolerance remains unknown. Here, we show that pumpkin- or figleaf gourd rootstock-enhanced cold tolerance of watermelon shoots is accompanied by increases in the accumulation of melatonin, methyl jasmonate (MeJA), and hydrogen peroxide (H2O2). Increased melatonin levels in leaves were associated with both increased melatonin in rootstocks and MeJA-induced melatonin biosynthesis in leaves of plants under cold stress. Exogenous melatonin increased the accumulation of MeJA and H2O2 and enhanced cold tolerance, while inhibition of melatonin accumulation attenuated rootstock-induced MeJA and H2O2 accumulation and cold tolerance. MeJA application induced H2O2 accumulation and cold tolerance, but inhibition of JA biosynthesis abolished rootstock- or melatonin-induced H2O2 accumulation and cold tolerance. Additionally, inhibition of H2O2 production attenuated MeJA-induced tolerance to cold stress. Taken together, our results suggest that melatonin is involved in grafting-induced cold tolerance by inducing the accumulation of MeJA and H2O2. MeJA subsequently increases melatonin accumulation, forming a self-amplifying feedback loop that leads to increased H2O2 accumulation and cold tolerance. This study reveals a novel regulatory mechanism of rootstock-induced cold tolerance."

Authors: Yu. E. Kolupaev and T. O. Yastreb.

Applied Biochemistry and Microbiology (2021)

Abstract: "The role of jasmonic acid (JA) and jasmonate signaling in the regulation of plant adaptive responses to the action of stressors is reviewed. The synthesis of JA in plants and the main pathway for the transduction of jasmonate signals are briefly described. The effect of JA on the content of other signaling mediators (calcium ions, reactive oxygen species, nitric oxide, hydrogen sulfide, and carbon monoxide) in cells is surveyed. Data on the involvement of jasmonate signaling components (in particular, COI1 and JIN1/MYC2 proteins) in the physiological effects of signaling mediator gasotransmitters are summarized. Data on changes in the endogenous content of JA under the action of stressors and information on the effect of exogenous JA and its derivatives on the plant resistance are given. The spectrum of jasmonate-dependent defense responses of plants and the mechanisms of their induction are analyzed. Particular attention is given to the role of JA in the activation of the antioxidant system and the regulation of stomata under stress conditions."

Authors:  Yosef Fichman and Ron Mittler.

The Plant Journal (2021)

Abstract: "The sensing of abiotic stress, mechanical injury, or pathogen attack by a single plant tissue results in the activation of systemic signals that travel from the affected tissue to the entire plant. This process is essential for plant survival during stress and is termed systemic signaling. Among the different signals triggered during this process are calcium, electric, reactive oxygen species (ROS) and hydraulic signals. These are thought to propagate at rapid rates through the plant vascular bundles and to regulate many of the systemic processes essential for plant survival. Although the different signals activated during systemic signaling are thought to be interlinked, their coordination and hierarchy remain to be determined. Here, using a combination of advanced whole-plant imaging and hydraulic pressure measurements, we studied the activation of all four systemic signals in wild type and different Arabidopsis thaliana mutants subjected to a local treatment of high light (HL) stress or wounding. Our findings reveal that activation of systemic membrane potential, calcium, ROS, and hydraulic pressure signals, in response to wounding, is dependent on glutamate receptor-like (GLR) proteins 3.3 and 3.6. In contrast, in response to HL stress, systemic changes in calcium and membrane potential depended on GLR 3.3 and 3.6, while systemic hydraulic signals did not. We further show that plasmodesmata functions are required for systemic changes in membrane potential and calcium during responses to HL stress or wounding. Our findings shed new light on the different mechanisms that integrate different systemic signals in plants during stress."

Authors: Francisco Javier Pérez, Ximena Noriega and Sebastián Rubio.

Antioxidants (2021)

Abstract: "Changes in the level of hydrogen peroxide (H2O2) is a good indicator to monitor fluctuations in cellular metabolism and in the stress responses. In this study, the changes in H2O2 content during bud endodormancy (ED) and budbreak were analysed in grapevine (Vitis vinifera L.). The results showed a gradual increase in the H2O2 content during the development of bud ED, which was mainly due to an increase in the activity of peroxidases (PODs). The maximum H2O2 content reached in the grapevine buds coincided with the maximum depth of bud ED. In contrast, during budbreak, the H2O2 content decreased. As the plant hormones cytokinin (CK) and auxin play an important role in budbreak and growth resumption in grapevine, the effect of exogenous applications of H2O2 on the expression of genes involved in CK and auxin metabolism was analysed. The results showed that H2O2 represses the expression of the CK biosynthesis genes VvIPT3a and VvLOG1 and induces the expression of the CK-inactivating gene VvCKX3, thus reducing potentially the CK content in the grapevine bud. On the other hand, H2O2 induced the expression of the auxin biosynthesis genes VvAMI1 and VvYUC3 and of the auxin transporter gene VvPIN3, thus increasing potentially the auxin content and auxin transport in grapevine buds. In general, the results suggest that H2O2 in grapevine buds is associated with the depth of ED and negatively regulates its budbreak."

