MdGH3.6 is targeted by MdMYB94 and plays a negative role in apple water-deficit stress tolerance

Involvement of endogenous IAA and ABA in the regulation of arbuscular mycorrhizal fungus on rooting of tea plant (Camellia sinensis L.) cuttings

BACKGROUND

Adventitious root (AR) formation is the key step for successful cutting propagation of tea plants (Camellia sinensis L.). Studies showed that arbuscular mycorrhizal fungus (AMF) can promote the rooting ability, and auxin pathway in basal stem of cuttings was involved in this process. However, auxin and abscisic acid (another important regulator on AR formation) in the other parts of cuttings at different rooting stages responding to AMF inoculation are not well studied. Therefore, in this paper, contents, enzymes and genes related to these two plant hormones were comprehensively determined aiming to unveil how endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) involve in the AMF regulating AR development of tea cuttings.

RESULTS

Inoculating with AMF significantly increased the proportion of cuttings at S2 stage (AR formation), which was more than twice as much as the control. And the total rooting rate in mycorrhizal treatment was also higher than that in the control with an increase of 8.66%. Enzyme activity assays showed that except for decreased polyphenol oxidase (PPO) activity at the S3 stage and peroxidase (POD) activity in middle stem of S3 stage, AMF inoculation increased activities of POD, PPO, superoxide dismutase (SOD) and catalase (CAT) to varying degrees in leaf, middle stem and basal stem of tea cuttings. After inoculation with AMF, the indoleacetic acid oxidase (IAAO) activity decreased to a certain extent in the first three stages of tea cuttings, which showed a trend of 'low-high-low' in the basal stem of all treatments. Besides, there was a significantly positive correlation between SOD activity and AR formation, especially for the proportion of cuttings at S2 and S3 stages. Higher IAA level and IAA/ABA ratio was found in basal stem of cuttings at S1 stage induced by AMF, which promoting the AR formation as revealed by correlation analysis. At the same time, AMF significantly elevated the level of IAA in leaf at S1 stage. By screening differentially expressed genes (DEGs) related to IAA and ABA pathways, together with redundant analysis, it was indicated that auxin biosynthesis and transport, as well as ABA transport and signal transduction, were involved in AMF regulating the rooting of tea cuttings.

CONCLUSIONS

Overall, both endogenous IAA and ABA played roles in the regulation of AR formation of tea cuttings by AMF inoculating, enriching the theoretical basis of AMF regulating rooting of cuttings and providing foundations for cutting propagation of tea plants.

Natural variation in an HD-ZIP factor identifies its role in controlling apple leaf cuticular wax deposition

Natural variation is an invaluable genetic resource for plant trait improvement. Here, we performed a genome-wide association study (GWAS) analysis and identified MdHDG5, which controls apple leaf cuticular wax. An A-to-G single-nucleotide polymorphism (SNP) on the HDG5 promoter is associated with HDG5 expression and hexacosanol content (a component of leaf cuticular wax). Furthermore, the single-nucleotide variation (G/G) within a MYB cis-regulatory element (CRE) can be directly bound by MYB62, which represses HDG5 expression and leaf wax deposition. In addition, MdPIAL2, a Small Ubiquitin-like Modifier (SUMO) E3 ligase, positively controls apple leaf wax deposition by stabilizing MdHDG5, while MdMIEL1 interacts with and degrades both MdHDG5 and MdPIAL2 to negatively control leaf wax deposition. Notably, MIEL1 expression is negatively associated with leaf hexacosanol deposition. Taken together, our results provide significant genetic insights into the natural variation of leaf cuticular wax loads in apple and identify the intricate molecular regulation of MdHDG5.

Comparative transcriptomic and hormonal analyses reveal potential regulation networks of adventitious root formation in Metasequoia glyptostroboides Hu et Cheng

BACKGROUND

The extract from Metasequoia glyptostroboides Hu et Cheng, a rare and endangered species native to China, exhibits numerous biological and pharmacological activities. The species is recalcitrant to rooting during micropropagation, a challenge that has yet to be resolved. In this study, transcriptomic and hormonal analyses were conducted to appreciate the molecular mechanism of adventitious root (AR) formation in optimized rooting conditions.

RESULTS

The use of 2/5-strength Woody Plant Medium (WPM) significantly promoted AR formation of M. glyptostroboides shoots while the content of endogenous auxin, cytokinins and gibberellins (GAs) varied at different stages of AR formation. Transcriptomic analysis showed the significant up- or down-regulation of differentially expressed genes (DEGs) associated with plant hormone signal transduction and the phenylpropanoid biosynthesis pathway in response to 2/5-strength WPM. DEGs related to the biosynthesis of indole-3-acetic acid, cytokinins and GAs were identified. Transcript factors involved in 13 families were also revealed. A weighted gene co-expression network analysis indicated a strong correlation between hormones and genes involved in plant hormone signal transduction and the phenylpropanoid biosynthetic pathway.

