Plant hormones (Literature sources on phytohormones and plant signalling)

Authors: Yalikunjiang Aizezi, Yinpeng Xie, Hongwei Guo and Kai Jiang.

Life (2022)

Abstract: "The apical hook is formed by dicot seedlings to protect the tender shoot apical meristem during soil emergence. Regulated by many phytohormones, the apical hook has been taken as a model to study the crosstalk between individual signaling pathways. Over recent decades, the roles of different phytohormones and environmental signals in apical hook development have been illustrated. However, key regulators downstream of canonical hormone signaling have rarely been identified via classical genetics screening, possibly due to genetic redundancy and/or lethal mutation. Chemical genetics that utilize small molecules to perturb and elucidate biological processes could provide a complementary strategy to overcome the limitations in classical genetics. In this review, we summarize current progress in hormonal regulation of the apical hook, and previously reported chemical tools that could assist the understanding of this complex developmental process. We also provide insight into novel strategies for chemical screening and target identification, which could possibly lead to discoveries of new regulatory components in apical hook development, or unidentified signaling crosstalk that is overlooked by classical genetics screening."

Authors: Khopeno Khuvung, Federico A. O. Silva Gutierrez and Didier Reinhardt.

Frontiers in Plant Science (2022)

Abstract: "Despite its central role in the control of plant architecture, strigolactone has been recognized as a phytohormone only 15 years ago. Together with auxin, it regulates shoot branching in response to genetically encoded programs, as well as environmental cues. A central determinant of shoot architecture is apical dominance, i.e., the tendency of the main shoot apex to inhibit the outgrowth of axillary buds. Hence, the execution of apical dominance requires long-distance communication between the shoot apex and all axillary meristems. While the role of strigolactone and auxin in apical dominance appears to be conserved among flowering plants, the mechanisms involved in bud activation may be more divergent, and include not only hormonal pathways but also sugar signaling. Here, we discuss how spatial aspects of SL biosynthesis, transport, and sensing may relate to apical dominance, and we consider the mechanisms acting locally in axillary buds during dormancy and bud activation."

Authors: Sumeera Asghar, Yao Xiong, Meng Che, Xingqiang Fan, Hui Li, Yi Wang, Xuefeng Xu, Wei Li and Zhenhai Han.

Plant Cell Reports (2022)

Key message: We have demonstrated that strigolactone inhibitor, Tis108, could be used to improve shoot regeneration of apple, and provided insights into the molecular mechanism of strigolactone-mediated inhibition of adventitious shoot formation. 

Abstract: "Lack of an efficient transformation system largely stagnated the application of transgenic and CRISPR technology in apple rootstock. High shoot regeneration ability is an important basis for establishing an effective transformation system. In this study, we first demonstrated the inhibitory effects of strigolactones on the adventitious shoot formation of apple rootstock M26. Next, we successfully verified that strigolactone-biosynthesis inhibitor, Tis108, could be used to improve the shoot regeneration of woody plants. Our results also suggest strigolactone-biosynthesis gene, MdCCD7, can be a target gene for biotechnological improvements of shoot regeneration capacity. Furthermore, we have employed transcriptome analysis to reveal the molecular mechanism of strigolactone-mediated inhibition of adventitious shoot formation. Differentially expressed genes associated with photosynthesis, secondary growth, and organ development were identified. WGCNA suggests SLs might affect shoot regeneration through interaction with other hormones, especially, auxin, cytokinin, and ethylene. We were able to identify important candidate genes mediating the cross-talk between strigolactone and other hormones during the process of adventitious shoot formation. Overall, our findings not only propose a useful chemical for improving shoot regeneration in practice but also provide insights into the molecular mechanism of strigolactone-mediated inhibition of adventitious shoot formation."

Authors: Jasmina Kurepa and Jan A. Smalle.

International Journal of Molecular Sciences (2022)

Abstract: "The hormones auxin and cytokinin regulate numerous aspects of plant development and often act as an antagonistic hormone pair. One of the more striking examples of the auxin/cytokinin antagonism involves regulation of the shoot/root growth ratio in which cytokinin promotes shoot and inhibits root growth, whereas auxin does the opposite. Control of the shoot/root growth ratio is essential for the survival of terrestrial plants because it allows growth adaptations to water and mineral nutrient availability in the soil. Because a decrease in shoot growth combined with an increase in root growth leads to survival under drought stress and nutrient limiting conditions, it was not surprising to find that auxin promotes, while cytokinin reduces, drought stress tolerance and nutrient uptake. Recent data show that drought stress and nutrient availability also alter the cytokinin and auxin signaling and biosynthesis pathways and that this stress-induced regulation affects cytokinin and auxin in the opposite manner. These antagonistic effects of cytokinin and auxin suggested that each hormone directly and negatively regulates biosynthesis or signaling of the other. However, a growing body of evidence supports unidirectional regulation, with auxin emerging as the primary regulatory component. This master regulatory role of auxin may not come as a surprise when viewed from an evolutionary perspective."

