Plant hormones (Literature sources on phytohormones and plant signalling)

Authors: Martijn de Roij, Jan Willem Borst and Dolf Weijers.

The Plant Cell (2024)

One-sentence summary: The authors review the deeply intertwined roles of proteolytic regulation in the auxin response.

Abstract: "The signaling molecule auxin sits at the nexus of plant biology and coordinates essentially all growth and developmental processes in plants. Auxin molecules are transported throughout plant tissues and are capable of evoking highly specific physiological responses in plant cells by inducing various molecular pathways. In many of these pathways, proteolysis plays a crucial role for correct physiological responses. This review provides a chronology of the discovery and characterisation of the auxin receptor, which is a fascinating example of separate research trajectories ultimately converging on the discovery of a core auxin signaling hub which relies on degradation of a family of transcriptional inhibitor proteins – the Aux/IAAs. Beyond describing the “classical” proteolysis-driven auxin response system, we explore more recent examples of the interconnection of proteolytic systems, which target a range of other auxin signaling proteins, and auxin response. By highlighting these emerging concepts, we provide potential future directions to further investigate the role of protein degradation within the framework of auxin response."

Authors: Changxia Li, Xuemei Hou, Zongxi Zhao, Huwei Liu, Panpan Huang, Meimei Shi, Xuetong Wu, Rong Gao, Zhiya Liu, Lijuan Wei, Yihua Li and Weibiao Liao.

Cellular & Molecular Biology Letters (2024)

Abstract: "In tomato (Solanum lycopersicum), the ripening of fruit is regulated by the selective expression of ripening-related genes, and this procedure is controlled by transcription factors (TFs). In the various plant-specific TF families, the no apical meristem (NAM), Arabidopsis thaliana activating factor 1/2 (ATAF1/2), and cup-shaped cotyledon 2 (CUC2; NAC) TF family stands out and plays a significant function in plant physiological activities, such as fruit ripening (FR). Despite the numerous genes of NAC found in the tomato genome, limited information is available on the effects of NAC members on FR, and there is also a lack of studies on their target genes. In this research, we focus on SlNAP1, which is a NAC TF that positively influences the FR of tomato. By employing CRISPR/Cas9 technology, compared with the wild type (WT), we generated slnap1 mutants and observed a delay in the ethylene production and color change of fruits. We employed the yeast one-hybrid (Y1H) and dual-luciferase reporter (DLR) assays to confirm that SlNAP1 directly binds to the promoters of two crucial genes involved in gibberellin (GA) degradation, namely SlGA2ox1 and SlGA2ox5, thus activating their expression. Furthermore, through a yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BIFC) and luciferase (LUC) assays, we established an interaction between SlNAP1 and SlGID1. Hence, our findings suggest that SlNAP1 regulates FR positively by activating the GA degradation genes directly. Additionally, the interaction between SlNAP1 and SlGID1 may play a role in SlNAP1-induced FR. Overall, our study provides important insights into the molecular mechanisms through which NAC TFs regulate tomato FR via the GA pathway."

Authors: Jihua Ding, Kejing Wang, Shashank Pandey, Mariano Perales, Isabel Allona, Md Rezaul Islam Khan, Victor B. Busov, Rishikesh P. Bhalerao

Journal of Experimental Botany (2024)

Abstract: "Seasonal bud dormancy in perennial woody plants is a crucial and intricate process that is vital for the survival and development of plants. Over the past few decades, significant advancements have been made in understanding many features of bud dormancy, particularly in model species, where certain molecular mechanisms underlying this process have been elucidated. In this review, we provide an overview of recent molecular progress in understanding bud dormancy in trees, with a specific emphasis on the integration of common signaling and molecular mechanisms identified across different tree species. Additionally, we address some challenges that have emerged in the in-depth understanding of bud dormancy and offer insights for future studies."

Authors: Tingliang Xu, Xiaowen Zheng, Yi Yang, Shumin Yang, Xingwan Yi, Chao Yu, Le Luo, Jia Wang, Tangren Cheng, Qixiang Zhang and Huitang Pan.

Planta (2024)

Main conclusion: IAA cooperates with JA to inhibit SA and negatively regulates rose black spot disease resistance. 