CONCLUSIONS

These results indicate that the AR-promoting potential of 2/5-strength WPM in M. glyptostroboides was due to complex interactions between hormones and the expression of genes related to plant hormone signal transduction and the phenylpropanoid biosynthetic pathway.

Understory Environmental Conditions Drive Leaf Level-Lipid Biosynthesis in a Deciduous and Evergreen Tree Species

Plants in the understory experience climatic conditions affected by the overstory canopy that influence physiological and biochemical processes. Here, we investigate the relationships of leaf lipid molecular abundances to leaf water content, transmitted irradiance, and free-air temperature (Tair) from deciduous angiosperm (Quercus buckleyi) and evergreen gymnosperm (Juniperus ashei) understory trees across an elevation gradient in a central Texas (USA) woodland. Monthly sampling from 04/2019 to 01/2020 revealed that long-chain leaf waxes (≥ C27) accumulated with leaf water deficit over the growing season for both tree species. Higher transmitted light during the hottest, driest months was due to a decreased leaf area index (LAI) in the canopy as leaf shedding is a common drought response. Isoprenoids (sesqui-, di-terpenoids, phytosterols) in leaves changed by month with changing LAI and transmittance associated with monthly Tair changes. The chain length of n-alkanols in Q. buckleyi shifted with seasonal LAI at different topographic positions. The unsaturation of fatty acids in both tree species decreased with increased seasonal Tair but showed topography sensitivity. Leaf-level metabolites responded to understory microclimatic variables that were influenced by seasonality and topography.

Promoting γ-aminobutyric acid accumulation to enhances saline-alkali tolerance in tomato

Saline-alkali stress is a widely distributed abiotic stress that severely limits plant growth. γ-Aminobutyric acid (GABA) accumulates rapidly in plants under saline-alkali stress, but the underlying molecular mechanisms and associated regulatory networks remain unclear. Here, we report a MYB-like protein, I-box binding factor (SlMYBI), which positively regulates saline-alkali tolerance through induced GABA accumulation by directly modulating the glutamate decarboxylase (GAD) gene SlGAD1 in tomato (Solanum lycopersicum L.). Overexpression of SlGAD1 increased GABA levels and decreased reactive oxygen species accumulation under saline-alkali stress, while silencing of SlGAD1 further suggested that SlGAD1 plays an active role in GABA synthesis and saline-alkali tolerance of tomato. In addition, we found that SlMYBI activates SlGAD1 transcription. Both overexpression of SlMYBI and editing of SlMYBI using CRISPR-Cas9 showed that SlMYBI regulates GABA synthesis by modulating SlGAD1 expression. Furthermore, the interaction of SlNF-YC1 with SlMYBI enhanced the transcriptional activity of SlMYBI on SlGAD1 to further improve saline-alkali tolerance in tomato. Interestingly, we found that ethylene signaling was involved in the GABA response to saline-alkali stress by RNA-seq analysis of SlGAD1-overexpressing lines. This study elucidates the involvement of SlMYBI in GABA synthesis regulation. Specifically, the SlMYBI-SlNF-YC1 module is involved in GABA accumulation in response to saline-alkali stress.

The yellowhorn MYB transcription factor MYB30 is required for wax accumulation and drought tolerance

Yellowhorn (Xanthoceras sorbifolium Bunge) is an economically important tree species in northern China, mainly distributed in arid and semi-arid areas where water resources are scarce. Drought affects its yield and the expansion of its suitable growth area. It was found that the wax content in yellowhorn leaves varied significantly among different germplasms, which had a strong correlation with the drought resistance of yellowhorn. In this study, XsMYB30 was isolated from 'Zhongshi 4' of yellowhorn, a new highly waxy variety. DAP-Seq technology revealed that the pathways associated with fatty acids were significantly enriched in the target genes of XsMYB30. Moreover, the results of electrophoretic mobility shift assay, yeast one hybrid assay and dual-luciferase assay demonstrated that XsMYB30 could directly and specifically bind with the promoters of genes involved in wax biosynthesis (XsFAR4, XsCER1 and XsKCS1), lipid transfer (XsLTPG1 and XsLTP1) and fatty acid synthesis (XsKASIII), thus enhancing their expression. In addition, the overexpression of XsMYB30 in poplar promoted the expression levels of these target genes and increased the wax deposition on poplar leaves leading to a notable improvement in the plant's ability to withstand drought. These findings indicate that XsMYB30 is an important regulatory factor in cuticular wax biosynthesis and the drought resistance of yellowhorn.

