Arabidopsis seedling establishment under waterlogging requires ABCG5-mediated formation of a dense cuticle layer

The Arabidopsis katamari2 Mutant Exhibits a Hypersensitive Seedling Arrest Response at the Phase Transition from Heterotrophic to Autotrophic Growth

Young seedlings use nutrients stored in the seeds to grow and acquire photosynthetic potential. This process, called seedling establishment, involves a developmental phase transition from heterotrophic to autotrophic growth. Some membrane-trafficking mutants of Arabidopsis (Arabidopsis thaliana), such as the katamari2 (kam2) mutant, exhibit growth arrest during seedling development, with a portion of individuals failing to develop true leaves on sucrose-free solid medium. However, the reason for this seedling arrest is unclear. In this study, we show that seedling arrest is a temporal growth arrest response that occurs not only in kam2 but also in wild-type (WT) Arabidopsis; however, the threshold for this response is lower in kam2 than in the WT. A subset of the arrested kam2 seedlings resumed growth after transfer to fresh sucrose-free medium. Growth arrest in kam2 on sucrose-free medium was restored by increasing the gel concentration of the medium or covering the surface of the medium with a perforated plastic sheet. WT Arabidopsis seedlings were also arrested when the gel concentration of sucrose-free medium was reduced. RNA sequencing revealed that transcriptomic changes associated with the rate of seedling establishment were observed as early as 4 d after sowing. Our results suggest that the growth arrest of both kam2 and WT seedlings is an adaptive stress response and is not simply caused by the lack of a carbon source in the medium. This study provides a new perspective on an environmental stress response under unfavorable conditions during the phase transition from heterotrophic to autotrophic growth in Arabidopsis.

Endomembrane-biased dimerization of ABCG16 and ABCG25 transporters determines their substrate selectivity in ABA-regulated plant growth and stress responses

ATP-binding cassette (ABC) transporters are integral membrane proteins that have evolved diverse functions fulfilled via the transport of various substrates. In Arabidopsis, the G subfamily of ABC proteins is particularly abundant and participates in multiple signaling pathways during plant development and stress responses. In this study, we revealed that two Arabidopsis ABCG transporters, ABCG16 and ABCG25, engage in ABA-mediated stress responses and early plant growth through endomembrane-specific dimerization-coupled transport of ABA and ABA-glucosyl ester (ABA-GE), respectively. We first revealed that ABCG16 contributes to osmotic stress tolerance via ABA signaling. More specifically, ABCG16 induces cellular ABA efflux in both yeast and plant cells. Using FRET analysis, we showed that ABCG16 forms obligatory homodimers for ABA export activity and that the plasma membrane-resident ABCG16 homodimers specifically respond to ABA, undergoing notable conformational changes. Furthermore, we demonstrated that ABCG16 heterodimerizes with ABCG25 at the endoplasmic reticulum (ER) membrane and facilitates the ER entry of ABA-GE in both Arabidopsis and tobacco cells. The specific responsiveness of the ABCG16-ABCG25 heterodimer to ABA-GE and the superior growth of their double mutant support an inhibitory role of these two ABCGs in early seedling establishment via regulation of ABA-GE translocation across the ER membrane. Our endomembrane-specific analysis of the FRET signals derived from the homo- or heterodimerized ABCG complexes allowed us to link endomembrane-biased dimerization to the translocation of distinct substrates by ABCG transporters, providing a prototypic framework for understanding the omnipotence of ABCG transporters in plant development and stress responses.

