A HAT1-DELLA signaling module regulates trichome initiation and leaf growth by achieving gibberellin homeostasis

Gibberellins regulate masculinization through the SpGAI-SpSTM module in dioecious spinach

The sex of dioecious plants is mainly determined by genetic factors, but it can also be converted by environmental cues such as exogenous phytohormones. Gibberellic acids (GAs) are well-known inducers of flowering and sexual development, yet the pathway of gibberellin-induced sex conversion in dioecious spinach (Spinacia oleracea L.) remains elusive. Based on sex detection before and after GA3 application using T11A and SSR19 molecular markers, we confirmed and elevated the masculinization effect of GA on a single female plant through exogenous applications of GA3, showing complete conversion and functional stamens. Silencing of GIBBERELLIC ACID INSENSITIVE (SpGAI), a single DELLA family protein that is a central GA signaling repressor, results in similar masculinization. We also show that SpGAI can physically interact with the spinach KNOX transcription factor SHOOT MERISTEMLESS (SpSTM), which is a homolog of the flower meristem identity regulator STM in Arabidopsis. The silencing of SpSTM also masculinized female flowers in spinach. Furthermore, SpSTM could directly bind the intron of SpPI to repress SpPI expression in developing female flowers. Overall, our results suggest that GA induces a female masculinization process through the SpGAI-SpSTM-SpPI regulatory module in spinach. These insights may help to clarify the molecular mechanism underlying the sex conversion system in dioecious plants while also elucidating the physiological basis for the generation of unisexual flowers so as to establish dioecy in plants.

The Ins and Outs of Homeodomain-Leucine Zipper/Hormone Networks in the Regulation of Plant Development

The generation of complex plant architectures depends on the interactions among different molecular regulatory networks that control the growth of cells within tissues, ultimately shaping the final morphological features of each structure. The regulatory networks underlying tissue growth and overall plant shapes are composed of intricate webs of transcriptional regulators which synergize or compete to regulate the expression of downstream targets. Transcriptional regulation is intimately linked to phytohormone networks as transcription factors (TFs) might act as effectors or regulators of hormone signaling pathways, further enhancing the capacity and flexibility of molecular networks in shaping plant architectures. Here, we focus on homeodomain-leucine zipper (HD-ZIP) proteins, a class of plant-specific transcriptional regulators, and review their molecular connections with hormonal networks in different developmental contexts. We discuss how HD-ZIP proteins emerge as key regulators of hormone action in plants and further highlight the fundamental role that HD-ZIP/hormone networks play in the control of the body plan and plant growth.

HD-Zip II transcription factors control distal stem cell fate in Arabidopsis roots by linking auxin signaling to the FEZ/SOMBRERO pathway

In multicellular organisms, specialized tissues are generated by specific populations of stem cells through cycles of asymmetric cell divisions, where one daughter undergoes differentiation and the other maintains proliferative properties. In Arabidopsis thaliana roots, the columella - a gravity-sensing tissue that protects and defines the position of the stem cell niche - represents a typical example of a tissue whose organization is exclusively determined by the balance between proliferation and differentiation. The columella derives from a single layer of stem cells through a binary cell fate switch that is precisely controlled by multiple, independent regulatory inputs. Here, we show that the HD-Zip II transcription factors (TFs) HAT3, ATHB4 and AHTB2 redundantly regulate columella stem cell fate and patterning in the Arabidopsis root. The HD-Zip II TFs promote columella stem cell proliferation by acting as effectors of the FEZ/SMB circuit and, at the same time, by interfering with auxin signaling to counteract hormone-induced differentiation. Overall, our work shows that HD-Zip II TFs connect two opposing parallel inputs to fine-tune the balance between proliferation and differentiation in columella stem cells.

Exogenously applied gibberellic acid and benzylamine modulate growth and chemical constituents of dwarf schefflera: a stepwise regression analysis