Cotton HD-Zip I transcription factor GhHB4-like regulates the plant response to salt stress

Soil salinity is a major environmental constraint to plant production. The homeodomain-leucine zipper I (HD-Zip I) transcription factors play a crucial role in growth, development and defence responses of plants. However, the function and underlying mechanism of HD-Zip I in cotton remain unexplored. This study investigated the role of GhHB4-like, a cotton HD-Zip I gene, in plant tolerance to salt stress. Ectopic expression of GhHB4-like gene enhanced, while its silencing impaired the salt tolerance in Arabidopsis. Y1H and effector-reporter assays revealed that GhHB4-like activated the expression of GhNAC007, which is essential for salt resistance. Knock-down of GhNAC007 also impaired salt resistance of cotton plants. In addition, GhHB4-like-GhNAC007 might have positively regulated the expression of GhMYB96 and ABA signalling-related genes, thereby leading to enhanced salt resistance. Interestingly, deleting motifs 3 and 5 near the 3'-end of GhHB4-like significantly enhanced GhNAC007 activation, indicating that both motifs acted as transcriptional activation inhibitory domains. The results suggest that GhHB4-like-GhNAC007 regulated plant response to salt stress, potentially by modulating GhMYB96 and ABA signalling-related genes.

The auxin-responsive CsSPL9-CsGH3.4 module finely regulates auxin levels to suppress the development of adventitious roots in tea (Camellia sinensis)

The cutting technique is extensively used in tea breeding, with key emphasis on promoting the growth of adventitious roots (ARs). Despite its importance in tea cultivation, the mechanisms underlying AR development in tea remain unclear. In this study, we demonstrated the essential role of auxins in the initiation and progression of AR and established that the application of exogenous 1-naphthaleneacetic acid-enhanced AR formation in tissue-cultured seedlings and cuttings. Then, we found that the auxin-responsive transcription factor CsSPL9 acted as a negative regulator of AR development by reducing the levels of free indole-3-acetic acid (IAA) in tea plants. Furthermore, we identified CsGH3.4 as a downstream target of CsSPL9, which was activated by direct binding to its promoter. CsGH3.4 also inhibited AR development and maintained low levels of free IAA. Thus, these results revealed the inhibitory effect of the auxin-responsive CsSPL9-CsGH3.4 module on AR development by reducing free IAA levels in tea. These findings have significant theoretical and practical value for enhancing tea breeding practices.

SlGAD2 is the target of SlTHM27, positively regulates cold tolerance by mediating anthocyanin biosynthesis in tomato

Cold stress significantly limits the yield and quality of tomato. Deciphering the key genes related to cold tolerance is important for selecting and breeding superior cold-tolerant varieties. γ-aminobutyric acid (GABA) responds to various types of stress by rapidly accumulating in plant. In this study, glutamic acid decarboxylase (GAD2) was a positive regulator to enhance cold stress tolerance of tomato. Overexpression of SlGAD2 decreased the extent of cytoplasmic membrane damage and increased the endogenous GABA content, antioxidant enzyme activities, and reactive oxygen species (ROS) scavenging capacity in response to cold stress, whereas Slgad2 mutant plants showed the opposite trend. In addition, SlGAD2 induced anthocyanin biosynthesis in response to cold stress by increasing the content of endogenous GABA. Further study revealed that SlGAD2 expression was negatively regulated by the transcription factor SlTHM27. However, the transcript levels of SlTHM27 were repressed under cold stress. Antioxidant enzyme activities, SlGAD2 transcript levels, GABA and anthocyanin contents were significantly increased in Slthm27 mutant plants. Further, our study demonstrated that SlTHM27 decreases SlGAD2-promoted cold resistance in tomato by repressing SlGAD2 transcription. Overall, our results showed that the SlTHM27-SlGAD2 model regulates the cold tolerance in tomato by regulating GABA and anthocyanin.

Identification of two key genes involved in flavonoid catabolism and their different roles in apple resistance to biotic stresses

Biosynthesis of flavonoid aglycones and glycosides is well established. However, key genes involved in their catabolism are poorly understood, even though the products of hydrolysis and oxidation play important roles in plant resistance to biotic stress. Here, we report on catabolism of dihydrochalcones (DHCs), the most abundant flavonoids in domesticated apple and wild Malus. Two key genes, BGLU13.1 and PPO05, were identified by activity-directed protein purification. BGLU13.1-A hydrolyzed phlorizin, (the most abundant DHC in domesticated apple) to produce phloretin which was then oxidized by PPO05. The process differed in some wild Malus, where trilobatin (a positional isomer of phlorizin) was mainly oxidized by PPO05. The effects of DHC catabolism on apple resistance to biotic stresses was investigated using transgenic plants. Either directly or indirectly, phlorizin hydrolysis affected resistance to the phytophagous pest two-spotted spider mite, while oxidation of trilobatin was involved in resistance to the biotrophic fungus Podosphaera leucotricha. DHC catabolism did not affect apple resistance to necrotrophic pathogens Valsa mali and Erwinia amylovara. These results suggest that different DHC catabolism pathways play different roles in apple resistance to biotic stresses. The role of DHC catabolism on apple resistance appeared closely related to the mode of invasion/damage used by pathogen/pest.