GmABCG5, an ATP-binding cassette G transporter gene, is involved in the iron deficiency response in soybean

Iron deficiency is a major nutritional problem causing iron deficiency chlorosis (IDC) and yield reduction in soybean, one of the most important crops. The ATP-binding cassette G subfamily plays a crucial role in substance transportation in plants. In this study, we cloned the GmABCG5 gene from soybean and verified its role in Fe homeostasis. Analysis showed that GmABCG5 belongs to the ABCG subfamily and is subcellularly localized at the cell membrane. From high to low, GmABCG5 expression was found in the stem, root, and leaf of young soybean seedlings, and the order of expression was flower, pod, seed stem, root, and leaf in mature soybean plants. The GUS assay and qRT-PCR results showed that the GmABCG5 expression was significantly induced by iron deficiency in the leaf. We obtained the GmABCG5 overexpressed and inhibitory expressed soybean hairy root complexes. Overexpression of GmABCG5 promoted, and inhibition of GmABCG5 retarded the growth of soybean hairy roots, independent of nutrient iron conditions, confirming the growth-promotion function of GmABCG5. Iron deficiency has a negative effect on the growth of soybean complexes, which was more obvious in the GmABCG5 inhibition complexes. The chlorophyll content was increased in the GmABCG5 overexpression complexes and decreased in the GmABCG5 inhibition complexes. Iron deficiency treatment widened the gap in the chlorophyll contents. FCR activity was induced by iron deficiency and showed an extraordinary increase in the GmABCG5 overexpression complexes, accompanied by the greatest Fe accumulation. Antioxidant capacity was enhanced when GmABCG5 was overexpressed and reduced when GmABCG5 was inhibited under iron deficiency. These results showed that the response mechanism to iron deficiency is more actively mobilized in GmABCG5 overexpression seedlings. Our results indicated that GmABCG5 could improve the plant's tolerance to iron deficiency, suggesting that GmABCG5 might have the function of Fe mobilization, redistribution, and/or secretion of Fe substances in plants. The findings provide new insights into the ABCG subfamily genes in the regulation of iron homeostasis in plants.

Air channels create a directional light signal to regulate hypocotyl phototropism

In plants, light direction is perceived by the phototropin photoreceptors, which trigger directional growth responses known as phototropism. The formation of a phototropin activation gradient across a photosensitive organ initiates this response. However, the optical tissue properties that functionally contribute to phototropism remain unclear. In this work, we show that intercellular air channels limit light transmittance through various organs in several species. Air channels enhance light scattering in Arabidopsis hypocotyls, thereby steepening the light gradient. This is required for an efficient phototropic response in Arabidopsis and Brassica. We identified an embryonically expressed ABC transporter required for the presence of air channels in seedlings and a structure surrounding them. Our work provides insights into intercellular air space development or maintenance and identifies a mechanism of directional light sensing in plants.

Air spaces bend light in plant stems

Intercellular air spaces are necessary for phototropism in Arabidopsis thaliana.

Identification and Characterization of ABCG15-A Gene Required for Exocarp Color Differentiation in Pear

Exocarp color is a commercially essential quality for pear which can be divided into two types: green and russet. The occurrence of russet color is associated with deficiencies and defects in the cuticular and epidermal layers, which affect the structure of the cell wall and the deposition of suberin. Until now, the genetic basics triggering this trait have not been well understood, and limited genes have been identified for the trait. To figure out the gene controlling the trait of exocarp color, we perform a comprehensive genome-wide association study, and we describe the candidate genes. One gene encoding the ABCG protein has been verified to be associated with the trait, using an integrative analysis of the metabolomic and transcriptomic data. This review covers a variety of omics resources, which provide a valuable resource for identifying gene-controlled traits of interest. The findings in this study help to elucidate the genetic components responsible for the trait of exocarp color in pear, and the implications of these findings for future pear breeding are evaluated.