Authors: Ilaria Fraudentali, Renato A. Rodrigues-Pousada, Riccardo Angelini, Sandip A. Ghuge and Alessandra Cona.

International Journal of Molecular Sciences (2021)

Abstract: "Polyamines are ubiquitous, low-molecular-weight aliphatic compounds, present in living organisms and essential for cell growth and differentiation. Copper amine oxidases (CuAOs) oxidize polyamines to aminoaldehydes releasing ammonium and hydrogen peroxide, which participates in the complex network of reactive oxygen species acting as signaling molecules involved in responses to biotic and abiotic stresses. CuAOs have been identified and characterized in different plant species, but the most extensive study on a CuAO gene family has been carried out in Arabidopsis thaliana. Growing attention has been devoted in the last years to the investigation of the CuAO expression pattern during development and in response to an array of stress and stress-related hormones, events in which recent studies have highlighted CuAOs to play a key role by modulation of a multilevel phenotypic plasticity expression. In this review, the attention will be focused on the involvement of different AtCuAOs in the IAA/JA/ABA signal transduction pathways which mediate stress-induced phenotypic plasticity events."

Authors: Yanning Su, Yizhi Huang, Xintan Dong, Ruijia Wang, Mingyu Tang, Jiabang Cai, Jiayi Chen, Xinquan Zhang and Gang Nie.

Frontiers in Plant Science (2021)

Abstract: "Perennial ryegrass (Lolium perenne L.) is an important cool-season grass species that is widely cultivated in temperate regions worldwide but usually sensitive to heat stress. Jasmonates (JAs) may have a positive effect on plant tolerance under heat stress. In this study, results showed that exogenous methyl jasmonic acid (MeJA) could significantly improve heat tolerance of perennial ryegrass through alteration of osmotic adjustment, antioxidant defense, and the expression of JA-responsive genes. MeJA-induced heat tolerance was involved in the maintenance of better relative water content (RWC), the decline of chlorophyll (Chl) loss for photosynthetic maintenance, as well as maintained lower electrolyte leakage (EL) and malondialdehyde (MDA) content under heat condition, so as to avoid further damage to plants. Besides, results also indicated that exogenous MeJA treatment could increase the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), thus enhancing the scavenging ability of reactive oxygen species, alleviating the oxidative damage caused by heat stress. Heat stress and exogenous MeJA upregulated transcript levels of related genes (LpLOX2, LpAOC, LpOPR3, and LpJMT) in JA biosynthetic pathway, which also could enhance the accumulation of JA and MeJA content. Furthermore, some NAC transcription factors and heat shock proteins may play a positive role in enhancing resistance of perennial ryegrass with heat stress."

Authors: Ralf Oelmüller

International Journal of Molecular Sciences (2021)

Abstract: "Adaptation and response to environmental changes require dynamic and fast information distribution within the plant body. If one part of a plant is exposed to stress, attacked by other organisms or exposed to any other kind of threat, the information travels to neighboring organs and even neighboring plants and activates appropriate responses. The information flow is mediated by fast-traveling small metabolites, hormones, proteins/peptides, RNAs or volatiles. Electric and hydraulic waves also participate in signal propagation. The signaling molecules move from one cell to the neighboring cell, via the plasmodesmata, through the apoplast, within the vascular tissue or—as volatiles—through the air. A threat-specific response in a systemic tissue probably requires a combination of different traveling compounds. The propagating signals must travel over long distances and multiple barriers, and the signal intensity declines with increasing distance. This requires permanent amplification processes, feedback loops and cross-talks among the different traveling molecules and probably a short-term memory, to refresh the propagation process. Recent studies show that volatiles activate defense responses in systemic tissues but also play important roles in the maintenance of the propagation of traveling signals within the plant. The distal organs can respond immediately to the systemic signals or memorize the threat information and respond faster and stronger when they are exposed again to the same or even another threat. Transmission and storage of information is accompanied by loss of specificity about the threat that activated the process. I summarize our knowledge about the proposed long-distance traveling compounds and discuss their possible connections."

Authors: Alexis Porcher, Vincent Guérin, Nathalie Leduc, Anita Lebrec, Jérémy Lothier and Alain Vian.