Authors:  Jian You Wang, Justine Braguy and Salim Al-Babili.

Pant Signaling & Behavior (2023)

Abstract: "The cleavage of plant carotenoids leads to apocarotenoids, a group of metabolites including precursors of the hormones strigolactones (SLs) and abscisic acid, regulatory and signaling molecules. Zaxinone is a recently discovered apocarotenoid growth regulator that improves growth and suppress SL biosynthesis in rice (Oryza sativa). To test if zaxinone also counteracts the growth regulatory effects of SLs in rice, we co-supplied zaxinone and the synthetic SL analog rac-GR24 to the rice SL-deficient DWARF17 (d17) mutant. Results showed that co-application of GR24 and zaxinone still rescued d17 phenotype, indicating that zaxinone and GR24 act independently in regulating root and shoot growth and development in rice."

Authors: Yumeng Chen, Jie Liu, Jieshun Lin, Yuda Purwana Roswanjaya, Marcin Nadzieja, Flavien Buron, Wouter Kohlen, Markus Geisler, Jens Stougaard and Dugald Reid.

bioRxiv (2022)

Abstract: "Development of symbiotic root nodules is a cytokinin-dependent process that is critical to nitrogen acquisition in legumes. The extent and manner in which root nodules contribute to whole-plant cytokinin and nitrogen supply signalling is unknown. Using a combination of genetic, biochemical and physiological approaches, we characterised the role of cytokinin synthesis, export and perception in coordination of symbiotic nodule development and shoot growth in the legume Lotus japonicus. LjPup1 encodes a plasma membrane localised cytokinin exporter with isopentenyladenine (iP) and trans-Zeatin (tZ) export capacity. LjPup1 shows a distinct nodule-specific expression pattern with greatest transcript levels detected in mature nodules. Mutants accumulate more isopentenyladenine riboside (iPR) in nodule tissues and demonstrate hallmarks of reduced cytokinin signalling. Despite normal nodule numbers and function, shoot growth is markedly reduced in Ljpup1 mutants, as well as in mutants impaired in tZ biosynthesis. We found symbiotic root nodules contribute to shoot growth via export of active cytokinins. A cytokinin exporter in the purine permease family thus contributes to long-distance cytokinin homeostasis regulating plant development."

Authors: Akie Shimotohno.

Plant Biotechnology (2022)

Abstract: "Unlike animals, terrestrial plants are sessile and able to give rise to new organs throughout their lifetime. In the most extreme cases, they can survive for over a thousand years. With such protracted life cycles, plants have evolved sophisticated strategies to adapt to variable environments by coordinating their morphology as well as their growth, and have consequently acquired a high degree of developmental plasticity, which is supported by small groups of long-lived stem cells found in proliferative centers called meristems. Shoot apical meristems (SAMs) contain multipotent stem cells and provide a microenvironment that ensures both a self-renewable reservoir, to produce primordia and sustain growth, and a differentiating population that develops into all of the above-ground organs of land plants. The homeodomain transcription factor WUSCHEL (WUS) is expressed in the organizing center and acts as a master regulator to govern shoot stem cell homeostasis. In this review, I highlight recent advances in our understanding of the molecular mechanisms and signaling networks that underlie SAM maintenance, and discuss how plants utilize WUS to integrate intrinsic and extrinsic cues."

Authors: Daniel DeGennaro, Ricardo A Urquidi-Camacho, Liang Zhang and Elena D. Shpak.

bioRxiv (2022)

Abstract: "Leaves and flowers are produced by the shoot apical meristem (SAM) at a certain distance from its center, a process that requires the hormone auxin. The amount of auxin and the pattern of its distribution in the initiation zone determine the size and spatial arrangement of organ primordia. Auxin gradients in the SAM are formed by PIN-FORMED (PIN) auxin efflux carriers whose polar localization in the plasma membrane depends on the protein kinase PINOID (PID). Previous work determined that ERECTA family genes (ERfs) control initiation of leaves. ERfs are plasma membrane receptors that enable cell-to-cell communications by sensing extracellular small proteins from Epidermal Patterning Factor/EPF-like (EPF/EPFL) family. Here, we investigate whether ERfs regulate initiation of organs by altering auxin distribution or signaling. Genetic and pharmacological data suggest that ERfs do not regulate organogenesis through PINs while transcriptomics data show ERfs do not alter primary transcriptional responses to auxin. Our results indicate that in the absence of ERf signaling, the peripheral zone cells inefficiently initiate leaves in response to auxin signals and that increased accumulation of auxin in the er erl1 erl2 SAM can partially rescue organ initiation defects. We propose that both auxin and ERfs are essential for leaf initiation, and that they have common downstream targets. Genetic data also indicate that the role of PID in initiation of cotyledons and leaves cannot be attributed solely to regulation of PIN polarity, and PID is likely to have other functions in addition to regulation of auxin distribution."