Ethylene enhanced waterlogging tolerance by changing root architecture and inducing aerenchyma formation in maize seedlings

Waterlogging negatively affects maize growth and yield. In this study, we found that ethylene played a vital role in plant adaptation to waterlogging. ET promotes better growth in seedlings under waterlogging conditions by altering root architecture and increasing lateral root formation by 42.1%. What's more, plants with high endogenous ethylene levels exhibited reduced sensitivity to waterlogging stress. ET also induced the formation of aerenchyma, a specialized tissue that facilitates gas exchange, in a different pattern compared to aerenchyma formed under waterlogging. Aerenchyma induced by ET was mainly located in the medial cortex of the roots and was not prone to decay. ethylene inhibited root elongation under normal conditions, but this inhibition was not alleviated under waterlogging stress. Upon activation of the ET signaling pathway, the transcription factor EREB90 promoted aerenchyma formation by enhancing the programmed cell death process. Overexpression of EREB90 resulted in increased waterlogging tolerance compared to wild type plants. Our findings suggest that pre-treatment of maize seedlings with ET before waterlogging stress can trigger the programmed cell death process and induce aerenchyma formation, thus improving waterlogging resistance.

Oxylipins and Reactive Carbonyls as Regulators of the Plant Redox and Reactive Oxygen Species Network under Stress

Reactive oxygen species (ROS), and in particular H2O2, serve as essential second messengers at low concentrations. However, excessive ROS accumulation leads to severe and irreversible cell damage. Hence, control of ROS levels is needed, especially under non-optimal growth conditions caused by abiotic or biotic stresses, which at least initially stimulate ROS synthesis. A complex network of thiol-sensitive proteins is instrumental in realizing tight ROS control; this is called the redox regulatory network. It consists of sensors, input elements, transmitters, and targets. Recent evidence revealed that the interplay of the redox network and oxylipins–molecules derived from oxygenation of polyunsaturated fatty acids, especially under high ROS levels–plays a decisive role in coupling ROS generation and subsequent stress defense signaling pathways in plants. This review aims to provide a broad overview of the current knowledge on the interaction of distinct oxylipins generated enzymatically (12-OPDA, 4-HNE, phytoprostanes) or non-enzymatically (MDA, acrolein) and components of the redox network. Further, recent findings on the contribution of oxylipins to environmental acclimatization will be discussed using flooding, herbivory, and establishment of thermotolerance as prime examples of relevant biotic and abiotic stresses.

Apple SUMO E3 ligase MdSIZ1 regulates cuticular wax biosynthesis by SUMOylating transcription factor MdMYB30

A key function of SUMOylation is the coordinated modification of numerous proteins to optimize plant growth and resistance to environmental stress. Plant cuticular wax is deposited on the surface of primary plant organs to form a barrier that provides protection against changes in terrestrial environments. Many recent studies have examined cuticular wax biosynthetic pathways and regulation. However, whether SUMOylation is involved in the regulation of cuticle wax deposition at the posttranslational level remains unclear. Here, we demonstrate that a small ubiquitin-like modifier (SUMO) E3 ligase, SAP AND MIZ1 DOMAIN CONTAINING LIGASE1 (MdSIZ1), regulates wax accumulation and cuticle permeability in apple (Malus domestica Borkh), SUMO E2 CONJUGATING ENZYME 1(MdSCE1) physically interacts with MdMYB30, a transcription factor involved in the regulation of cuticle wax accumulation. MdSIZ1 mediates the SUMOylation and accumulation of MdMYB30 by inhibiting its degradation through the 26S proteasome pathway. Furthermore, MdMYB30 directly binds to the β-KETOACYL-COA SYNTHASE 1 (MdKCS1) promoter to activate its expression and promote wax biosynthesis. These findings indicate that the MdSIZ1-MdMYB30-MdKCS1 module positively regulates cuticular wax biosynthesis in apples. Overall, the findings of our study provide insights into the regulation pathways involved in cuticular wax biosynthesis.

The Role of Aquaporins in Plant Growth under Conditions of Oxygen Deficiency

Plants frequently experience hypoxia due to flooding caused by intensive rainfall or irrigation, when they are partially or completely submerged under a layer of water. In the latter case, some resistant plants implement a hypoxia avoidance strategy by accelerating shoot elongation, which allows lifting their leaves above the water surface. This strategy is achieved due to increased water uptake by shoot cells through water channels (aquaporins, AQPs). It remains a puzzle how an increased flow of water through aquaporins into the cells of submerged shoots can be achieved, while it is well known that hypoxia inhibits the activity of aquaporins. In this review, we summarize the literature data on the mechanisms that are likely to compensate for the decline in aquaporin activity under hypoxic conditions, providing increased water entry into cells and accelerated shoot elongation. These mechanisms include changes in the expression of genes encoding aquaporins, as well as processes that occur at the post-transcriptional level. We also discuss the involvement of hormones, whose concentration changes in submerged plants, in the control of aquaporin activity.