Multi-omics analysis identifies EcCS4 is negatively regulated in response to phytotoxin isovaleric acid stress in Echinochloa crus-galli

BACKGROUND

Knowledge of herbicidal targets is critical for weed management and food safety. The phytotoxin isovaleric acid (ISA) is effective against weeds with a broad spectrum, carries low environmental risks, and is thus an excellent herbicide lead. However, the biochemical and molecular mechanisms underlying the action of ISA remain unclear.

RESULTS

Multi-omics data showed that acetyl coenzyme A (acetyl-CoA) was the key affected metabolite, and that citrate synthase (CS) 4 was substantially down-regulated under ISA treatment in Echinochloa crus-galli leaves. In particular, the transcript level of EcCS4 was the most significantly regulated among the six genes involved in the top 10 different pathways. The EcCS4 encodes a protein of 472 amino acids and is localized to the cell membrane and mitochondria, similar to the CS4s of other plants. The protein content of EcCS4 was down-regulated after ISA treatment at 0.5 h. ISA markedly inhibited the CS4 activity in vitro in a concentration-dependent manner (IC50  = 41.35 μM). In addition, the transgenic rice plants overexpressing EcCS4 (IC50  = 111.8 mM for OECS4-8 line) were more sensitive, whereas loss-of-function rice mutant lines (IC50  = 746.5 mM for oscs4-19) were more resistant to ISA, compared to wild type (WT) plants (IC50  = 355.6 mM).

CONCLUSION

CS4 was first reported as a negative regulator of plant responses to ISA. These results highlight that CS4 is a candidate target gene for the development of novel herbicides and for breeding herbicide-resistant crops. © 2023 Society of Chemical Industry.

CmoPIP1-4 confers drought tolerance in pumpkin by altering hydrogen sulfide signaling

Drought is a major limiting factor for the growth and development of pumpkins. Plasma membrane intrinsic proteins (PIPs) are major water channels that play a crucial role in the regulation of cellular water status and solute trafficking during drought conditions. CmoPIP1-4 is a plasma membrane-localized protein that is significantly upregulated in roots and leaves under drought-stress conditions. In this study, the overexpression of CmoPIP1-4 enhances drought resistance in yeast. In contrast, CRISPR-mediated CmoPIP1-4 knockout in pumpkin roots increased drought sensitivity. This increased drought sensitivity of CmoPIP1-4 knockout plants is associated with a decline in the levels of hydrogen sulfide (H2S) and abscisic acid (ABA), accompanied by an increase in water loss caused by greater levels of transpiration and stomatal conductance. In addition, the sensitivity of CmoPIP1-4 CRISPR plants is further aggravated by reduced antioxidative enzyme activity, decreased proline and sugar contents, and extensive root damage. Furthermore, expression profiles of genes such as CmoHSP70s, CmoNCED3, CmoNCED4, and others involved in metabolic activities were markedly reduced in CmoPIP1-4 CRISPR plants. Moreover, we also discovered an interaction between the drought-responsive gene CmoDCD and CmoPIP1-4, indicating their potential role in activating H2S-mediated signaling in pumpkin, which could confer drought tolerance. The findings of our study collectively demonstrate CmoPIP1-4 plays a crucial role in the regulation of H2S-mediated signaling, influencing stomatal density and aperture in pumpkin plants, and thereby enhancing their drought tolerance.

The chromatin remodeller MdRAD5B enhances drought tolerance by coupling MdLHP1-mediated H3K27me3 in apple

RAD5B belongs to the Rad5/16-like group of the SNF2 family, which often functions in chromatin remodelling. However, whether RAD5B is involved in chromatin remodelling, histone modification, and drought stress tolerance is largely unclear. We identified a drought-inducible chromatin remodeler, MdRAD5B, which positively regulates apple drought tolerance. Transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) analysis showed that MdRAD5B affects the expression of 466 drought-responsive genes through its chromatin remodelling function in response to drought stress. In addition, MdRAD5B interacts with and degrades MdLHP1, a crucial regulator of histone H3 trimethylation at K27 (H3K27me3), through the ubiquitin-independent 20S proteasome. Chromatin immunoprecipitation-sequencing (ChIP-seq) analysis revealed that MdRAD5B modulates the H3K27me3 deposition of 615 genes in response to drought stress. Genetic interaction analysis showed that MdRAD5B mediates the H3K27me3 deposition of drought-responsive genes through MdLHP1, which causes their expression changes under drought stress. Our results unravelled a dual function of MdRAD5B in gene expression modulation in apple in response to drought, that is, via the regulation of chromatin remodelling and H3K27me3.

