Hormone signaling
Auxin is an important plant hormone that is essential for growth and development due to its effects on organogenesis, morphogenesis, tropisms, and apical dominance. The functional diversity of auxin highlights the importance of its biosynthesis, transport, and associated responses. In this study, we show that a NAC transcription factor, ANAC092 (also named AtNAC2 and ORESARA1), known to positively regulate leaf senescence and contribute to abiotic stress responses, also affects primary root development. Plants overexpressing ANAC092 had altered root meristem lengths and shorter primary roots compared with the wild‐type control. Additionally, expression of the proANAC092::GUS was strongly induced by indole‐3‐acetic acid. Quantitative real‐time RT‐PCR (qRT‐PCR) analysis revealed that the YUCCA2, PIN, and ARF expression levels were downregulated in ANAC092‐overexpressing plants. Moreover, yeast one‐hybrid and chromatin immunoprecipitation assays confirmed that ANAC092 binds to the promoters of AUXIN RESPONSE FACTOR 8 (ARF8) and PIN‐FORMED 4 (PIN4). Furthermore, a dual‐luciferase assay indicated that ANAC092 decreases ARF8 and PIN4 promoter activities. We also applied a CRISPR/Cas9 system to mutate ANAC092. The roots of three of the analyzed mutants were longer than normal. Collectively, our findings indicate that ANAC092 negatively affects root development by controlling the auxin pathway.
The formation of lateral branches has an important and fundamental contribution to the remarkable developmental plasticity of plants, which allows plants to alter their architecture to adapt to the challenging environment conditions. The Gibberellin (GA) phytohormones have been known to regulate the outgrowth of axillary meristems (AMs), but the specific molecular mechanisms remain unclear. Here we show that DELLA proteins regulate axillary bud formation by interacting and regulating the DNA‐binding ability of SQUAMOSA‐PROMOTER BINDING PROTEIN LIKE 9 (SPL9), a microRNA156‐targeted squamosa promoter binding protein‐like transcription factor. SPL9 participates in the initial regulation of axillary buds by repressing the expression of LATERAL SUPPRESSOR (LAS), a key regulator in the initiation of AMs, and LAS contributes to the specific expression pattern of the GA deactivation enzyme GA2ox4, which is specifically expressed in the axils of leaves to form a low‐GA cell niche in this anatomical region. Nevertheless, increasing GA levels in leaf axils by ectopically expressing the GA‐biosynthesis enzyme GA20ox2 significantly impaired axillary meristem initiation. Our study demonstrates that DELLA‐SPL9‐LAS‐GA2ox4 defines a core feedback regulatory module that spatially pattern GA content in the leaf axil and precisely control the axillary bud formation in different spatial and temporal.
Jasmonic acid (JA) and some of its derivatives are important signals in plant stress responses such as biotic or abiotic stresses during alteration of the environment and are signals in several developmental processes such as trichome development, growth/defense balance, root growth, senescence, or light signaling (Wasternack and Hause 2013). Several specific roles of JA are known for proper organ development of flowers. The most prominent example was found by identification of the JA insensitive mutant of Arabidopsis, CORONATINE INSENSITIVE1 (coi1) which is mutated in the F‐Box protein COI1, a component of the JA receptor complex, the Skp1/Cullin/F‐box (SCF) complex. The mutant coi1 is JA‐insensitive and male sterile (cf. rev. Wasternack and Hause 2013). Surprisingly, the corresponding counterpart jai1 of tomato is also JA‐insensitive and altered in the F‐box protein, but is female sterile (cf. rev. Wasternack and Hause 2013).
Adventitious root (AR) formation from leafy stem cuttings is critical for breeding of many forest and horticultural species. In addition to the plant hormone auxin, wound‐induced signaling caused by the cutting excision is also essential for AR initiation. Here we found that reactive oxygen species (ROS) are rapidly generated at the excision site as a wound‐induced signal and propagated throughout the hypocotyl cutting after excision of the Arabidopsis (Arabidopsis thaliana ) primary root. ROS propagation was not observed in the presence of an NADPH oxidase inhibitor (diphenylene iodonium chloride) or in a knockout mutant of the NADPH oxidase gene respiratory burst oxidase homolog protein D (RBOHD ). Respiratory burst oxidase homolog protein D was specifically upregulated in hypocotyl cuttings at 0.5 h post excision (hpe). Together, these data suggest that RBOHD mediates ROS propagation in hypocotyl cuttings. We also found that auxin levels increased significantly in the shoot apex at 5 hpe and at the base of the cutting at 6 hpe; these effects were blocked by treatment with ROS scavengers. Consistent with this, transcript levels of auxin biosynthesis and polar‐transport genes generally increased between 1 to 6 hpe. Collectively, our results suggest that wound‐induced ROS participate in AR induction through regulation of auxin biosynthesis and transport.
