Plant hormones (Literature sources on phytohormones and plant signalling)

Authors: Benjamin Hubert, Olivier Leprince and Julia Buitink.

Journal of Experimental Botany (2024)

Abstract: "To ensure their vital role in disseminating the species, dormant seeds have developed adaptive strategies to protect themselves against pathogens and predators. This is orchestrated through the synthesis of an array of constitutive defenses that are put in place in a developmentally regulated manner, which are the focus of this review. We summarize the defense activity and the nature of the molecules coming from the exudate of imbibing seeds that leak into its vicinity, also referred to as the spermosphere. As a second layer of protection, the dual role of the seed coat will be discussed; as a physical barrier and a multi-layered reservoir of defense compounds that are synthesized during seed development. Since imbibed dormant seeds can persist in the soil for extended times, we address the question if during this period, a constitutively regulated defense program is switched on to provide further protection, using the well-defined pathogenesis-related (PR) protein family. In addition, we review the hormonal and signaling pathways that might be involved in the interplay between dormancy and defense and point out questions that need further attention."

Authors: Jiahong Chen, Lei Yang, Hehua Zhang, Junbin Ruan and Yuan Wang.

Plant Cell Reports (2024) 

Key message: The sugar supply in the medium affects the apical hook development of Arabidopsis etiolated seedlings. In addition, we provided the mechanism insights of this process. 

Abstract: "Dicotyledonous plants form an apical hook structure to shield their young cotyledons from mechanical damage as they emerge from the rough soil. Our findings indicate that sugar molecules, such as sucrose and glucose, are crucial for apical hook development. The presence of sucrose and glucose allows the apical hooks to be maintained for a longer period compared to those grown in sugar-free conditions, and this effect is dose-dependent. Key roles in apical hook development are played by several sugar metabolism pathways, including oxidative phosphorylation and glycolysis. RNA-seq data revealed an up-regulation of genes involved in starch and sucrose metabolism in plants grown in sugar-free conditions, while genes associated with phenylpropanoid metabolism were down-regulated. This study underscores the significant role of sugar metabolism in the apical hook development of etiolated Arabidopsis seedlings."

Authors: Kiyoshi Yamazaki, Yoshihiro Ohmori, Hirokazu Takahashi, Atsushi Toyoda, Yutaka Sato, Mikio Nakazono and Toru Fujiwara.

Plant and Cell Physiology (2024)

Abstract: "Nutritropism is a positive tropism toward nutrients in plant roots. An NH4+ gradient is a nutritropic stimulus in rice (Oryza sativa L.). When rice roots are exposed to an NH4+ gradient generated around nutrient sources, root tips bend toward and coil around the sources. The molecular mechanisms are largely unknown. Here, we analyzed the transcriptomes of the inside and outside of bending root tips exhibiting nutritropism to reveal nutritropic signal transduction. Tissues facing the nutrient sources (inside) and away (outside) were separately collected by laser microdissection. Principal component analysis revealed distinct transcriptome patterns between the two tissues. Annotations of 153 differentially expressed genes implied that auxin, gibberellin and ethylene signaling were activated differentially between the sides of the root tips under nutritropism. Exogenous application of transport and/or biosynthesis inhibitors of these phytohormones largely inhibited the nutritropism. Thus, signaling and de novo biosynthesis of the three phytohormones are necessary for nutritropism. Expression patterns of IAA genes implied that auxins accumulated more in the inside tissues, meaning that ammonium stimulus is transduced to auxin signaling in nutritropism similar to gravity stimulus in gravitropism. SAUR and expansin genes, which are known to control cell wall modification and to promote cell elongation in shoot gravitropism, were highly expressed in the inside tissues rather than the outside tissues, and our transcriptome data are unexplainable for differential elongation in root nutritropism."

Authors: Kenshiro Watanabe, Kenji Hashimoto, Kota Hasegawa, Hiroki Shindo, Yushin Tsuruda, Kamila Kupisz, Mateusz Koselski, Piotr Wasko, Kazimierz Trebacz and Kazuyuki Kuchitsu.