MdbHLH93 confers drought tolerance by activating MdTyDC expression and promoting dopamine biosynthesis

Dopamine and its biosynthesis-limiting enzyme tyrosine decarboxylase (TyDC) play a vital part in mediating plant growth and the response to drought stress. However, the underlying molecular mechanism remains poorly understood. Here, drought stress markedly induced the expression of Malus domestica bHLH93 (MdbHLH93), the apple basic helix-loop-helix transcription factor. Moreover, MdbHLH93 directly bound to the Malus domestica TyDC (MdTyDC) promoter and positively regulated its expression, which resulted in dopamine synthesis and enhanced drought tolerance. Furthermore, the additive effect of overexpressing MdbHLH93 and MdTyDC simultaneously promoted dopamine synthesis and drought tolerance in apples, while the interference of MdbHLH93 inhibited this effect, indicating that MdTyDC-regulated dopamine synthesis and drought tolerance were positively regulated by MdbHLH93. Taken together, these findings suggest the positive regulation of dopamine accumulation by MdbHLH93 through its transcriptional regulation of MdTyDC and show that increased dopamine content confers drought tolerance in apples.

MdBT2 regulates nitrogen-mediated cuticular wax biosynthesis via a MdMYB106-MdCER2L1 signalling pathway in apple

Cuticular waxes play important roles in plant development and the interaction between plants and their environment. Researches on wax biosynthetic pathways have been reported in several plant species. Also, wax formation is closely related to environmental condition. However, the regulatory mechanism between wax and environmental factors, especially essential mineral elements, is less studied. Here we found that nitrogen (N) played a negative role in the regulation of wax synthesis in apple. We therefore analysed wax content, composition and crystals in BTB-TAZ domain protein 2 (MdBT2) overexpressing and antisense transgenic apple seedlings and found that MdBT2 could downregulate wax biosynthesis. Furthermore, R2R3-MYB transcription factor 16-like protein (MdMYB106) interacted with MdBT2, and MdBT2 mediated its ubiquitination and degradation through the 26S proteasome pathway. Finally, HXXXD-type acyl-transferase ECERIFERUM 2-like1 (MdCER2L1) was confirmed as a downstream target gene of MdMYB106. Our findings reveal an N-mediated apple wax biosynthesis pathway and lay a foundation for further study of the environmental factors associated with wax regulatory networks in apple.

Histone deacetylase MdHDA6 is an antagonist in regulation of transcription factor MdTCP15 to promote cold tolerance in apple

Understanding the molecular regulation of plant cold response is the basis for cold resistance germplasm improvement. Here, we revealed that the apple histone deacetylase MdHDA6 can perform histone deacetylation on cold-negative regulator genes and repress their expression, leading to the positive regulation of cold tolerance in apples. Moreover, MdHDA6 directly interacts with the transcription factor MdTCP15. Phenotypic analysis of MdTCP15 transgenic apple lines and wild types reveals that MdTCP15 negatively regulates cold tolerance in apples. Furthermore, we found that MdHDA6 can facilitate histone deacetylation of MdTCP15 and repress the expression of MdTCP15, which positively contributes to cold tolerance in apples. Additionally, the transcription factor MdTCP15 can directly bind to the promoter of the cold-negative regulator gene MdABI1 and activate its expression, and it can also directly bind to the promoter of the cold-positive regulator gene MdCOR47 and repress its expression. However, the co-expression of MdHDA6 and MdTCP15 can inhibit MdTCP15-induced activation of MdABI1 and repression of MdCOR47, suggesting that MdHDA6 suppresses the transcriptional regulation of MdTCP15 on its downstream genes. Our results demonstrate that histone deacetylase MdHDA6 plays an antagonistic role in the regulation of MdTCP15-induced transcriptional activation or repression to positively regulate cold tolerance in apples, revealing a new regulatory mechanism of plant cold response.