Abscisic acid (ABA) is an important phytohormone regulating plant growth, development, and stress responses. It has an essential role in multiple physiological processes of plants, such as stomatal closure, cuticular wax accumulation, leaf senescence, bud dormancy, seed germination, osmotic regulation, and growth inhibition among many others. Abscisic acid controls downstream responses to abiotic and biotic environmental changes through both transcriptional and posttranscriptional mechanisms. During the past 20 years, ABA biosynthesis and many of its signaling pathways have been well characterized. Here we review the dynamics of ABA metabolic pools and signaling that affects many of its physiological functions.
Endophytic fungi can be beneficial to plant growth. However, the molecular mechanisms underlying colonization of Acremonium spp. remain unclear. In this study, a novel endophytic Acremonium strain was isolated from the buds of Panax notoginseng and named Acremonium sp. D212. The Acremonium sp. D212 could colonize the roots of P. notoginseng, enhance the resistance of P. notoginseng to root rot disease, and promote root growth and saponin biosynthesis in P. notoginseng. Acremonium sp. D212 could secrete indole‐3‐acetic acid (IAA) and jasmonic acid (JA), and inoculation with the fungus increased the endogenous levels of IAA and JA in P. notoginseng. Colonization of the Acremonium sp. D212 in the roots of the rice line Nipponbare was dependent on the concentration of methyl jasmonate (MeJA) (2–15 μmol/L) and 1‐naphthalenacetic acid (NAA) (10–20 μmol/L). Moreover, the roots of the JA signaling‐defective coi1‐18 mutant were colonized by Acremonium sp. D212 to a lesser degree than those of the wild‐type Nipponbare and miR393b‐overexpressing lines, and the colonization was rescued by MeJA but not by NAA. It suggests that the cross‐talk between JA signaling and the auxin biosynthetic pathway plays a crucial role in the colonization of Acremonium sp. D212 in host plants.
Brassinosteroids (BRs) are plant‐specific steroid hormones which regulate plant growth, development, and adaptation. Transcriptional regulation plays key roles in plant hormone signaling. A mediator can serve as a bridge between gene‐specific transcription factors and the RNA polymerase machinery, functioning as an essential component in regulating the transcriptional process. However, whether a mediator is involved in BR signaling is unknown. Here, we discovered that Oryza sativa mediator subunit 25 (OsMED25) is an important regulator of rice BR signaling. Phenotypic analyses showed that the OsMED25‐RNAi and osmed25 mutant presented erect leaves, as observed in BR‐deficient mutants. In addition, the OsMED25‐RNAi and osmed25 mutant exhibited decreased BR sensitivity. Genetic analysis indicated that OsMED25‐RNAi could suppress the enhanced BR signaling phenotype of Osbzr1‐D . Further biochemical analysis showed that OsMED25 interacts with OsBZR1 in vivo , and OsMED25 is enriched on the promoter of OsBZR1 target genes. RNA sequencing analysis indicated that OsMED25 affects the expression of approximately 45% of OsBZR1‐regulated genes and mainly functions as a corepressor of OsBZR1. Together, these findings revealed that OsMED25 regulates rice BR signaling by interacting with OsBZR1 and modulating the expression of OsBZR1 target genes, thus expanding our understanding of the roles of mediators in plant hormone signaling.