Abstract: "Black spot disease caused by the fungus Marssonina rosae is the most prevalent and severe ailment in rose cultivation, leading to the appearance of black spots on leaves and eventual leaf fall, significantly impacting the utilization of roses in gardens. Salicylic acid (SA) and jasmonic acid (JA) are pivotal hormones that collaborate with indole-3 acetic acid (IAA) in regulating plant defense responses; however, the detailed mechanisms underlying the induction of black spot disease resistance by IAA, JA, and SA remain unclear. In this study, transcript analysis was conducted on resistant (R13–54) and susceptible (R12–26) lines following M. rosae infection. In addition, the impact of exogenous interference with IAA on SA- and JA-mediated disease resistance was examined. The continuous accumulation of JA, in synergy with IAA, inhibited activation of the SA signaling pathway in the early infection stage, thereby negatively regulating the induction of effective resistance to black spot disease. IAA administration alleviated the inhibition of SA on JA to negatively regulate the resistance of susceptible strains by further enhancing the synthesis and accumulation of JA. However, IAA did not contribute to the negative regulation of black spot resistance when high levels of JA were inhibited. Virus-induced gene silencing of RcTIFY10A, an inhibitor of the JA signaling pathway, further suggested that IAA upregulation led to a decrease in disease resistance, a phenomenon not observed when the JA signal was inhibited. Collectively, these findings indicate that the IAA-mediated negative regulation of black spot disease resistance relies on activation of the JA signaling pathway."

Authors: Masato Homma, Kiyono Uchida, Takatoshi Wakabayashi, Masaharu Mizutani, Hirosato Takikawa and Yukihiro Sugimoto.

Frontiers in Plant Science (2024)

Abstract: "Strigolactones (SLs), a class of plant apocarotenoids, serve dual roles as rhizosphere-signaling molecules and plant hormones. Orobanchol, a major naturally occurring SL, along with its various derivatives, has been detected in the root exudates of plants of the Fabaceae family. Medicaol, fabacyl acetate, and orobanchyl acetate were identified in the root exudates of barrel medic (Medicago truncatula), pea (Pisum sativum), and cowpea (Vigna unguiculata), respectively. Although the biosynthetic pathway leading to orobanchol production has been elucidated, the biosynthetic pathways of the orobanchol derivatives have not yet been fully elucidated. Here, we report the identification of 2-oxoglutarate-dependent dioxygenases (DOXs) and BAHD acyltransferases responsible for converting orobanchol to these derivatives in Fabaceae plants. First, the metabolic pathways downstream of orobanchol were analyzed using substrate feeding experiments. Prohexadione, an inhibitor of DOX inhibits the conversion of orobanchol to medicaol in barrel medic. The DOX inhibitor also reduced the formation of fabacyl acetate and fabacol, a precursor of fabacyl acetate, in pea. Subsequently, we utilized a dataset based on comparative transcriptome analysis to select a candidate gene encoding DOX for medicaol synthase in barrel medic. Recombinant proteins of the gene converted orobanchol to medicaol. The candidate genes encoding DOX and BAHD acyltransferase for fabacol synthase and fabacol acetyltransferase, respectively, were selected by co-expression analysis in pea. The recombinant proteins of the candidate genes converted orobanchol to fabacol and acetylated fabacol. Furthermore, fabacol acetyltransferase and its homolog in cowpea acetylated orobanchol. The kinetics and substrate specificity analyses revealed high affinity and strict recognition of the substrates of the identified enzymes. These findings shed light on the molecular mechanisms underlying the structural diversity of SLs."

Authors: Zhana Chagan, Genki Nakata, Shin Suzuki, Ayumi Yamagami, Ryo Tachibana, Surina Surina, Shozo Fujioka, Minami Matsui, Tetsuo Kushiro, Takuya Miyakawa, Tadao Asami and Takeshi Nakano.

Plant Physiology (2024)

One-sentence summary: The regulatory factor BIL8 promotes brassinosteroid signaling and plant growth by inhibiting the phosphorylation activity of BIN2 kinase.