Ornamental foliage plants that have a dense appearance are highly valued. One way to achieve this is by using plant growth regulators as a tool for plant growth management. In a greenhouse with a mist irrigation system, a study was conducted on dwarf schefflera, an ornamental foliage plant, which was exposed to foliar application of gibberellic acid and benzyladenine hormones. The hormones were sprayed on dwarf schefflera leaves at 0, 100, and 200 mg/l concentrations, at 15-day intervals in three stages. The experiment was conducted as a factorial based on a completely randomized design, with four replicates. The combination of gibberellic acid and benzyladenine at 200 mg/l concentration had a significant effect on leaf number, leaf area, and plant height. The treatment also resulted in the highest content of photosynthetic pigments. Furthermore, the highest soluble carbohydrate to reducing sugars ratio was observed in treatments of 100 and 200 mg/l benzyladenine, and 200 mg/l gibberellic acid + benzyladenine. Stepwise regression analysis showed that root volume was the first variable to enter the model, explaining 44% of variations. The next variable was root fresh weight, and the two-variable model explained 63% of variations in leaf number. The greatest positive effect on leaf number was related to root fresh weight (0.43), which had a positive correlation with leaf number (0.47). The results showed that 200 mg/l concentration of gibberellic acid and benzyladenine significantly improved morphological growth, chlorophyll and carotenoid synthesis, and reducing sugar and soluble carbohydrate contents in dwarf schefflera.

Leaf growth - complex regulation of a seemingly simple process

Understanding the underlying mechanisms of plant development is crucial to successfully steer or manipulate plant growth in a targeted manner. Leaves, the primary sites of photosynthesis, are vital organs for many plant species, and leaf growth is controlled by a tight temporal and spatial regulatory network. In this review, we focus on the genetic networks governing leaf cell proliferation, one major contributor to final leaf size. First, we provide an overview of six regulator families of leaf growth in Arabidopsis: DA1, PEAPODs, KLU, GRFs, the SWI/SNF complexes, and DELLAs, together with their surrounding genetic networks. Next, we discuss their evolutionary conservation to highlight similarities and differences among species, because knowledge transfer between species remains a big challenge. Finally, we focus on the increase in knowledge of the interconnectedness between these genetic pathways, the function of the cell cycle machinery as their central convergence point, and other internal and environmental cues.

The heat response regulators HSFA1s promote Arabidopsis thermomorphogenesis via stabilizing PIF4 during the day

During summer, plants often experience increased light inputs and high temperatures, two major environmental factors with contrasting effects on thermomorphological traits. The integration of light and temperature signaling to control thermomorphogenesis in plants is critical for their acclimation in such conditions, but the underlying mechanisms remain largely unclear. We found that heat shock transcription factor 1d (HSFA1d) and its homologs are necessary for plant thermomorphogenesis during the day. In response to warm daytime temperature, HSFA1s markedly accumulate and move into the nucleus where they interact with phytochrome-interacting factor 4 (PIF4) and stabilize PIF4 by interfering with phytochrome B-PIF4 interaction. Moreover, we found that the HSFA1d nuclear localization under warm daytime temperature is mediated by constitutive photomorphogenic 1-repressed GSK3-like kinase BIN2. These results support a regulatory mechanism for thermomorphogenesis in the daytime mediated by the HSFA1s-PIF4 module and uncover HSFA1s as critical regulators integrating light and temperature signaling for a better acclimation of plants to the summer high temperature.

The DELLA-ABI4-HY5 module integrates light and gibberellin signals to regulate hypocotyl elongation

Plant growth is coordinately controlled by various environmental and hormonal signals, of which light and gibberellin (GA) signals are two critical factors with opposite effects on hypocotyl elongation. Although interactions between the light and GA signaling pathways have been studied extensively, the detailed regulatory mechanism of their direct crosstalk in hypocotyl elongation remains to be fully clarified. Previously, we reported that ABA INSENSITIVE 4 (ABI4) controls hypocotyl elongation through its regulation of cell-elongation-related genes, but whether it is also involved in GA signaling to promote hypocotyl elongation is unknown. In this study, we show that promotion of hypocotyl elongation by GA is dependent on ABI4 activation. DELLAs interact directly with ABI4 and inhibit its DNA-binding activity. In turn, ABI4 combined with ELONGATED HYPOCOTYL 5 (HY5), a key positive factor in light signaling, feedback regulates the expression of the GA2ox GA catabolism genes and thus modulates GA levels. Taken together, our results suggest that the DELLA-ABI4-HY5 module may serve as a molecular link that integrates GA and light signals to control hypocotyl elongation.