Comprehensive Analysis of GH3 Gene Family in Potato and Functional Characterization of StGH3.3 under Drought Stress

As an important hormone response gene, Gretchen Hagen 3 (GH3) maintains hormonal homeostasis by conjugating excess auxin with amino acids during plant stress-related signaling pathways. GH3 genes have been characterized in many plant species, but they are rarely reported in potato. Here, 19 StGH3 genes were isolated and characterized. Phylogenetic analysis indicated that StGH3s were divided into two categories (group I and group III). Analyses of gene structure and motif composition showed that the members of a specific StGH3 subfamily are relatively conserved. Collinearity analysis of StGH3 genes in potato and other plants laid a foundation for further exploring the evolutionary characteristics of the StGH3 genes. Promoter analysis showed that most StGH3 promoters contained hormone and abiotic stress response elements. Multiple transcriptome studies indicated that some StGH3 genes were responsive to ABA, water deficits, and salt treatments. Moreover, qRT-PCR analysis indicated that StGH3 genes could be induced by phytohormones (ABA, SA, and MeJA) and abiotic stresses (water deficit, high salt, and low temperature), although with different patterns. Furthermore, transgenic tobacco with transient overexpression of the StGH3.3 gene showed positive regulation in response to water deficits by increasing proline accumulation and reducing the leaf water loss rate. These results suggested that StGH3 genes may be involved in the response to abiotic stress through hormonal signal pathways. Overall, this study provides useful insights into the evolution and function of StGH3s and lays a foundation for further study on the molecular mechanisms of StGH3s in the regulation of potato drought resistance.

Insight into regulation of adventitious root formation by arbuscular mycorrhizal fungus and exogenous auxin in tea plant (Camellia sinensis L.) cuttings

Introduction

Adventitious root (AR) development, affected by various biotic and abiotic factors, is the most important procedure in tea plant (Camellia sinensis L.) cutting propagation. Establishing symbiotic relationships with most terrestrial plants, AMF (Arbuscular mycorrhizal fungus) can mediate the AR formation of several herbaceous and woody plants in previous studies.

Methods

In this paper, effects of combined application of AMF and exogenous auxin on AR formation of cuttings from different tea plant varieties ('Pingyangtezao', 'Longjing 43' and 'Longjingchangye') were studied. Then we also performed RNA-Seq analysis with 'Pingyangtezao' cuttings aiming to find the possible auxin-related pathway of AM fungal regulation on AR formation. To accurately uncover the regulatory mechanism of AMF on AR formation of tea cuttings, rooting process were separated into four stages (S0, non-rooting; S1, AR protrusion; S2, AR formation and S3, AR elongation) at the same sampling time.

Results and Discussion

Results showed that IBA treatment increased the mycorrhizal colonization rate, especially in 'Pingyangtezao' variety (from 37.58% to 46.29%). Both inoculating AMF and addition of IBA promoted the AR formation, and rooting of different tea plant varieties showed different dependence on auxin. AMF could alleviate the effect of auxin-related inhibitors (2,3,5-triiodobenzoic acid, L-α-(Aminooxy)-β-phenylpropionic acid and α-(phenylethyl-2-oxo)-IAA) on rooting of tea cuttings, even though the colonization of AMF was hindered at various degrees. Transcriptomic analysis showed that different numbers of differentially expressed genes (DEGs) at various rooting stages of tea cuttings with the most at S2 stage (1360 DEGs), indicating the increasing regulation by AMF with the development of AR. Similar trend was found in auxin-related DEGs, and family genes of YUC, GH, PIN, LAX, SAUR, AUX, and ABP involved in the AM fungal regulation on AR formation of tea cuttings. Additionally, AMF strongly mediated auxin transport and signal transduction pathways in tea cuttings as showed by the results of correlation analysis. Overall, interaction of AMF and exogenous auxin in promoting rooting and the preliminary mechanism of AMF regulating AR formation of tea cuttings was deciphered in this paper, which may provide a basis for further deep mechanistic research and cutting propagation of tea production.

PtrABR1 Increases Tolerance to Drought Stress by Enhancing Lateral Root Formation in Populus trichocarpa

Roots are the main organ for water uptake and the earliest part of a plant's response to drought, making them of great importance to our understanding of the root system's response to drought. However, little is known about the underlying molecular mechanisms that control root responses to drought stress. Here, we identified and functionally characterized the AP2/ERF family transcription factor (TF) PtrABR1 and the upstream target gene zinc-finger protein TF PtrYY1, which respond to drought stress by promoting the growth and development of lateral roots in Populus trichocarpa. A root-specific induction of PtrABR1 under drought stress was explored. The overexpression of PtrABR1 (PtrABR1-OE) promoted root growth and development, thereby increasing tolerance to drought stress. In addition, PtrYY1 is directly bound to the promoter of PtrABR1 under drought stress, and the overexpression of PtrYY1 (PtrYY1-OE) promoted lateral root growth and development and increased tolerance to drought stress. An RNA-seq analysis of PtrABR1-OE with wild-type (WT) poplar identified PtrGH3.6 and PtrPP2C44, which share the same pattern of expression changes as PtrABR1. A qRT-PCR and cis-element analysis further suggested that PtrGH3.6 and PtrPP2C44 may act as potential downstream targets of PtrABR1 genes in the root response pathway to drought stress. In conclusion, these results reveal a novel drought regulatory pathway in which PtrABR1 regulates the network through the upstream target gene PtrYY1 and the potential downstream target genes PtrGH3.6 and PtrPP2C44, thereby promoting root growth and development and improving tolerance to drought stress.