Fusarium ear rot, caused by Fusarium verticillioides, is a devastating fungal disease in maize that reduces yield and quality; moreover, F. verticillioides produces fumonisin mycotoxins, which pose serious threats to human and animal health. Here, we performed a genome‐wide association study (GWAS) under three environmental conditions and identified 34 single‐nucleotide polymorphisms (SNPs) that were significantly associated with Fusarium ear rot resistance. With reference to the maize B73 genome, 69 genes that overlapped with or were adjacent to the significant SNPs were identified as potential resistance genes to Fusarium ear rot. Comparing transcriptomes of the most resistant and most susceptible lines during the very early response to Fusarium ear rot, we detected many differentially expressed genes enriched for pathways related to plant immune responses, such as plant hormone signal transduction, phenylpropanoid biosynthesis, and cytochrome P450 metabolism. More than one‐fourth of the potential resistance genes detected in the GWAS were differentially expressed in the transcriptome analysis, which allowed us to predict numbers of candidate genes for maize resistance to ear rot, including genes related to plant hormones, a MAP kinase, a PR5‐like receptor kinase, and heat shock proteins. We propose that maize plants initiate early immune responses to Fusarium ear rot mainly by regulating the growth‐defense balance and promoting biosynthesis of defense compounds.
Brassinosteroids (BRs) play important roles in regulating plant reproductive processes. BR signaling or BR biosynthesis null mutants do not produce seeds under natural conditions, but the molecular mechanism underlying this infertility is poorly understood. In this study, we report that outer integument growth and embryo sac development were impaired in the ovules of the Arabidopsis thaliana BR receptor null mutant bri1‐116 . Gene expression and RNA‐seq analyses showed that the expression of INNER NO OUTER (INO ), an essential regulator of outer integument growth, was significantly reduced in the bri1‐116 mutant. Increased INO expression due to overexpression or increased transcriptional activity of BRASSINAZOLE‐RESISTANT 1 (BZR1) in the mutant alleviated the outer integument growth defect in bri1‐116 ovules, suggesting that BRs regulate outer integument growth partially via BZR1‐mediated transcriptional regulation of INO . Meanwhile, INO expression in bzr‐h , a null mutant for all BZR1 family genes, was barely detectable; and the outer integument of bzr‐h ovules had much more severe growth defects than those of the bri1‐116 mutant. Together, our findings establish a new role for BRs in regulating ovule development and suggest that BZR1 family transcription factors might regulate outer integument growth through both BRI1‐dependent and BRI1‐independent pathways.
Crosstalk between plant hormone signaling pathways is vital for controlling the immune response during pathogen invasion. Salicylic acid (SA) and jasmonic acid (JA) often play important but antagonistic roles in the immune responses of higher plants. Here, we identify a basic helix‐loop‐helix transcription activator, OsbHLH6, which confers disease resistance in rice by regulating SA and JA signaling via nucleo‐cytosolic trafficking in rice (Oryza sativa). OsbHLH6 expression was upregulated during Magnaporthe oryzae infection. Transgenic rice plants overexpressing OsbHLH6 display increased JA responsive gene expression and enhanced disease susceptibility to the pathogen. Nucleus‐localized OsbHLH6 activates JA signaling and suppresses SA signaling; however, the SA regulator OsNPR1 (Nonexpressor of PR genes 1) sequesters OsbHLH6 in the cytosol to alleviate its effect. Our data suggest that OsbHLH6 controls disease resistance by dynamically regulating SA and JA signaling.
Hormones are important signaling molecules regulating developmental processes and responses to environmental stimuli in higher plants. Rice endosperm, the portion of the seed surrounding the embryo, is the main determinant of rice grain shape and yield; however, the dynamics and exact functions of phytohormones in developing endosperm remain elusive. Through a systemic study including transcriptome analysis, hormone measurement, and transgene‐based endosperm‐specific expression of phytohormone biosynthetic enzymes, we demonstrated that dynamic phytohormone levels play crucial roles in the developing rice endosperm, particularly in regard to grain shape and quality. We detected diverse, differential, and dramatically changing expression patterns of genes related to hormone biosynthesis and signaling during endosperm development, especially at early developmental stages. Liquid chromatography measurements confirmed the dynamic accumulation of hormones in developing endosperm. Further transgenic analysis performed on plants expressing hormone biosynthesis genes driven by an endosperm‐specific promoter revealed differential effects of the hormones, especially auxin and brassinosteroids, in regulating grain shape and quality. Our studies help elucidate the distinct roles of hormones in developing endosperm and provide novel and useful tools for influencing crop seed shape and yield.