Abstract: "Glycogen synthase kinase 3 (GSK3) is an evolutionarily conserved serine/threonine protein kinase in eukaryotes. In plants, the GSK3-like kinase BRASSINOSTEROID-INSENSITIVE 2 (BIN2) functions as a central signaling node through which hormonal and environmental signals are integrated to regulate plant development and stress adaptation. BIN2 plays a major regulatory role in brassinosteroid (BR) signaling and is critical for phosphorylating/inactivating BRASSINAZOLE-RESISTANT 1 (BZR1), also known as BRZ-INSENSITIVE-LONG HYPOCOTYL 1 (BIL1), a master transcription factor of BR signaling, but the detailed regulatory mechanism of BIN2 action has not been fully revealed. In this study, we identified BIL8 as a positive regulator of BR signaling and plant growth in Arabidopsis (Arabidopsis thaliana). Genetic and biochemical analyses showed that BIL8 is downstream of the BR receptor BRASSINOSTEROID-INSENSITIVE 1 (BRI1) and promotes the dephosphorylation of BIL1/BZR1. BIL8 interacts with and inhibits the activity of the BIN2 kinase, leading to the accumulation of dephosphorylated BIL1/BZR1. BIL8 suppresses the cytoplasmic localization of BIL1/BZR1, which is induced via BIN2-mediated phosphorylation. Our study reveals a regulatory factor, BIL8, that positively regulates BR signaling by inhibiting BIN2 activity."

Author: Leiyun Yang

The Plant Cell (2024)

Excerpts: "Simultaneously, low temperature stimulates plant immunity by activating salicylic acid (SA)-mediated signaling (Wu et al. 2019). However, the mechanism by which plants integrate cold signals and immune signaling remain elusive. To investigate this question, Shaoqin Li and colleagues (Li et al. 2024) studied the potential involvement of ICE1 in cold-induced immunity in Arabidopsis."

"To elucidate how ICE1 regulates cold-induced immunity, the authors conducted a yeast-two-hybrid screen and identified NON-EXPRESSER OF PR GENES 1 (NPR1), a master transcriptional co-activator of PATHOGENESIS-RELATED GENE 1 (PR1) in SA-activated immunity, as an ICE1 interactor.....Additionally, the authors found that ICE1 directly binds to the PR1 promoter for gene activation. These results illustrate that ICE1 interacts with NPR1 and is required for SA-mediated immunity by directly promoting PR1 expression."

"These results support the conclusion that ICE1 and TGA3 work synergistically to promote PR1 transcription. This study not only shed light on a new role of ICE1 in SA-mediated immunity at low temperature, but also revealed NPR1-TGA3/ICE1 as an important nexus integrating SA signaling and cold signals in plant immunity (see Figure)."

Authors: Tomoaki Sakamoto, Shuka Ikematsu, Hokuto Nakayama, Terezie Mandáková, Gholamreza Gohari, Takuya Sakamoto, Gaojie Li, Hongwei Hou, Sachihiro Matsunaga, Martin A. Lysak and Seisuke Kimura

Communications Biology (2024)

One-sentence summary: A chromosome-level genome assembly for the amphibious plant Rorippa aquatica uncovers its tetraploid origin and an involvement of ethylene in heterophylly upon submergence.

Abstract: "The ability to respond to varying environments is crucial for sessile organisms such as plants. The amphibious plant Rorippa aquatica exhibits a striking type of phenotypic plasticity known as heterophylly, a phenomenon in which leaf form is altered in response to environmental factors. However, the underlying molecular mechanisms of heterophylly are yet to be fully understood. To uncover the genetic basis and analyze the evolutionary processes driving heterophylly in R. aquatica, we assembled the chromosome-level genome of the species. Comparative chromosome painting and chromosomal genomics revealed that allopolyploidization and subsequent post-polyploid descending dysploidy occurred during the speciation of R. aquatica. Based on the obtained genomic data, the transcriptome analyses revealed that ethylene signaling plays a central role in regulating heterophylly under submerged conditions, with blue light signaling acting as an attenuator of ethylene signal. The assembled R. aquatica reference genome provides insights into the molecular mechanisms and evolution of heterophylly."