Mastering Plant Hormones: Insights into Synthesis, Transport, and Cutting-Edge Research

Plant hormones, or phytohormones, are crucial biochemical messengers that profoundly influence plant physiology, governing growth, development, and environmental responses. This article provides an in-depth exploration of the synthesis, transport, and signaling mechanisms of plant hormones, as well as a review of the latest research findings in the field. Aimed at professionals in biotechnology and agriculture, it delivers foundational knowledge and cutting-edge insights essential for advancing crop science and horticultural practices.

I. What is Hormones and PhytoHormones?

Just as animal hormones are chemical messengers that coordinate the activities of various organs, plant hormones, or phytohormones, serve a similar purpose within plants. These substances are pivotal in regulating a multitude of functions from cellular activity and nutritional assimilation to reproductive development and stress responses.

Historically, the study of plant hormones has identified several key types that are essential to plant growth and health. The five typical plant hormones discovered by the mid-20th century—auxins (often referred to as growth hormones), cytokinins, gibberellins, ethylene, and abscisic acid—have been extensively studied for their roles in plant physiology.

More recently, researchers have identified additional phytohormones that play critical roles in plant health and development:

• Oleoresinosterol and Monocotyledonin Lactone: These hormones are newer discoveries that are still being explored for their functions and effects within various plants.
• Jasmonic Acid: Involved in plant immune responses, particularly in response to mechanical injuries and certain pathogenic attacks.
• Salicylic Acid: Plays a crucial role in the plant's disease resistance mechanism, enhancing its ability to fight off pathogens.

II. Hormone synthesis, transport and signaling

(1) Synthesis and accumulation: the synthesis of plant hormones is subject to strict regulation, feedback regulation is the main mechanism of action of plant hormones, divided into positive feedback and negative feedback.

(2) Transportation: plant hormones are transported through the xylem or phloem to play a role in the corresponding parts.

(3) Perception: plant hormones play a role by binding to receptor proteins and through changes in protein denaturation.

(4) Signal transduction: The ubiquitin-mediated protein degradation pathway dependent on the 26S proteasome plays an important role in almost all hormone signaling pathways, and cellular responses to hormones are realized through the ubiquitin protein degradation pathway by regulating the stability of key transcriptional regulators.

III. Functions of hormones

(1) Regulation of plant nutrient growth, reproductive development

(2) Regulation of seed maturation, dormancy and germination

(3) Research in abiotic stress response

(4) Research in biotic stress response

(6) Phytohormone interactions in regulatory networks

IV. Unveiling Plant Hormone Secrets: 3 New Research Studies Explored

Article 1: Unlocking Potato Tuberization: StAST1 Gene Regulation and Phytohormone Insights

The research result entitled 'TCP transcription factor StAST1 represses potato tuberization by regulating tuberigen complex activity (doi.org/10.1093/plphys/kiae138)' was published online on March 15, 2024, in which a new TCP transcription factor, StAST1, was found to repress potato tuberization by regulating tuberigen complex activity, as well as regulating GA response through activating the expression of the potato GA20 oxidase gene (StGA20ox1). The results suggest that StAST1 functions as a tuber formation suppressor by regulating phytohormone levels. MetwareBio provided the phytohormone detection and analysis service for this study.

The emergence of new types of organs is an adaptation of plants to their environment, ensuring that they can cope with unfavorable conditions or survive in extreme environments. Potato forms tubers through underground meristematic or stolon differentiation, which allows the plant to survive cold winters. Potato is a model system for studying the formation of specialized organs underground. Understanding tuberization in potato plants will not only help to secure potato production under climate change conditions, but also contribute to a better understanding of storage organ formation in other plants. Similar to flowering, the stolon expressed flowering locus T-like (FT-like) protein SELF-PRUNING 6A (StSP6A) induces transcriptional reprogramming in the proximal tip of the stolon by binding to the bZIP transcription factors StABI5-like 1 (StABL1) and StFD-like 1 (StFDL1), thus playing an important role in tuber formation process of tuber formation. However, the molecular mechanisms regulating the widely conserved FT-bZIP interaction remain largely unexplored.

1. Silencing of the StAST1 gene leads to tuber formation and shortened life cycle

To investigate the gene function of StAST1 during tuberization, the authors generated StAST1 overexpressing plants, OE-StAST1, and RNA-disturbed plants of StAST1, Ri-StAST1. 4-week-old plants, StAST1-silenced plants tubered within 5 d, while wild-type and overexpressing plants tubered later. After 120 d of growth, the number of tubers of StAST1-silenced plants was significantly lower than that of overexpressed and wild plants. StAST1-silenced and StAST1-overexpressing plants exhibited earlier and later leaf senescence, respectively, and shorter and longer plant life cycles, respectively, compared with the wild type (Fig. 1). Taken together, these findings suggest that StAST1 negatively regulates tuberization time and plant maturity. The authors again assessed the dormancy phenotype of newly harvested tubers. It was found that StAST1 overexpressing tubers released dormancy for the longest time, followed by the wild type, and StAST1-silenced plants sprouted for the shortest time. This suggests that StAST1 also negatively regulates tuber sprouting.