The Roles of Gibberellins in Regulating Leaf Development

Plant growth and development are correlated with many aspects, including phytohormones, which have specific functions. However, the mechanism underlying the process has not been well elucidated. Gibberellins (GAs) play fundamental roles in almost every aspect of plant growth and development, including cell elongation, leaf expansion, leaf senescence, seed germination, and leafy head formation. The central genes involved in GA biosynthesis include GA20 oxidase genes (GA20oxs), GA3oxs, and GA2oxs, which correlate with bioactive GAs. The GA content and GA biosynthesis genes are affected by light, carbon availability, stresses, phytohormone crosstalk, and transcription factors (TFs) as well. However, GA is the main hormone associated with BR, ABA, SA, JA, cytokinin, and auxin, regulating a wide range of growth and developmental processes. DELLA proteins act as plant growth suppressors by inhibiting the elongation and proliferation of cells. GAs induce DELLA repressor protein degradation during the GA biosynthesis process to control several critical developmental processes by interacting with F-box, PIFS, ROS, SCLl3, and other proteins. Bioactive GA levels are inversely related to DELLA proteins, and a lack of DELLA function consequently activates GA responses. In this review, we summarized the diverse roles of GAs in plant development stages, with a focus on GA biosynthesis and signal transduction, to develop new insight and an understanding of the mechanisms underlying plant development.

Plant Development and Crop Yield: The Role of Gibberellins

Gibberellins have been classically related to a few key developmental processes, thus being essential for the accurate unfolding of plant genetic programs. After more than a century of research, over one hundred different gibberellins have been described. There is a continuously increasing interest in gibberellins research because of their relevant role in the so-called "Green Revolution", as well as their current and possible applications in crop improvement. The functions attributed to gibberellins have been traditionally restricted to the regulation of plant stature, seed germination, and flowering. Nonetheless, research in the last years has shown that these functions extend to many other relevant processes. In this review, the current knowledge on gibberellins homeostasis and mode of action is briefly outlined, while specific attention is focused on the many different responses in which gibberellins take part. Thus, those genes and proteins identified as being involved in the regulation of gibberellin responses in model and non-model species are highlighted. The present review aims to provide a comprehensive picture of the state-of-the-art perception of gibberellins molecular biology and its effects on plant development. This picture might be helpful to enhance our current understanding of gibberellins biology and provide the know-how for the development of more accurate research and breeding programs.

Salicylic acid-activated BIN2 phosphorylation of TGA3 promotes Arabidopsis PR gene expression and disease resistance

The plant defense hormone, salicylic acid (SA), plays essential roles in immunity and systemic acquired resistance. Salicylic acid induced by the pathogen is perceived by the receptor nonexpressor of pathogenesis-related genes 1 (NPR1), which is recruited by TGA transcription factors to induce the expression of pathogenesis-related (PR) genes. However, the mechanism by which post-translational modifications affect TGA's transcriptional activity by salicylic acid signaling/pathogen infection is not well-established. Here, we report that the loss-of-function mutant of brassinosteroid insensitive2 (BIN2) and its homologs, bin2-3 bil1 bil2, causes impaired pathogen resistance and insensitivity to SA-induced PR gene expression, whereas the gain-of-function mutant, bin2-1, exhibited enhanced SA signaling and immunity against the pathogen. Our results demonstrate that salicylic acid activates BIN2 kinase, which in turn phosphorylates TGA3 at Ser33 to enhance TGA3 DNA binding ability and NPR1-TGA3 complex formation, leading to the activation of PR gene expression. These findings implicate BIN2 as a new component of salicylic acid signaling, functioning as a key node in balancing brassinosteroid-mediated plant growth and SA-induced immunity.

Interaction of BES1 and LBD37 transcription factors modulates brassinosteroid-regulated root forging response under low nitrogen in arabidopsis

Brassinosteriod (BR) plays important roles in regulation of plant growth, development and environmental responses. BR signaling regulates multiple biological processes through controlling the activity of BES1/BZR1 regulators. Apart from the roles in the promotion of plant growth, BR is also involved in regulation of the root foraging response under low nitrogen, however how BR signaling regulate this process remains unclear. Here we show that BES1 and LBD37 antagonistically regulate root foraging response under low nitrogen conditions. Both the transcriptional level and dephosphorylated level of BES1, is significant induced by low nitrogen, predominantly in root. Phenotypic analysis showed that BES1 gain-of-function mutant or BES1 overexpression transgenic plants exhibits progressive outgrowth of lateral root in response to low nitrogen and BES1 negatively regulates repressors of nitrate signaling pathway and positively regulates several key genes required for NO₃ ^- uptake and signaling. In contrast, BES1 knock-down mutant BES1-RNAi exhibited a dramatical reduction of lateral root elongation in response to low N. Furthermore, we identified a BES1 interacting protein, LBD37, which is a negative repressor of N availability signals. Our results showed that BES1 can inhibit LBD37 transcriptional repression on N-responsive genes. Our results thus demonstrated that BES1-LBD37 module acts critical nodes to integrate BR signaling and nitrogen signaling to modulate the root forging response at LN condition.