ATP-Binding Cassette G Transporters and Their Multiple Roles Especially for Male Fertility in Arabidopsis, Rice and Maize

ATP-binding cassette subfamily G (ABCG) transporters are extensive in plants and play essential roles in various processes influencing plant fitness, but the research progress varies greatly among Arabidopsis, rice and maize. In this review, we present a consolidated nomenclature and characterization of the whole 51 ABCG transporters in maize, perform a phylogenetic analysis and classification of the ABCG subfamily members in maize, and summarize the latest research advances in ABCG transporters for these three plant species. ABCG transporters are involved in diverse processes in Arabidopsis and rice, such as anther and pollen development, vegetative and female organ development, abiotic and biotic stress response, and phytohormone transport, which provide useful clues for the functional investigation of ABCG transporters in maize. Finally, we discuss the current challenges and future perspectives for the identification and mechanism analysis of substrates for plant ABCG transporters. This review provides a basic framework for functional research and the potential application of ABCG transporters in multiple plants, including maize.

The Role of Phytohormones in Plant Response to Flooding

Climatic variations influence the morphological, physiological, biological, and biochemical states of plants. Plant responses to abiotic stress include biochemical adjustments, regulation of proteins, molecular mechanisms, and alteration of post-translational modifications, as well as signal transduction. Among the various abiotic stresses, flooding stress adversely affects the growth of plants, including various economically important crops. Biochemical and biological techniques, including proteomic techniques, provide a thorough understanding of the molecular mechanisms during flooding conditions. In particular, plants can cope with flooding conditions by embracing an orchestrated set of morphological adaptations and physiological adjustments that are regulated by an elaborate hormonal signaling network. With the help of these findings, the main objective is to identify plant responses to flooding and utilize that information for the development of flood-tolerant plants. This review provides an insight into the role of phytohormones in plant response mechanisms to flooding stress, as well as different mitigation strategies that can be successfully administered to improve plant growth during stress exposure. Ultimately, this review will expedite marker-assisted genetic enhancement studies in crops for developing high-yield lines or varieties with flood tolerance.

Regulatory mechanisms underlying cuticular wax biosynthesis

Plants are sessile organisms that have developed hydrophobic cuticles that cover their aerial epidermal cells to protect them from terrestrial stresses. The cuticle layer is mainly composed of cutin, a polyester of hydroxy and epoxy fatty acids, and cuticular wax, a mixture of very-long-chain fatty acids (>20 carbon atoms) and their derivatives, aldehydes, alkanes, ketones, alcohols, and wax esters. During the last 30 years, forward and reverse genetic, transcriptomic, and biochemical approaches have enabled the identification of key enzymes, transporters, and regulators involved in the biosynthesis of cutin and cuticular waxes. In particular, cuticular wax biosynthesis is significantly influenced in an organ-specific manner or by environmental conditions, and is controlled using a variety of regulators. Recent studies on the regulatory mechanisms underlying cuticular wax biosynthesis have enabled us to understand how plants finely control carbon metabolic pathways to balance between optimal growth and development and defense against abiotic and biotic stresses. In this review, we summarize the regulatory mechanisms underlying cuticular wax biosynthesis at the transcriptional, post-transcriptional, post-translational, and epigenetic levels.

