Abstract
In citrus, lignin overaccumulation in the juice sac results in granulation and an unpleasant fruit texture and taste. By integrating metabolic phenotyping and transcriptomic analyses, we found 702 differentially expressed genes (DEGs), including 24 transcription factors (TFs), to be significantly correlated with lignin content. CgMYB58 was further identified as a critical R2R3 MYB TF involved in lignin overaccumulation owing to its high transcript levels in Huanong Red-fleshed pummelo (HR, Citrus grandis) fruits. Transient expression of CgMYB58 led to an increase in the lignin content in the pummelo fruit mesocarp, whereas its stable overexpression significantly promoted lignin accumulation and upregulated 19 lignin biosynthetic genes. Among these genes, CgPAL1, CgPAL2, Cg4CL1, and CgC3H were directly modulated by CgMYB58 through interaction with their promoter regions. Moreover, we showed that juice sac granulation in pummelo fruits could be affected by indole-3-acetic acid (IAA) and abscisic acid (ABA) treatments. In HR pummelo, ABA significantly accelerated this granulation, whereas IAA effectively inhibited this process. Taken together, these results provide novel insight into the lignin accumulation mechanism in citrus fruits. We also revealed the theoretical basis via exogenous IAA application, which repressed the expression of CgMYB58 and its target genes, thus alleviating juice sac granulation in orchards.
Highlights
Lignin, which normally accumulates in secondary cell walls, has been reported to play a critical role in maintaining mechanical strength and facilitating the transport of water and nutrients in plants[1,2], and is crucial for plant growth and responses to environmental stress[3,4]
We found that indole-3-acetic acid (IAA)- and abscisic acid (ABA)-responsive CgMYB58 played a critical role in lignin biosynthesis
Lignin excessively overaccumulated during juice sac granulation in pummelo We investigated juice sac granulation in four pummelo cultivars, Huanong Red-fleshed (HR), HB, FH, and Kao Pan (KP), at 205 days post anthesis (DPA) and found that HR exhibited the most severe granulation (Fig. 1a)
Summary
Lignin, which normally accumulates in secondary cell walls, has been reported to play a critical role in maintaining mechanical strength and facilitating the transport of water and nutrients in plants[1,2], and is crucial for plant growth and responses to environmental stress[3,4]. In plants, the lignin biosynthetic pathway involves a series of enzymes, including phenylalanine ammonia lyase (PAL), cinnamate 4-hydroxylase (C4H), 4coumarate CoA ligase (4CL), hydroxy cinnamoyl CoA, shikimate/quinate hydroxy cinnamoyl transferase (HCT), p-coumarate 3-hydroxylase (C3H), caffeoyl CoA 3-Omethyltransferase (CCoAOMT), cinnamoyl CoA reductase (CCR), caffeic acid O-methyltransferase, cinnamyl alcohol dehydrogenase, and ferulate 5-hydroxylase. Through reactions catalyzed by these enzymes, plants produce three types of lignin monomers, phydroxyphenyl (H) monomers, guaiacyl (G) monomers, Shi et al Horticulture Research (2020)7:139 and syringyl (S) monomers, which are polymerized by laccases (LACs) or peroxidase[2,3,11]. Transcription factors (TFs) regulating lignin biosynthetic genes have been well studied in various plant species, such as Arabidopsis thaliana[12], Populus trichocarpa[13], and switchgrass (Panicum virgatum)[14]. In Arabidopsis, AC elements bound by MYBs are widely distributed in the promoters of PAL1, PAL2, 4CL1, 4CL2, HCT, C3H1, CCoAOMT1, CCR1, and CAD52
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