Genetic transformation of LoHDZ2 and analysis of its function to enhance stress resistance in Larix olgensis

To study the function of LoHDZ2 in larch, we first constructed a VB191103-LoHDZ2::GUS overexpression vector. Through Agrobacterium-mediated infection, the expression vector was transferred into a larch embryogenic cell line. A stable resistant cell line was subsequently screened, and mature embryos were induced to grow until they developed into seedlings. Antagonistic cell lines were identified at both the DNA and RNA levels. The transgenic cell lines were then subjected to GUS staining, and transgenic cell lines were ultimately identified and obtained. These transgenic cell lines were sequenced to identify differentially expressed genes, and a cluster analysis was performed. The resistant cell lines were cultured under stress conditions involving 20% PEG₆₀₀₀ and 200 mM NaCl proliferation media (1/10-BM). After the stress treatment, the contents of peroxidase (POD), malondialdehyde (MDA) and superoxide dismutase (SOD) in both wild-type and transgenic cell lines were measured. The results are summarized below: (1) When the specific fragment of the target gene in the genome of the resistant cell line was amplified. At the RNA level, the expression of the fragment in four resistant lines increased. In addition, GUS staining showed a blue reaction, indicating that LoHDZ2 was successfully integrated into the larch embryonic cell lines. (2) To verify the accuracy and reliability of the transcriptome data, 10 differentially expressed genes (5 upregulated and 5 down regulated genes) were subjected to qRT-PCR verification. The results showed that the expression trend of the 10 differentially expressed genes was the same as that revealed by RNA-Seq, indicating that the transcriptome data were reliable. (3) The transcriptome sequencing showed that 176 genes were upregulated and that 140 genes were down regulated. Through GO enrichment analysis and KEGG metabolic pathway analysis, the screened differentially expressed genes were related to biological processes such as larch metabolism and response to stimuli, indicating that these genes may be closely involved in the regulation of the larch response to external stimuli, including heat stress, drought stress, metal ion stress and bacterial infection, and may participate in the growth process. (4) After 20% PEG₆₀₀₀ treatment, the POD enzyme activity of the transgenic cell line was greater than that of the wild-type; this activity could effectively remove the amount of peroxide produced. The MDA content of the transgenic cell lines was lower than that of the wild-type cell lines, and the accumulation degree of harmful substances was low, indicating that the degree of oxidative damage of the transgenic cell lines was lower than that of the wild-type cell lines. The SOD content of the transgenic cell lines was lower than that of the wild-type cell lines, indicating that the drought resistance of the transgenic cell lines was enhanced. After 200 mM NaCl treatment, although the increase in SOD content was not obvious, the same trend was detected, indicating that the resistance of the transgenic cell lines was indeed stronger than that of the wild-type cell lines. According to the results of previous experiments, after this gene was overexpressed in tobacco, the transformed plants showed obvious dwarfing, which may indicate that the stress resistance of the plant was enhanced. In conclusion, a transgenic larch cell line was successfully obtained, and transgenic larch seedlings were successfully induced. LoHDZ2 may participate in the response of plants to the external environment, and may participate in the growth and development of Larixolgensis by affecting plant metabolic pathways.

The HB40-JUB1 transcriptional regulatory network controls gibberellin homeostasis in Arabidopsis

The gibberellins (GAs) are phytohormones that play fundamental roles in almost every aspect of plant growth and development. Although GA biosynthetic and signaling pathways are well understood, the mechanisms that control GA homeostasis remain largely unclear in plants. Here, we demonstrate that the homeobox transcription factor (TF) HB40 of the HD-Zip family regulates GA content at two additive control levels in Arabidopsis thaliana. We show that HB40 expression is induced by GA and in turn reduces the levels of endogenous bioactive GAs by simultaneously reducing GA biosynthesis and increasing GA deactivation. Consistently, HB40 overexpression leads to typical GA-deficiency traits, such as small rosettes, reduced plant height, delayed flowering, and male sterility. By contrast, a loss-of-function hb40 mutation enhances GA-controlled growth. Genome-wide RNA sequencing combined with molecular-genetic analyses revealed that HB40 directly activates the transcription of JUNGBRUNNEN1 (JUB1), a key TF that represses growth by suppressing GA biosynthesis and signaling. HB40 also activates genes encoding GA 2-oxidases (GA2oxs), which are major GA-catabolic enzymes. The effect of HB40 on plant growth is ultimately mediated through the induction of nuclear growth-repressing DELLA proteins. Collectively, our results reveal the important role of the HB40-JUB1 regulatory network in controlling GA homeostasis during plant growth.