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.
Fatty acid elongase (FAE), which catalyzes the synthesis of very-long-chain fatty acids (VLCFAs), is a multiprotein complex; however, little is known about its quaternary structure. In this study, BiFC and/or yeast two hybrid (Y2H) assays showed that 1) homo-interactions were observed in β-ketoacyl-CoA synthases (KCS2, KCS9, and KCS6), Eceriferum2-like proteins [CER2 and CER2-Like2 (C2L2)], and FAE complex proteins (KCR1, PAS2, ECR, and PAS1), except for CER2-Like1 (C2L1). 2) Hetero-interactions were observed between KCSs (KCS2, KCS9, and KCS6), between CER2-LIKEs (CER2, C2L2, and C2L1), and between FAE complex proteins (KCR1, PAS2, ECR, and PAS1). 3) PAS1 interacts with FAE complex proteins (KCR1, PAS2, and ECR), but not with KCSs (KCS2, KCS9, and KCS6) and CER2-LIKEs (CER2, C2L2, and C2L1). 4) Asp (308 aa) and three Arg (309 to 311 aa) residues of KCS9 were essential for the homo-interactions of KCS9 and hetero-interactions between KCS9 and PAS2 or ECR. An Asp (339 aa) residue of KCS9 is involved in its homo- and hetero-interactions with ECR. Complementation analysis of Arabidopsis kcs9 mutant by the expression of amino acid-substituted KCS9 mutant genes showed that 5) Two Asp residues (308 and 339 aa) of KCS9 are involved in the synthesis of C24 VLCFAs from C22. This study suggests that protein-protein interaction (PPI) in FAE complexes is important for VLCFA synthesis and provides insight into the quaternary structure of FAE complexes for efficient synthesis of VLCFAs.
Flavonols and anthocyanins are the two major classes of flavonoids in Brassica rapa. To elucidate the flavonoid biosynthetic pathway in Chinese cabbage (B. rapa L. subsp. pekinensis), we analyzed flavonoid contents in two varieties of Chinese cabbage with normal green (5546) and purple (8267) leaves. The 8267 variety accumulates significantly higher levels of quercetin, isorhamnetin, and cyanidin than the 5546 variety, indicating that 3′-dihydroxylated flavonoids are more prevalent in the purple than in the green variety. Gene expression analysis showed that the expression patterns of most phenylpropanoid pathway genes did not correspond to the flavonoid accumulation patterns in 5546 and 8267 varieties, except for BrPAL1.2 while most early and late flavonoid biosynthetic genes are highly expressed in 8267 variety. In particular, the flavanone 3′-hydroxylase BrF3′H (Bra009312) is expressed almost exclusively in 8267. We isolated the coding sequences of BrF3′H from the two varieties and found that both sequences encode identical amino acid sequences and are highly conserved with F3'H genes from other species. An in vitro enzymatic assay demonstrated that the recombinant BrF3′H protein catalyzes the 3′-hydroxylation of a wide range of 4′-hydroxylated flavonoid substrates. Kinetic analysis showed that kaempferol is the most preferred substrate and dihydrokaempferol (DHK) is the poorest substrate for recombinant BrF3′H among those tested. Transient expression of BrF3′H in Nicotiana benthamiana followed by infiltration of naringenin and DHK as substrates resulted in eriodictyol and quercetin production in the infiltrated leaves, demonstrating the functionality of BrF3′H in planta. As the first functional characterization of BrF3′H, our study provides insight into the molecular mechanism underlying purple coloration in Chinese cabbage.
Wheat (Triticum aestivum L.) is one of the world's three staple crops and accounts for approximately 20% of the total calories consumed by the world's population. It is known that wheat is a difficult crop to introduce foreign genes into, having a large genome (16 Gb) containing three highly related subgenomes (AABBDD). Owing to the low transformation efficiency of wheat, it is difficult to apply new technologies such as genome editing and basic research based on molecular biology, such as the discovery of useful genes and functional analysis. Recently, the completion of a wheat genome map by the International Wheat Genome Sequencing Consortium (IWGSC) and the development of a stable and reproducible wheat transformation system have accelerated research regarding the expression and control of useful genes. In this review, we introduce in detail the recently developed highly efficient Agrobacterium-mediated wheat transformation system and its applications in plant biotechnology, such as RNA interference (RNAi), overexpression, and gene editing using this system.
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