Plant and Cell Physiology (2024)

Abstract: "In response to both biotic and abiotic stresses, vascular plants transmit long-distance Ca2+ and electrical signals from localized stress sites to distant tissues through their vasculature. Various models have been proposed for the mechanisms underlying the long-distance signaling, primarily centered around the presence of vascular bundles. We here demonstrate that the non-vascular liverwort Marchantia polymorpha possesses a mechanism for propagating Ca2+ waves and electrical signals in response to wounding. The propagation velocity of these signals was approximately 1–2 mm s-1, equivalent to that observed in vascular plants. Both Ca2+ waves and electrical signals were inhibited by La3+ as well as tetraethylammonium chloride, suggesting the crucial importance of both Ca2+ channel(s) and K+ channel(s) in wound-induced membrane depolarization as well as the subsequent long-distance signal propagation. Simultaneous recordings of Ca2+ and electrical signals indicated a tight coupling between the dynamics of these two signaling modalities. Furthermore, molecular genetic studies revealed that a GLUTAMATE RECEPTOR-LIKE (GLR) channel plays a central role in the propagation of both Ca2+ waves and electrical signals. Conversely, none of the three two-pore channels were implicated in either signal propagation. These findings shed light on the evolutionary conservation of rapid long-distance Ca2+ wave and electrical signal propagation involving GLRs in land plants, even in the absence of vascular tissue."

Authors: Takuya Uragami, Takatoshi Kiba, Mikiko Kojima, Yumiko Takebayashi, Yuzuru Tozawa, Yuki Hayashi, Toshinori Kinoshita and Hitoshi Sakakibara.

bioRxiv (2024)

Abstract: "The directional and sequential flow of cytokinin in plants is organized by a complex network of transporters. Genes involved in several aspects of cytokinin transport have been characterized, but a large part of the elaborate system remains elusive. In this study, we have identified ABCC4 as a cytokinin efflux transporter gene. Using a transient expression system in tobacco leaves, we screened Arabidopsis transporter genes and isolated ATP-BINDING CASSETTE TRANSPORTER C4 (ABCC4). Further validation through drug-induced expression in Arabidopsis and heterologous expression in budding yeast revealed that ABCC4 effluxes the active form of cytokinins. During the seedling stage, ABCC4 was highly expressed in roots, and its expression was up-regulated in response to cytokinin application. Loss-of-function mutants of ABCC4 displayed enhanced primary root elongation, similar to mutants impaired in cytokinin biosynthesis or signaling, which was suppressed by exogenous trans-zeatin treatment. In contrast, overexpression of the gene led to suppression of root elongation. These results suggest that ABCC4 plays a role in the efflux of active cytokinin, thereby contributing to root growth regulation. Our findings contribute to unraveling the many complexities of cytokinin flow and enhance our understanding of the regulatory mechanisms underlying root system development in plants."

Authors: Ivan Kulich, Julia Schmid, Anastasia Teplova, Linlin Qi and Jiří Friml.

eLife (2024)

One-sentence summary: "Negative Gravitropic Response of roots (NGRs), pivotal for root gravitropic bending, are indispensable for the gravity-induced translocation of D6 protein kinase, a key regulator of PIN3 auxin efflux carrier activity."

