Overexpression of the Novel Arabidopsis Gene At5g02890 Alters Inflorescence Stem Wax Composition and Affects Phytohormone Homeostasis

BrKCS6 mutation conferred a bright glossy phenotype to Chinese cabbage

KEY MESSAGE

BrKCS6 encoding 3-ketoacyl-CoA synthases was cloned through MutMap and KASP analysis, and its function was verified via allelic mutants in Chinese cabbage. Bright and glossy green appearance is an attractive commodity character for leafy vegetables and is mainly caused by the absence of epicuticular wax crystals. In this study, two allelic epicuticular wax crystal deficiency mutants, wdm9 and wdm10, were obtained from an EMS mutagenesis population of Chinese cabbage (Brassica rapa L. ssp. pekinensis). Genetic analysis revealed that the mutant phenotype was controlled by a recessive nuclear gene. BrKCS6 encoding 3-ketoacyl-CoA synthases was identified as the candidate gene by MutMap and KASP analysis. A SNP (G to A) on BrKCS6 in wdm9 led to the amino acid substitution from serine (S) to phenylalanine (F), and another SNP (G to A) in wdm10 resulted in the amino acid substitution from serine (S) to leucine (L). Both SNPs are located in the ACP_syn_III_C conserved domain, corresponding to two highly conserved sites among BrKCS6 and its homologs. These two amino acid substitutions changed the secondary structure and the three-dimensional structure of BrKCS6 protein. qRT-PCR results showed that the relative expression of BrKCS6 significantly decreased in the flower, stem, and leaves in mutant, and the relative expressions of the downstream key genes of BrKCS6 were down-regulated in mutant. We were the first to clone the precious glossy bright gene BrKCS6 which has a great potential for commodity quality breeding in Chinese cabbage.

BrWAX2 plays an essential role in cuticular wax biosynthesis in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

Map-based cloning was used to identify the BrWAX2 gene, which was involved in the cuticular wax biosynthesis. The malfunction of BrWAX2 together with other reduced expression of genes in alkane-forming pathway caused the glossy phenotype. Cuticular wax covering the outer plant surface plays various roles in protecting against biotic and abiotic stresses. Wax-less mutant shows glossy in stem and leaf surface and plays important roles in enriching Chinese cabbage germplasm resources for breeding brilliant green varieties. However, genes responsible for the glossy trait in Chinese cabbage are rarely reported. In this study, we identified a glossy Chinese cabbage line Y1211-1. Genetic analysis indicated that the glossy trait in Y1211-1 was controlled by a single recessive locus, BrWAX2 (Brassica rapa WAX 2). Using bulked segregant sequencing (BSA-Seq) and kompetitive allele-specific PCR (KASP) assays, BrWAX2 was fine-mapped to an interval of 100.78 kb. Functional annotation analysis, expression analysis, and sequence variation analysis revealed that Bra032670, homologous to CER1 in Arabidopsis, was the most likely candidate gene for BrWAX2. The gene Bra032670 was absent in glossy mutant. Cuticular wax composition analysis and RNA-Seq analysis suggested that the absence of BrWAX2 together with the decreased expression of other genes in alkane-forming pathway reduced the wax amount and caused the glossy phenotype. Furthermore, we developed and validated the functional marker BrWAX2-sp for BrWAX2. Overall, these results provide insight into the molecular mechanism underlying cuticular wax biosynthesis and reveal valuable information for marker-assisted selection (MAS) breeding in Chinese cabbage.

BnA1.CER4 and BnC1.CER4 are redundantly involved in branched primary alcohols in the cuticle wax of Brassica napus

The mutations BnA1.CER4 and BnC1.CER4 produce disordered wax crystals types and alter the composition of epidermal wax, causing increased cuticular permeability and sclerotium resistance. The aerial surfaces of land plants are coated with a cuticle, comprised of cutin and wax, which is a hydrophobic barrier for preventing uncontrolled water loss and environmental damage. However, the mechanisms by which cuticle components are formed are still unknown in Brassica napus L. and were therefore assessed here. BnA1.CER4 and BnC1.CER4, encoding fatty acyl-coenzyme A reductases localizing to the endoplasmic reticulum and highly expressed in leaves, were identified and functionally characterized. Expression of BnA1.CER4 and BnC1.CER4 cDNA in yeast (Saccharomyces cerevisiae) induced the accumulation of primary alcohols with chain lengths of 26 carbons. The mutant line Nilla glossy2 exhibited reduced wax crystal types, and wax composition analysis showed that the levels of branched primary alcohols were decreased, whereas those of the other branched components were increased. Further analysis showed that the mutant had reduced water retention but enhanced resistance to Sclerotinia sclerotiorum. Collectively, our study reports that BnA1.CER4 and BnC1.CER4 are fatty acyl-coenzyme A reductase genes in B. napus with a preference for branched substrates that participate in the biosynthesis of anteiso-primary alcohols.