Modes of secretion of plant lipophilic metabolites via ABCG transporter-dependent transport and vesicle-mediated trafficking

Many lipophilic metabolites produced by terrestrial plants are deposited on plant surfaces to protect them from abiotic and biotic stresses. Plant-derived lipophilic metabolites include apoplastic biopolymers, such as wax, cutin, sporopollenin, suberin, and lignin, as well as low-molecular-weight secondary metabolites. These secreted molecules confer adaptive toughness and robustness on plants. The mechanisms responsible for the secretion of these lipophilic metabolites remain unclear, although two pathways, mediated by transporters and vesicles, have been proposed. Recent genetic and biochemical studies have shown that G-type ATP-binding cassette (ABCG) transporters and membrane trafficking factors are involved in the apoplastic accumulation of lipophilic metabolites in plants. These two distinctive modes of secretion may be either exclusive or collaborative. This review describes these transporter-dependent and vesicle-mediated mechanisms underlying the secretion of lipophilic metabolites.

Accelerated remodeling of the mesophyll-bundle sheath interface in the maize C4 cycle mutant leaves

C4 photosynthesis in the maize leaf involves the exchange of organic acids between mesophyll (M) and the bundle sheath (BS) cells. The transport is mediated by plasmodesmata embedded in the suberized cell wall. We examined the maize Kranz anatomy with a focus on the plasmodesmata and cell wall suberization with microscopy methods. In the young leaf zone where M and BS cells had indistinguishable proplastids, plasmodesmata were simple and no suberin was detected. In leaf zones where dimorphic chloroplasts were evident, the plasmodesma acquired sphincter and cytoplasmic sleeves, and suberin was discerned. These modifications were accompanied by a drop in symplastic dye mobility at the M-BS boundary. We compared the kinetics of chloroplast differentiation and the modifications in M-BS connectivity in ppdk and dct2 mutants where C4 cycle is affected. The rate of chloroplast diversification did not alter, but plasmodesma remodeling, symplastic transport inhibition, and cell wall suberization were observed from younger leaf zone in the mutants than in wild type. Our results indicate that inactivation of the C4 genes accelerated the changes in the M-BS interface, and the reduced permeability suggests that symplastic transport between M and BS could be regulated for normal operation of C4 cycle.

Poplar's Waterlogging Resistance Modeling and Evaluating: Exploring and Perfecting the Feasibility of Machine Learning Methods in Plant Science

Floods, as one of the most common disasters in the natural environment, have caused huge losses to human life and property. Predicting the flood resistance of poplar can effectively help researchers select seedlings scientifically and resist floods precisely. Using machine learning algorithms, models of poplar's waterlogging tolerance were established and evaluated. First of all, the evaluation indexes of poplar's waterlogging tolerance were analyzed and determined. Then, significance testing, correlation analysis, and three feature selection algorithms (Hierarchical clustering, Lasso, and Stepwise regression) were used to screen photosynthesis, chlorophyll fluorescence, and environmental parameters. Based on this, four machine learning methods, BP neural network regression (BPR), extreme learning machine regression (ELMR), support vector regression (SVR), and random forest regression (RFR) were used to predict the flood resistance of poplar. The results show that random forest regression (RFR) and support vector regression (SVR) have high precision. On the test set, the coefficient of determination (R²) is 0.8351 and 0.6864, the root mean square error (RMSE) is 0.2016 and 0.2780, and the mean absolute error (MAE) is 0.1782 and 0.2031, respectively. Therefore, random forest regression (RFR) and support vector regression (SVR) can be given priority to predict poplar flood resistance.