Plant Physiology (2021)

Abstract: "Rosebush (Rosa ‘Radrazz’) plants are an excellent model to study light control of bud outgrowth since bud outgrowth only arises in the presence of light and never occurs in darkness. Recently, we demonstrated high levels of H2O2 present in the quiescent axillary buds strongly repress the outgrowth process. In light, the outgrowing process occurred after H2O2 scavenging through the promotion of Ascorbic acid-Glutathione (AsA-GSH) dependent pathways and the continuous decrease in H2O2 production. Here we showed Respiratory Burst Oxidase Homologs (RBOH) expression decreased in buds during the outgrowth process in light. In continuous darkness, the same decrease was observed although H2O2 remained at high levels in axillary buds, as a consequence of the strong inhibition of AsA-GSH cycle and GSH synthesis preventing the outgrowth process. Cytokinin (CK) application can evoke bud outgrowth in light as well as in continuous darkness. Furthermore, CKs are the initial targets of light in the photocontrol process. We showed CK application to cultured buds in darkness decreases bud H2O2 to a level that is similar to that observed in light. Furthermore, this treatment restores GSH levels and engages bud burst. We treated plants with buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, to solve the sequence of events involving H2O2/GSH metabolisms in the photocontrol process. This treatment prevented bud burst, even in the presence of CK, suggesting the sequence of actions starts with the positive CK effect on GSH that in turn stimulates H2O2 scavenging, resulting in initiation of bud outgrowth."

Authors:  Xiamusiya Kakan, Yanwen Yu, Shenghui Li, Xiaoying Li, Rongfeng Huang and Juan Wang.

BMC Plant Biology (2021)

Abstract: "Background - Abscisic acid (ABA) plays an important role in plant abiotic stress responses, and ABA INSENSITIVE 4 (ABI4) is a pivotal transcription factor in the ABA signaling pathway. In Arabidopsis, ABI4 negatively regulates salt tolerance; however, the mechanism through which ABI4 regulates plant salt tolerance is poorly understood. Our previous study showed that ABI4 directly binds to the promoter of the VITAMIN C DEFECTIVE 2 (VTC2) gene, inhibiting the transcription of VTC2 and ascorbic acid (AsA) biosynthesis. Results - In the present study, we found that treatment with exogenous AsA could alleviate salt stress sensitivity of ABI4-overexpressing transgenic plants. The decreased AsA content and increased reactive oxygen species (ROS) levels in ABI4-overexpressing seedlings under salt treatment indicated that AsA-promoted ROS scavenging was related to ABI4-mediated salt tolerance. Gene expression analysis showed that ABI4 was induced at the early stage of salt stress, giving rise to reduced VTC2 expression. Accordingly, the abundance of the VTC2 protein decreased under the same salt stress conditions, and was absent in the ABI4 loss-of-function mutants, suggesting that the transcriptional inhibition of ABI4 on VTC2 resulted in the attenuation of VTC2 function. In addition, other encoding genes in the AsA biosynthesis and recycling pathways showed different responses to salt stress, demonstrating that AsA homeostasis is complicated under salinity stress. Conclusions - This study elucidates the negative modulation of ABI4 in salt stress tolerance through the regulation of AsA biosynthesis and ROS accumulation in plants."

Authors: L. O. Omoarelojie, M. G. Kulkarni, J. F. Finnie and J. van Staden.

Plant Growth Regulation (2021)

Abstract: "Strigolactones regulate root development and are known to inhibit adventitious root formation. However, the hormonal targets of strigolactones during adventitious rooting are yet to be determined. As a signal molecule, H2O2 stimulates adventitious rooting. On the other hand, plasma membrane H+-ATPase is involved in several hormonal signals that are linked to root development. This study evaluated the influence of a synthetic strigolactone (rac-GR24), H2O2, and plasma membrane H+-ATPase on adventitious root formation in mung bean hypocotyls. We also determined if H2O2 and plasma membrane H+-ATPases are possible hormonal targets in strigolactone-mediated inhibition of adventitious rooting. The results confirm the inhibitory role of strigolactones and the enhancing influence of H2O2 on adventitious rooting while also suggesting that plasma membrane H+-ATPase activity is necessary and may be employed to regulate adventitious rooting. Also, rac-GR24 diminished endogenous H2O2 content by inhibiting NOX and SOD activities while also inhibiting plasma membrane H+-ATPase activity. Therefore, we conclude that interference with H2O2 signaling and plasma membrane H+-ATPase activity may serve as a potent hormonal mechanism that may be employed by strigolactones to downregulate adventitious rooting in mung bean hypocotyls."