Arabidopsis ECERIFERUM2-LIKEs Are Mediators of Condensing Enzyme Function

Condensing enzymes catalyze the committed reaction of fatty acid elongation and determine the chain length of fatty acids accepted and produced by the elongation complex. While necessary for the elongation of very-long-chain fatty acids (VLCFAs), identified plant condensing enzymes cannot efficiently produce VLCFAs longer than 28 carbons, which are precursors for the most abundant cuticular waxes of most plant species that have been surveyed. The eceriferum2 (cer2) mutant of Arabidopsis thaliana has a severe wax-deficient phenotype and specifically lacks waxes longer than 28 carbons, but the CER2 protein does not share sequence similarity with condensing enzymes. Instead, CER2 is homologous to BAHD acyltransferases. Heterologous expression in yeast previously demonstrated that CER2, and a small clade of BAHD acyltransferases with high sequence identity to CER2, can extend the chain-length specificity of the condensing enzyme CER6. This biochemical function is distinct from that of the broader BAHD acyltransferase family. The product specificity and physiological functions of individual CER2-LIKE proteins are unique. Here, we demonstrate that CER2 physically interacts with the fatty acid elongase. We cloned chimeric CER2-LIKE proteins and expressed these in yeast cells to identify the features that define the substrate specificities of CER2-LIKEs. We generated homology-based structural models to compare CER2-LIKEs and BAHD acyltransferases. In addition, based on the current phylogenetic analysis of the CER2-LIKE clade, we describe two further Arabidopsis CER2-LIKE genes, CER2-LIKE3 and CER2-LIKE4. We used yeast expression and mutant analysis to characterize these genes. Collectively, these results expand our knowledge of the functions of CER2-LIKEs, the BAHD acyltransferase family and cuticular wax metabolism.

Silencing against the conserved NAC domain of the potato StNAC103 reveals new NAC candidates to repress the suberin associated waxes in phellem

Both suberin and its associated waxes contribute to the formation of apoplastic barriers that protect plants from the environment. Some transcription factors have emerged as regulators of the suberization process. The potato StNAC103 gene was reported as a repressor of suberin polyester and suberin-associated waxes deposition because its RNAi-mediated downregulation (StNAC103-RNAi) over-accumulated suberin and associated waxes in the tuber phellem concomitantly with the induction of representative biosynthetic genes. Here, to explore if other genes of the large NAC gene family participate to this repressive function, we extended the silencing to other NAC members by targeting the conserved NAC domain of StNAC103 (StNAC103-RNAi-c). Transcript profile of the StNAC103-RNAi-c phellem indicated that StNAC101 gene was an additional potential target. In comparison with StNAC103-RNAi, the silencing with StNAC103-RNAi-c construct resulted in a similar effect in suberin but yielded an increased load of associated waxes in tuber phellem, mainly alkanes and feruloyl esters. Globally, the chemical effects in both silenced lines are supported by the transcript accumulation profile of genes involved in the biosynthesis, transport and regulation of apoplastic lipids. In contrast, the genes of polyamine biosynthesis were downregulated. Altogether these results point out to StNAC101 as a candidate to repress the suberin-associated waxes.

Evolutionarily conserved function of the sacred lotus (Nelumbo nucifera Gaertn.) CER2-LIKE family in very-long-chain fatty acid elongation

Identification of NnCER2 and NnCER2-LIKE from Nelumbo nucifera, which are required for the very-long-chain fatty acid elongation, provides new evidence that CER2 proteins are evolutionarily conserved across the eudicots. CER2-LIKE family proteins have been described as core components of the fatty acid elongase complex in Arabidopsis, maize, and rice, having specific function in synthesis of the C₃₀ to C₃₄ fatty acyl-CoA precursors of cuticular waxes. Little is known about the functional conservation in this gene family across species. In this study, two CER2-LIKE family proteins, NnCER2 and NnCER2-LIKE, were characterized from sacred lotus (Nelumbo nucifera), which is an ancient basal eudicot. The transcriptional expression of NnCER2 and NnCER2-LIKE was found in floating leaf blades, emergent petioles and vertical leaves, petals, and anthers. The NnCER2 and NnCER2-LIKE proteins were localized to the endoplasmic reticulum and nucleus. Overexpressing NnCER2 and NnCER2-LIKE in Arabidopsis led to alteration of cuticle wax structure in inflorescence stems, and this was associated with elevated 30, 32, and 34 carbon length wax compounds, and their derivatives. The different substrate specificities of NnCER2 and NnCER2-LIKE were explored using co-expression with AtCER6 in yeast cells. These findings provide clear evidence that the function of CER2 family proteins in producing VLCFAs is highly conserved across the eudicots.