Insights into the molecular mechanisms underlying responses of apple trees to abiotic stresses

Apple (Malus[Formula: see text]domestica) is a popular temperate fruit crop worldwide. However, its growth, productivity, and quality are often adversely affected by abiotic stresses such as drought, extreme temperature, and high salinity. Due to the long juvenile phase and highly heterozygous genome, the conventional breeding approaches for stress-tolerant cultivars are time-consuming and resource-intensive. These issues may be resolved by feasible molecular breeding techniques for apples, such as gene editing and marker-assisted selection. Therefore, it is necessary to acquire a more comprehensive comprehension of the molecular mechanisms underpinning apples' response to abiotic stress. In this review, we summarize the latest research progress in the molecular response of apples to abiotic stressors, including the gene expression regulation, protein modifications, and epigenetic modifications. We also provide updates on new approaches for improving apple abiotic stress tolerance, while discussing current challenges and future perspectives for apple molecular breeding.

Fruit crops combating drought: Physiological responses and regulatory pathways

Drought is a common stress in agricultural production. Thus, it is imperative to understand how fruit crops respond to drought and to develop drought-tolerant varieties. This paper provides an overview of the effects of drought on the vegetative and reproductive growth of fruits. We summarize the empirical studies that have assessed the physiological and molecular mechanisms of the drought response in fruit crops. This review focuses on the roles of calcium (Ca2+) signaling, abscisic acid (ABA), reactive oxygen species signaling, and protein phosphorylation underlying the early drought response in plants. We review the resulting downstream ABA-dependent and ABA-independent transcriptional regulation in fruit crops under drought stress. Moreover, we highlight the positive and negative regulatory mechanisms of microRNAs in the drought response of fruit crops. Lastly, strategies (including breeding and agricultural practices) to improve the drought resistance of fruit crops are outlined.

Orchard Management and Incorporation of Biochemical and Molecular Strategies for Improving Drought Tolerance in Fruit Tree Crops

Water scarcity is one of the greatest concerns for agronomy worldwide. In recent years, many water resources have been depleted due to multiple factors, especially mismanagement. Water resource shortages lead to cropland expansion, which likely influences climate change and affects global agriculture, especially horticultural crops. Fruit yield is the final aim in commercial orchards; however, drought can slow tree growth and/or decrease fruit yield and quality. It is therefore necessary to find approaches to solve this problem. The main objective of this review is to discuss the most recent horticultural, biochemical, and molecular strategies adopted to improve the response of temperate fruit crops to water stress. We also address the viability of cultivating fruit trees in dry areas and provide precise protection methods for planting fruit trees in arid lands. We review the main factors involved in planting fruit trees in dry areas, including plant material selection, regulated deficit irrigation (DI) strategies, rainwater harvesting (RWH), and anti-water stress materials. We also provide a detailed analysis of the molecular strategies developed to combat drought, such as Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) through gene overexpression or gene silencing. Finally, we look at the molecular mechanisms associated with the contribution of the microbiome to improving plant responses to drought.

Strigolactone regulates adventitious root formation via the MdSMXL7-MdWRKY6-MdBRC1 signaling cascade in apple

Propagation through stem cuttings is a popular method worldwide for species such as fruit tree rootstocks and forest trees. Adventitious root (AR) formation from stem cuttings is crucial for effective and successful clonal propagation of apple rootstocks. Strigolactones (SLs) are newly identified hormones involved in AR formation. However, the regulatory mechanisms underpinning this process remain elusive. In the present study, weighted gene co-expression network analysis, as well as rooting assays using stable transgenic apple materials, revealed that MdBRC1 served as a key gene in the inhibition of AR formation by SLs. We have demonstrated that MdSMXL7 and MdWRKY6 synergistically regulated MdBRC1 expression, depending on the interactions of MdSMXL7 and MdWRKY6 at the protein level downstream of SLs as well as the direct promoter binding on MdBRC1 by MdWRKY6. Furthermore, biochemical studies and genetic analysis revealed that MdBRC1 inhibited AR formation by triggering the expression of MdGH3.1 in a transcriptional activation pathway. Finally, the present study not only proposes a component, MdWRKY6, that enables MdSMXL7 to regulate MdBRC1 during the process of SL-controlled AR formation in apple, but also provides prospective target genes to enhance AR formation capacity using CRISPR (i.e. clustered regularly interspaced short palindromic repeats) technology, particularly in woody plants.