ABCG11 modulates cytokinin responses in Arabidopsis thaliana

The Arabidopsis ABC transporter ABCG11 transports lipidic precursors of surface coating polymers at the plasma membrane of epidermal cells. Mutants in ABCG11 exhibit severe developmental defects, suggesting that ABCG11 might also participate in phytohormone-mediated development. Here, we report that ABCG11 is involved in cytokinin-mediated development. The roots of abcg11 mutant seedlings failed to respond to cytokinins and accumulated more cytokinins than wild-type roots. When grown under short-day conditions, abcg11 exhibited longer roots and shorter hypocotyls compared to wild type, similar to abcg14, a knockout mutant in a cytokinin transporter. Treatment with exogenous trans-zeatin, which inhibits primary root elongation in the wild type, enhanced abcg11 primary root elongation. It also increased the expression of cytokinin-responsive Arabidopsis response regulator (ARR) genes, and the signal of the TCS::GFP reporter in abcg11 roots compared to wild-type roots, suggesting that cytokinin signaling was enhanced in abcg11 roots. When we treated only the roots of abcg11 with trans-zeatin, their shoots showed lower ARR induction than the wild type. The abcg14 abcg11 double mutant did not have additional root phenotypes compared to abcg11. Together, these results suggest that ABCG11 is necessary for normal cytokinin-mediated root development, likely because it contributes to cytokinin transport, either directly or indirectly.

Changes of Morphology, Chemical Compositions, and the Biosynthesis Regulations of Cuticle in Response to Chilling Injury of Banana Fruit During Storage

The plant cuticle covers almost all the outermost surface of aerial plant organs, which play a primary function in limiting water loss and responding to the environmental interactions. Banana fruit is susceptible to thermal changes with chilling injury below 13°C and green ripening over 25°C. Herein, the changes of surface morphology, chemical compositions of cuticle, and the relative expression of cuticle biosynthesis genes in banana fruit under low-temperature storage were investigated. Banana fruit exhibited chilling injury rapidly with browned peel appearance stored at 4°C for 6 days. The surface altered apparently from the clear plateau with micro-crystals to smooth appearance. As compared to normal ones, the overall coverage of the main cuticle pattern of waxes and cutin monomers increased about 22% and 35%, respectively, in browned banana stored under low temperature at 6 days. Fatty acids (C₁₆-C₁₈) and ω-OH, mid-chain-epoxy fatty acids (C₁₈) dominated cutin monomers. The monomers of fatty acids, the low abundant ω, mid-chain-diOH fatty acids, and 2-hydroxy fatty acids increased remarkably under low temperature. The cuticular waxes were dominated by fatty acids (> C₁₉), n-alkanes, and triterpenoids; and the fatty acids and aldehydes were shifted to increase accompanied by the chilling injury. Furthermore, RNA-seq highlighted 111 cuticle-related genes involved in fatty acid elongation, biosynthesis of very-long-chain (VLC) aliphatics, triterpenoids, and cutin monomers, and lipid-transfer proteins were significantly differentially regulated by low temperature in banana. Results obtained indicate that the cuticle covering on the fruit surface was also involved to respond to the chilling injury of banana fruit after harvest. These findings provide useful insights to link the cuticle on the basis of morphology, chemical composition changes, and their biosynthesis regulations in response to the thermal stress of fruit during storage.

2021 update on ATP-binding cassette (ABC) transporters: how they meet the needs of plants

Recent developments in the field of ABC proteins including newly identified functions and regulatory mechanisms expand the understanding of how they function in the development and physiology of plants.

The Arabidopsis mature endosperm promotes seedling cuticle formation via release of sulfated peptides

In Arabidopsis mature seeds, the onset of the embryo-to-seedling transition is nonautonomously controlled, being blocked by endospermic abscisic acid (ABA) release under unfavorable conditions. Whether the mature endosperm governs additional nonautonomous developmental processes during this transition is unknown. Mature embryos have a more permeable cuticle than seedlings, consistent with their endospermic ABA uptake capability. Seedlings acquire their well-sealing cuticles adapted to aerial lifestyle during germination. Endosperm removal prevents seedling cuticle formation, and seed reconstitution by endosperm grafting onto embryos shows that the endosperm promotes seedling cuticle development. Grafting different endosperm and embryo mutant combinations, together with biochemical, microscopy, and mass spectrometry approaches, reveal that the release of tyrosylprotein sulfotransferase (TPST)-sulfated CIF2 and PSY1 peptides from the endosperm promotes seedling cuticle development. Endosperm-deprived embryos produced nonviable seedlings bearing numerous developmental defects, not related to embryo malnutrition, all restored by exogenously provided endosperm. Hence, seedling establishment is nonautonomous, requiring the mature endosperm.