2. activation of StGA20ox1 by StAST1 regulates the GA response in potato

To determine the transcriptional changes of StAST1 in early tuberization, the authors performed transcriptome sequencing of Ri-StAST1 plants and wild-type material. 1,906 differentially expressed genes were identified in Ri-StAST1 plants, and among the hormone-regulated pathways, DEGs were predominantly enriched in the ABA pathway, suggesting that interfering with StAST1 may affect the plant response to ABA. In vitro application of 5 μM ABA significantly promoted early tuberization in Ri-StAST1 plants (Fig. 2) In addition to this, genes related to tuber function, such as StPP2C21 and ATSPS3F-like, and the sugar efflux transporters StSWEET12e and StSWEET11 were up-regulated in silenced StAST1 plants, which together suggest that interference with StAST1 results in multiple differentially expressed genes with the potential to promote ABA. StAST1 results in the up-regulation of several genes with tuberization-promoting functions.

Among the down-regulated expressed genes, StGA20ox1 and StGA20ox3 were significantly down-regulated in silenced plants.GA20ox encodes a key enzyme involved in GA biosynthesis, which catalyzes the production of biologically active direct precursors of GA. Silencing of StGA20ox1 promotes early tuber formation. To investigate the effect of StAST1 on metabolites in the GA synthesis pathway, wild-type and Ri-StAST1 stolon tips were collected and used for GA content determination. Sixteen of the 18 metabolites were detected. The levels of GA20, GA9, and GA19 synthesized by StGA20ox were significantly lower in Ri-StAST1 stolons (Fig. 2). In contrast, biologically active GA (e.g., GA1) was increased, whereas GA3 did not change significantly (Fig. 2). These results suggest that StAST1 acts during tuber formation by influencing the GA synthesis pathway. To test the GA response, GA3 was applied to Ri-StAST1 plants under tuber induction medium, and all Ri-StAST1 plants produced tubers. It was shown that disturbing StAST1 resulted in reduced sensitivity to GA. Subsequently, the authors demonstrated by dual luciferase reporter assay and EMSA that StAST1 specifically binds to StGA20ox1 and activates it.

Article 2: Peroxysomal Metabolic Cascade: SA-IAA Interplay in Rice Germination

On April 2 2024, Developmental Cell published online the research results entitled 'A peroxisomal cinnamate:CoA ligase-dependent phytohormone metabolic cascade in submerged rice germination cascade in submerged rice germination (doi.org/10.1016/j.devcel.2024.03.023)', in which transcriptome, phytohormone, enzyme activity, and subcellular localization analyses revealed two homologous peroxisomal CA-coenzyme a (CoA) ligases (OsCNL1/2), whose gene expression is highly induced under flooded and swollen conditions, to promote SA synthesis and to promote the development of the peroxisomal cinnamate:CoA ligase-dependent phytohormone metabolism. highly induced to promote SA synthesis, and the SA accumulation induced by submerged water uptake promotes IAA catabolism through several GH3 enzymes involved in IAA-amino acid coupling, thereby releasing the inhibitory effect of IAA on germination. MetwareBio provided phytohormone detection and analysis services for this research.

The mechanisms underlying the underwater germination capacity of rice are largely enigmatic, but are key research questions highly relevant to rice cultivation. Furthermore, although rice is known to accumulate salicylic acid (SA), the biosynthesis of salicylic acid is poorly understood and its role in underwater germination is unclear. Peroxisomes are generalized, multifunctional organelles whose metabolism often exhibits species specificity in plants. These organelles are required for germination in oilseed species such as Arabidopsis and soybean and for SA biosynthesis in rice. However, how peroxisome metabolism is involved in high-amylose seeds or underwater seed germination in species is largely unexplored.

In this paper, comparative transcriptome analysis of rice seeds under submerged and aerobic suckering conditions, as well as enzyme and subcellular localization analyses, identified two homologous peroxisomal CA-coenzyme a (CoA) ligases (OsCNL1/2), whose gene expression is highly induced under submerged suckering conditions. We further revealed the role of OsCNLs in SA biosynthesis in the PAL pathway in rice seeds and found that this strong accumulation of OsCNL-dependent SA is essential for submerged germination. Submergence uptake-induced SA accumulation promotes IAA catabolism through several GH3 enzymes involved in IAA-amino acid coupling, thereby releasing the inhibitory effect of IAA on germination. The discovery of this peroxisome-dependent cascade of phytohormone metabolism and the promotion of submerged germination by exogenously applied SA will contribute to future agricultural efforts to improve rice breeding and cultivation.