Genome-Scale Identification, Classification, and Expression Profiling of MYB Transcription Factor Genes in Cinnamomum camphora

The camphor tree (Cinnamomum camphora (L.) Presl.) is the representative species of subtropical evergreen broadleaved forests in eastern Asia and an important raw material for essential oil production worldwide. Although MYBs have been comprehensively characterized and their functions have been partially resolved in many plants, it has not been explored in C. camphora. In this study, 121 CcMYBs were identified on 12 chromosomes in the whole genome of C. camphora and found that CcMYBs were mainly expanded by segmental duplication. They were divided into 28 subgroups based on phylogenetic analysis and gene structural characteristics. In the promoter regions, numerous cis-acting elements were related to biological processes. Analysis of RNA sequencing data from seven tissues showed that CcMYBs exhibited different expression profiles, suggesting that they have various roles in camphor tree development. In addition, combined with the correlation analysis of structural genes in the flavonoid synthesis pathway, we identified CcMYBs from three subgroups that might be related to the flavonoid biosynthesis pathway. This study systematically analyzed CcMYBs in C. camphora, which will set the stage for subsequent research on the functions of CcMYBs during their lifetime and provide valuable insights for the genetic improvement of camphor trees.

PtoMPO1, a negative mediator, functions in poplar drought tolerance

Drought, as one of the most severe abiotic stresses in nature, adversely affects plant growth and development. Poplar is a woody plant which is prone to water-deficit sensitivity. Therefore, it is important to improve our understanding of how poplar responds to drought stress. Here, we cloned a gene from Populus tomentosa, namely PtoMPO1. PtoMPO1 encodes a DUF962 domain protein that is a homolog of yeast dioxygenase Mpo1 and Arabidopsis MHP1. The transcripts of PtoMPO1 were repressed by drought stress and ABA. Atmhp1-1 was a T-DNA insertion mutant lacking AtMHP1, and heteroexpression of PtoMPO1 in Atmhp1-1 significantly alleviated the sensitivity of Atmhp1-1 to ABA and NaCl, implying the functional replacement of PtoMPO1 to AtMHP1. PtoMPO1 overexpression decreased but PtoMPO1 mutation enhanced poplar drought tolerance. Furthermore, the expression of drought-related gene PtoRD26 is markedly lower in PtoMPO1-overexpressing plants and notably higher in Ptompo1 mutants compared to that in the wild type. Overall, these results suggested that PtoMPO1 functions as a novel negative mediator for drought tolerance in poplar.

Roles of Auxin in the Growth, Development, and Stress Tolerance of Horticultural Plants

Auxin, a plant hormone, regulates virtually every aspect of plant growth and development. Many current studies on auxin focus on the model plant Arabidopsis thaliana, or on field crops, such as rice and wheat. There are relatively few studies on what role auxin plays in various physiological processes of a range of horticultural plants. In this paper, recent studies on the role of auxin in horticultural plant growth, development, and stress response are reviewed to provide novel insights for horticultural researchers and cultivators to improve the quality and application of horticultural crops.

Engineering drought-tolerant apple by knocking down six GH3 genes and potential application of transgenic apple as a rootstock

Drought poses a major threat to apple fruit production and quality. Because of the apple's long juvenile phase, developing varieties with improved drought tolerance using biotechnology approaches is needed. Here, we used the RNAi approach to knock down six GH3 genes in the apple. Under prolonged drought stress, the MdGH3 RNAi plants performed better than wild-type plants and had stronger root systems, higher root-to-shoot ratio, greater hydraulic conductivity, increased photosynthetic capacity, and increased water use efficiency. Moreover, MdGH3 RNAi plants promoted the drought tolerance of the scion when they were used as rootstock, compared with wild-type and M9-T337 rootstocks. Scions grafted onto MdGH3 RNAi plants showed increased plant height, stem diameter, photosynthetic capacity, specific leaf weight, and water use efficiency. The use of MdGH3 RNAi plants as rootstocks can also increase the C/N ratio of the scion and achieve the same effect as the M9-T337 rootstock in promoting the flowering and fruiting of the scion. Notably, using MdGH3 RNAi plants as rootstocks did not reduce fruit weight and scion quality compared with using M9-T337 rootstock. Our research provides candidate genes and demonstrates a general approach that could be used to improve the drought tolerance of fruit trees without sacrificing the yield and quality of scion fruits.