Abstract: "Root gravitropic bending represents a fundamental aspect of terrestrial plant physiology. Gravity is perceived by sedimentation of starch-rich plastids (statoliths) to the bottom of the central root cap cells. Following gravity perception, intercellular auxin transport is redirected downwards leading to an asymmetric auxin accumulation at the lower root side causing inhibition of cell expansion, ultimately resulting in downwards bending. How gravity-induced statoliths repositioning is translated into asymmetric auxin distribution remains unclear despite PIN auxin efflux carriers and the Negative Gravitropic Response of roots (NGR) proteins polarize along statolith sedimentation, thus providing a plausible mechanism for auxin flow redirection. In this study, using a functional NGR1-GFP construct, we visualized the NGR1 localization on the statolith surface and plasma membrane (PM) domains in close proximity to the statoliths, correlating with their movements. We determined that NGR1 binding to these PM domains is indispensable for NGR1 functionality and relies on cysteine acylation and adjacent polybasic regions as well as on lipid and sterol PM composition. Detailed timing of the early events following graviperception suggested that both NGR1 repolarization and initial auxin asymmetry precede the visible PIN3 polarization. This discrepancy motivated us to unveil a rapid, NGR-dependent translocation of PIN-activating AGCVIII kinase D6PK towards lower PMs of gravity-perceiving cells, thus providing an attractive model for rapid redirection of auxin fluxes following gravistimulation."

Author: Masatsugu Toyota.

Plant and Cell Physiology (2024)

Excerpts: "The precise nature of this rapid, long-range communication system remains incompletely understood in plants. However, events such as the propagation of increases in cytosolic Ca2+ concentration ([Ca2+]cyt), reactive oxygen species (ROS) and electrical signals spatiotemporally correlate with the systemic spread of wound responses (Farmer et al. 2020, Suda and Toyota 2022)."

"In this issue, Watanabe et al. (2024) simultaneously recorded changes in [Ca2+]cyt and surface potential in M. polymorpha and demonstrated a spatiotemporal coupling between long-range Ca2+ and electrical signals in response to mechanical injury and the underlying molecular machinery conserved in land plants."

"Mechanical wounding of thallus branch 1 caused an immediate increase in [Ca2+]cyt in the wounded region, and this [Ca2+]cyt change was transmitted within 1–2 min to distal unwounded branch 2 (Fig. 1). Interestingly, this Ca2+ wave did not extend beyond the intermediate site to branches 3 and 4, originating from distinct meristematic zones (Fig. 1)."

"In summary, Watanabe et al. (2024) experimentally demonstrated that the non-vascular liverwort Marchantia propagates Ca2+ and electrical signals in response to wounding, with velocities similar to those observed in vascular plants (i.e. 1–2 mm/s). Pharmacological and genetic analyses suggest that these signals were tightly coupled and required MpGLR but not MpTPC (Fig. 1), supporting the concept that GLR-mediated long-range signaling is widely conserved in land plants."

Authors: s Qi Wu, Jinyan Xu, Yingdi Zhao, Yuancong Wang, Ling Zhou, Lihua Ning, Sergey Shabala and Han Zhao.

Plant Physiology (2024)

One-sentence summary: An ethylene response transcription factor plays an important role in the nitrogen-signaling network and in optimizing nitrate uptake in maize.

Abstract: "Maize (Zea mays L.) has very strong requirements for nitrogen. However, the molecular mechanisms underlying the regulations of nitrogen uptake and translocation in this species are not fully understood. Here, we report that an APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) transcription factor ZmEREB97 functions as an important regulator in the N-signaling network in maize. Predominantly expressed and accumulated in main root and lateral root primordia, ZmEREB97 rapidly responded to nitrate treatment. By overlapping the analyses of differentially expressed genes and conducting a DAP-seq assay, we identified 1446 potential target genes of ZmEREB97. Among these, 764 genes were co-regulated in two lines of zmereb97 mutants. Loss of function of ZmEREB97 substantially weakened plant growth under both hydroponic and soil conditions. Physiological characterization of zmereb97 mutant plants demonstrated that reduced biomass and grain yield were both associated with reduced nitrate influx, decreased nitrate content and less N accumulation. We further demonstrated that ZmEREB97 directly targets and regulates the expression of six ZmNRT genes by binding to the GCC box-related sequences in gene promoters. Collectively, these data suggest that ZmEREB97 is a major positive regulator of the nitrate response and that it plays an important role in optimizing nitrate uptake, offering a target for improvement of nitrogen use efficiency in crops."