The hypoxia-reoxygenation stress in plants

Plants are very plastic in adapting growth and development to changing adverse environmental conditions. This feature will be essential for plants to survive climate changes characterized by extreme temperatures and rainfall. Although plants require molecular oxygen (O2) to live, they can overcome transient low-O2 conditions (hypoxia) until return to standard 21% O2 atmospheric conditions (normoxia). After heavy rainfall, submerged plants in flooded lands undergo transient hypoxia until water recedes and normoxia is recovered. The accumulated information on the physiological and molecular events occurring during the hypoxia phase contrasts with the limited knowledge on the reoxygenation process after hypoxia, which has often been overlooked in many studies in plants. Phenotypic alterations during recovery are due to potentiated oxidative stress generated by simultaneous reoxygenation and reillumination leading to cell damage. Besides processes such as N-degron proteolytic pathway-mediated O2 sensing, or mitochondria-driven metabolic alterations, other molecular events controlling gene expression have been recently proposed as key regulators of hypoxia and reoxygenation. RNA regulatory functions, chromatin remodeling, protein synthesis, and post-translational modifications must all be studied in depth in the coming years to improve our knowledge on hypoxia-reoxygenation transition in plants, a topic with relevance in agricultural biotechnology in the context of global climate change.

Identification and functional analysis of the MdLTPG gene family in apple

Cuticular wax is synthesized from intracellular lipids that are exported by epidermal cells, and plant lipid transfer proteins (LTPs) play an important role in this process. The glycosylphosphatidylinositol (GPI)-anchored LTPs (LTPGs) are a large subgroup within the LTP family and function in lipid transport and wax formation. Although LTPG family members have been identified in several plant species, the LTPG gene family of apple (Malus domestica) remains uncharacterized. In this paper, we identified 26 potential LTPG genes by searching apple whole-genome annotation files using "GPI-anchored" and "lipid transferase" as keywords. Twenty of the 26 putative LTPG genes were confirmed as MdLTPG family members based on their subcellular localization predictions. The MdLTPGs were divided into four classes based on phylogenetic analysis and functional domain prediction. One member of each class was analyzed for subcellular localization, and all identified members were located on the plasma membrane. Most MdLTPG genes were induced by abiotic stress treatments such as low temperature, NaCl, and ABA. Finally, the MdLTPG17 protein was shown to interact with the lysine-rich arabinogalactan protein MdAGP18 to perform its function in wax transport during plant growth and development.

Lupeol Accumulation Correlates with Auxin in the Epidermis of Castor

Lupeol, a natural lupane-type pentacyclic triterpene, possesses various pharmacological properties, and its production attracts attention. Significant quantities of lupeol are deposited on the castor aerial organ surface and are easily extractable as a predominant wax constituent. Thus, castor might be considered as a potential bioreactor for the production of lupeol. The lupeol biosynthesis pathway is well known, but how it is regulated remains largely unknown. Among large numbers of castor cultivars, we targeted one accession line (337) with high levels of lupeol on its stem surface and low levels thereof on its hypocotyl surface, implicating that lupeol synthesis is differentially regulated in the two organs. To explore the underlying mechanisms, we did comparative transcriptome analysis of the first internode of 337 stem and the upper hypocotyl. Our results show that large amounts of auxin-related genes are differentially expressed in both parts, implying some possible interactions between auxin and lupeol production. We also found that several auxin-responsive cis-elements are present in promoter regions of HMGR and LUS genes encoding two key enzymes involved in lupeol production. Furthermore, auxin treatments apparently induced the expression levels of RcHMGR and RcLUS. Furthermore, we observed that auxin treatment significantly increased lupeol contents, whereas inhibiting auxin transport led to an opposite phenotype. Our study reveals some relationships between hormone activity and lupeol synthesis and might provide a promising way for improving lupeol yields in castor.