Plants depend on gravity to provide the constant landmark for downward root growth and upward shoot growth. The phytohormone auxin and its cell‐to‐cell transport machinery are central determinants ensuring gravitropic growth. Statolith sedimentation toward gravity is sensed in specialized cells. This positional cue is translated into the polar distribution of PIN auxin efflux carriers at the plasma membrane, leading to asymmetric auxin distribution and consequently, differential growth and organ bending. While we have started to understand the general principles of how primary organs execute gravitropism, we currently lack basic understanding of how lateral plant organs can defy gravitropic responses. Here we briefly review the establishment of the oblique gravitropic set point angle in lateral roots and particularly discuss the emerging role of asymmetric cytokinin signaling as a central anti‐gravitropic signal. Differential cytokinin signaling is co‐opted in gravitropic lateral and hydrotropic primary roots to counterbalance gravitropic root growth.
MYB transcription factors play vital roles in plant growth and metabolism. The phytohormone methyl jasmonate (MeJA) promotes phenolic acid accumulation in the medicinal herb Salvia miltiorrhiza, but the regulatory mechanism is poorly understood. Here, we identified the MeJA‐responsive R2R3‐MYB transcription factor gene SmMYB2 from a transcriptome library produced from MeJA‐treated S. miltiorrhiza hairy roots. SmMYB2 expression was tightly correlated with the expression of key salvianolic acid biosynthetic genes including CYP98A14. SmMYB2 was highly expressed in the periderm of S. miltiorrhiza and SmMYB2 localized to the nucleus. Overexpressing SmMYB2 in S. miltiorrhiza hairy roots significantly increased the levels of salvianolic acids (including rosmarinic acid and salvianolic acid B) by upregulating salvianolic acid biosynthetic genes such as CYP98A14. SmMYB2 binds to the MYB‐binding motifs in the promoter of CYP98A14, as confirmed by a dual‐luciferase assay and electrophoretic mobility shift assays. Anthocyanin contents were significantly higher in SmMYB2‐overexpressing hairy root lines than the control, primarily due to the increased expression of CHI, DFR, and ANS. These findings reveal the novel regulatory role of SmMYB2 in MeJA‐mediated phenolic acid biosynthesis, providing a useful target gene for metabolic engineering and shedding light on the salvianolic acid regulatory network.
High salinity is one of the major limiting factors that reduces crop productivity and quality. Herein, we report that small SALT TOLERANCE ENHANCER1 (STE1) protein without any known conserved domains is required for tomato salt tolerance. Overexpression (OE) of SlSTE1 enhanced the tolerance to multiple chloride salts (NaCl, KCl, and LiCl) and oxidative stress, along with elevated antioxidant enzyme activities, increased abscisic acid (ABA) and chlorophyll contents, and reduced malondialdehyde (MDA) and reactive oxygen species (ROS) accumulations compared to that of wild‐type (WT) plants. Moreover, decreased K+ efflux and increased H+ efflux were detected in the OE plants, which induced a higher K+/Na+ ratio. In contrast, SlSTE1‐RNAi plants displayed decreased tolerance to salt stress. RNA‐seq data revealed 1 330 differentially expressed genes in the OE plants versus WT plants under salt stress, and the transcription of numerous and diverse genes encoding transcription factors, stress‐related proteins, secondary metabolisms, kinases, and hormone synthesis/signaling‐related proteins (notably ABA and 1‐aminocyclopropane‐1‐carboxylate) was greatly elevated. Furthermore, SlSTE1‐OE plants showed increased sensitivity to ABA, and the results suggest that SlSTE1 promotes ABA‐dependent salt stress‐responsive pathways by interacting with SlPYLs and SlSnRK2s. Collectively, our findings reveal that the small SlSTE1 protein confers salt tolerance via ABA signaling and ROS scavenging and improves ion homeostasis in tomato.
Seed dormancy is an adaptive trait in plants. Breaking seed dormancy determines the timing of germination and is, thereby essential for ensuring plant survival and agricultural production. Seed dormancy and the subsequent germination are controlled by both internal cues (mainly hormones) and environmental signals. In the past few years, the roles of plant hormones in regulating seed dormancy and germination have been uncovered. However, we are only beginning to understand how light signaling pathways modulate seed dormancy and interaction with endogenous hormones. In this review, we summarize current views of the molecular mechanisms by which light controls the induction, maintenance and release of seed dormancy, as well as seed germination, by regulating hormone metabolism and signaling pathways.
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