The chemical NJ15 affects hypocotyl elongation and shoot gravitropism via cutin polymerization

We previously found a chemical, designated as NJ15, which inhibited both auxin and brassinosteroid responses in dark-grown Arabidopsis. To study its mode of action, we performed a phenotypic screening of NJ15-low-sensitive lines among mutant pools of Arabidopsis. One line (f127) showed clear NJ15-low-sensitivity in terms of hypocotyl elongation and shoot gravitropism. After further testing, it was determined that DCR, an enzyme involved in cutin polymerization, had lost its function in the mutant, which caused its low sensitivity to NJ15. Fatty acids are the base materials for polymers such as cutin and cuticular wax. We confirmed that NJ15 affects fatty acid biosynthesis, and that it does differently from cafenstrole, a known inhibitor of cuticular wax formation. Based on these results, we propose that the target of NJ15 is likely located within the cutin polymer formation pathway.

ABBREVIATIONS

Caf: cafenstrole; DEG: differentially expressed gene; FDR: false discovery rate; FOX: full length cDNA-overexpressor; VLCFA: very-long-chain fatty acid.

DEWAX2 Transcription Factor Negatively Regulates Cuticular Wax Biosynthesis in Arabidopsis Leaves

The aerial parts of terrestrial plants are covered with hydrophobic wax layers, which represent the primary barrier between plant cells and the environment and act to protect plants from abiotic and biotic stresses. Although total wax loads are precisely regulated in an environmental- or organ-specific manner, regulatory mechanisms underlying cuticular wax biosynthesis remain largely unknown. In this study, we characterized DEWAX2 (DECREASE WAX BIOSYNTHESIS2) which encodes an APETALA 2 (AP2)/ethylene response element-binding factor (ERF)-type transcription factor and is predominantly expressed in young seedlings, and rosette and cauline leaves. Total wax loads increased by approximately 12% and 16% in rosette and cauline leaves of dewax2, respectively, but were not significantly altered in the stems of dewax2 relative to the wild type (WT). The excess wax phenotype of dewax2 leaves was rescued upon expression of DEWAX2 driven by its own promoter. Overexpression of DEWAX2 decreased total wax loads by approximately 15% and 26% in the stems and rosette leaves compared with those of the WT, respectively. DEWAX2:eYFP (enhanced yellow fluorescent protein) was localized to the nucleus in Arabidopsis roots and hypocotyls. DEWAX2 possessed transcriptional repression activity in tobacco protoplasts. Transcriptome and quantitative real-time PCR analyses showed that the transcript levels of CER1, ACLA2, LACS1, LACS2 and KCS12 were down-regulated in DEWAX2 overexpression lines compared with the WT. Transient transcriptional assays showed that DEWAX2 represses the expression of its putative target genes. Quantitative chromatin immunoprecipitation-PCR revealed that DEWAX2 binds directly to the GCC motifs of the LACS1, LACS2, KCS12 and CER1 promoters. These results suggest that DEWAX2-mediated transcriptional repression may contribute to the total wax load in Arabidopsis leaves.

The Unique Role of the ECERIFERUM2-LIKE Clade of the BAHD Acyltransferase Superfamily in Cuticular Wax Metabolism

The elongation of very-long-chain fatty acids is a conserved process used for the production of many metabolites, including plant cuticular waxes. The elongation of precursors of the most abundant cuticular wax components of some plants, however, is unique in requiring ECERIFERUM2-LIKE (CER2-LIKE) proteins. CER2-LIKEs are a clade within the BAHD superfamily of acyltransferases. They are known to be required for cuticular wax production in both Arabidopsis and maize based on mutant studies. Heterologous expression of Arabidopsis and rice CER2-LIKEs in Saccharomyces cerevisiae has demonstrated that they modify the chain-length specificity of elongation when paired with particular condensing enzymes. Despite sequence homology, CER2-LIKEs are distinct from the BAHD superfamily in that they do not appear to use acyl transfer activity to fulfill their biological function. Here, we review the discovery and characterization of CER2-LIKEs, propose several models to explain their function, and explore the importance of CER2-LIKE proteins for the evolution of plant cuticles.

Molecular and Evolutionary Mechanisms of Cuticular Wax for Plant Drought Tolerance

Cuticular wax, the first protective layer of above ground tissues of many plant species, is a key evolutionary innovation in plants. Cuticular wax safeguards the evolution from certain green algae to flowering plants and the diversification of plant taxa during the eras of dry and adverse terrestrial living conditions and global climate changes. Cuticular wax plays significant roles in plant abiotic and biotic stress tolerance and has been implicated in defense mechanisms against excessive ultraviolet radiation, high temperature, bacterial and fungal pathogens, insects, high salinity, and low temperature. Drought, a major type of abiotic stress, poses huge threats to global food security and health of terrestrial ecosystem by limiting plant growth and crop productivity. The composition, biochemistry, structure, biosynthesis, and transport of plant cuticular wax have been reviewed extensively. However, the molecular and evolutionary mechanisms of cuticular wax in plants in response to drought stress are still lacking. In this review, we focus on potential mechanisms, from evolutionary, molecular, and physiological aspects, that control cuticular wax and its roles in plant drought tolerance. We also raise key research questions and propose important directions to be resolved in the future, leading to potential applications of cuticular wax for water use efficiency in agricultural and environmental sustainability.