1. OsCNLs are key enzymes in the submergence-induced SA biosynthesis in rice seeds

To verify whether peroxisomal OsCNLs are involved in the biosynthesis of 4-HBA and/or SA in rice seeds, the authors analyzed the contents of 4-HBA and SA in dry seeds and in seeds submerged in water for 20 hr. In WT, imbibition resulted in a significant increase in 4-HBA and SA content, with greater changes in SA (Figs. 1C and 1D). Surprisingly, after submergence, SA levels in cnl1 cnl2 were reduced by 95% compared to WT, but 4-HBA levels were only reduced by 27% (Figs. 1C and 1D). Based on these observations, it was concluded that the peroxisome CNL plays a more dominant role in SA biosynthesis than 4-HBA production during rice seed submerged germination. It was further hypothesized that since BA is an important intermediate in cellular metabolism, the partitioning of CNL and AIM1 in the peroxisome may help to separate the BA pools in the peroxisome and cytoplasm for a more specific control of SA biosynthesis. To determine whether underwater immersion-induced, OsCNL-dependent SA production is an essential prerequisite for underwater germination, we treated cnl1 cnl2 knockout seeds with SA, which resulted in complete rescue of the severe underwater germination defect (Figs. 1E and 1F). It was further hypothesized that after activation of CA to CACoA by OsCNLs, CA-CoA undergoes b-oxidation via the OsAIM1-dependent pathway to generate BA, which is then converted to SA. Therefore, the shortage of BA in cnl1 cnl2 seeds limits SA biosynthesis during imbibition, and thus an additional supply of BA should reestablish SA biosynthesis.

2. SA upregulates the expression of genes related to hormone metabolism during submerged seed suckering and swelling

To understand the effect of SA on cellular pathways during submerged germination, the authors performed a series of transcriptomic analyses on WT, and cnl1cnl2 knockout seeds, which were in the state of being dry or submerged, with or without exogenous SA, respectively. compared to the dry condition, submerged suckering resulted in the up-regulation of 5,850 genes and the down-regulation of 4,652 genes in the WT seeds, and the up-regulation of 4,560 genes and down-regulation of 3,354 genes in the cnl1 cnl2 4560 genes were up-regulated and 3354 genes were down-regulated, suggesting that the reduction in SA accumulation in cnl1 cnl2 seeds reduced the extent of transcriptome changes to some extent. To determine the functional categories of transcriptomic changes induced by endogenous or exogenous SA dependent on OsCNL, we performed two KEGG enrichment analyses of DEGs induced by the cnl1 cnl2 mutation or exogenous SA using untreated WT seeds as a reference (Fig. 2A). In both analyses, DEGs were enriched in the categories of "phytohormone signaling" and "plant-pathogen interactions," which is consistent with the idea that SA is a major phytohormone involved in plant-pathogen interactions. The authors screened 13 highly expressed genes, including 13 genes, from seed dipping, exogenous SA treatment, and OsCNL-dependent SA produced during seed dipping, which may induce SA signaling, starch degradation, and metabolism of hormones such as JA and IAA.

Article 3: BONZAI Proteins: Key Players in Brassinosteroid Signaling Across Maize and Arabidopsis

On March 8, Nature Communications published online the research results entitled 'Copine proteins are required for brassinosteroid signaling in maize and Arabidopsis (doi.org/10.1038/s41467-024-46289-6)'. The article found that Copine proteins (BONZAI), as new members of the BR receptor protein complex, are found in the monocotyledon maize and dicotyledon Arabidopsis thaliana and play an important role in the BR signaling pathway. This discovery provides a new perspective for understanding the molecular mechanisms of plant growth and development. MetwareBio is proud to contribute to this study with our phytohormone detection and analysis services.

Copine proteins are highly conserved and ubiquitous in eukaryotes, with indispensable roles in different species. However, their exact functions are unknown. The phytohormone oleuropein lactones (BRs) play a crucial role in plant growth and development and environmental response. A key event for effective BR signaling is the formation of a functional BRI1-SERK receptor complex and its transphosphorylation upon ligand binding. Mutations in the Copine gene in the Col-0 ecotype background of Arabidopsis lead to a dwarf phenotype in plants, and the Copine gene was therefore named BONZAI.In this study, knockout mutants of the maize BONZAI homologous genes, ZmBON1 and ZmBON3, were created by CRISPR/Cas9 gene editing. The mutants exhibited a distinct dwarf phenotype and morphological characteristics of BR-deficient mutants. The authors performed BR sensitivity tests on mutants of Arabidopsis BONZAI family members whose biological functions involved in the BR signaling pathway are conserved in the dicotyledonous plant Arabidopsis. Biochemical and molecular analyses showed that the BON proteins directly interact with the SERK kinase, thus ensuring efficient BRI1-SERK interaction and transphosphorylation. This study not only enriches the understanding of BR signaling and provides important targets for optimizing valuable agronomic traits, but also opens up a broader perspective on eukaryotic steroid hormone signaling and copine proteins.

Harnessing Phytohormonal Insights for Advanced Agricultural Practices

In conclusion, the comprehensive understanding of plant hormone dynamics not only enhances fundamental plant science but also supports agricultural innovation, particularly in improving crop resilience and productivity. MetwareBio, as a leading provider of metabolomics and proteomics services, offers sophisticated analytical tools that aid in decoding the complex interactions of phytohormones. Explore the possibilities with MetwareBio and elevate your agricultural methodologies with our state-of-the-art technologies and expert insights.