Tissue-specific transcriptomics analysis is widely used to identify regulators involved in plant development, growth, and responses to environmental stress. Lignification of specific fruit tissues is an evolutionary significant process that affects seed dispersal, whose regulation is also of great economic importance in fruit crops. Although a vast number of transcriptomic studies of various fruit types have been reported in recent years, the comprehensive analysis of fruit tissue-specific transcriptomics remains relatively scarce. In the present work, we performed a detailed tissue-specific transcriptome analysis of C. chekiangoleosa based on its fruit lignification pattern (Fig. 1a). The DEGs’ analysis focused on those genes associated with the highly lignified EN and SC tissue, which revealed thousands of them that were potentially involved in the fruit lignification process (Fig. 1). Functional analysis suggested the enriched DEGs were related to various biological pathways including the phenylpropanoid biosynthesis pathway (Fig. 2c & d), which implicates a central role for lignin biosynthesis during fruit tissue patterning. In peach fruit, for example, a genome-wide characterization of its transcriptome during the phase of stone cell formation in endocarp found evidence for the induction of prominent phenylpropanoid, lignin, and flavonoid pathway genes[40,41]. Likewise, a transcriptomics study of three developmental stages of pear fruit demonstrated that up-regulation of Cinnamoyl-CoA Reductase (CCR) was involved in stone cell formation. Our results from the gene expression analyses are largely consistent with previous work (Fig. 3), which suggests that common regulatory pathways are involved in establishing fruit lignification patterns.
The formation of a specific lignification pattern in fruits is regulated by the coordination of several types of TFs active during the developmental stages of fruit. Camellia plants form typical capsular fruits that undergo two independent lignification events, that of fruit peels and that of the seed coat. A genetic model for how lignification of C. chekiangoleosa fruits is directed has been proposed: a cascade of TFs, starting with the SHP-FUL MADS-box TFs through to bHLH-type TFs, NAC, MYB, and BLH TFs work together to regulate the biosynthesis of the cell wall and secondary metabolites during fruit development. We showed that the expression patterns of different types of TFs, including NAC, MYB, and BLH-like families, are correlated with the lignin accumulation in C. chekiangoleosa fruits (Fig. 3b). Hence, our results provided empirical evidence of the transcriptional network underlying that fruit’s lignification pattern. We also found that Camellia fruits are diverse in their size, secondary metabolites, and seed oil contents. But little is known about genetic regulation of fruit development in Camellia species, probably because of insufficient molecular biology tools. In the future, the functional characterizations of those TFs in C. chekiangoleosa will be essential for elucidating the regulatory mechanism responsible for that plant’s specific lignification pattern.
Lignification is a unique process contributing critically toward the maintenance and regulation of plants growth and development and their responses to biotic/abiotic stresses. Although lignin biosynthesis and its transcriptional regulation have been extensively studied for wood formation, lignification's regulation during fruit development is not yet well characterized, especially in fruit crops. The genus Camellia contains many species whose seed oil production is economically valuable. The fruit lignification is also a critical breeding trait associated with fruit size, seed dispersal, and oil yield. Recent work on C. japonica characterized the homolog gene of SHP1/2 (CjPLE) and revealed its potential role in regulating the pattern of fruit lignification; however, based on the callus-transformation assay, direct activation of lignin biosynthesis genes by CjPLE was not proved. Here we evaluated the key lignin biosynthesis genes and TFs in C. chekiangoleosa, finding that major lignin-related genes were highly expressed in both EN and SC tissues (Fig. 3). Therefore, we proposed that the activation of lignin biosynthesis in specified tissues requires a hierarchical interaction of TFs during fruit development.
The NST-like TFs are recognized as master regulators in the regulation of lignin biosynthesis for secondary cell wall formation in two well studied plants, A. thaliana and P. trichocarpa. Research on fruit crops has uncovered conserved functions of homologs of NST-like NAC genes for regulating the fruit lignification process. In loquat fruits, four NAC TFs (EjNAC1-4) are correlated with lignin accumulation in response to low temperature storage and heat stress. Functional analyses showed that EjNAC1 and EjNAC3 are capable of directly activating the expression of lignin biosynthesis genes. We found that CcNST1 was highly expressed in EN and SC tissues, whose levels correlated with the lignification pattern (Figs 1 & 3). Further, we showed that ectopic expression of CcNST1 in A. thaliana and hybrid poplar augmented tissue lignification (Figs 5 & 6). These results provided evidence that CcNST1 acts as a positive regulator of lignin biosynthesis in C. chekiangoleosa. Also, in the transgenic lines of poplar, the expression of SND1, MYB21, and MYB74 — downstream TFs of the poplar NST gene — was significantly up-regulated (Fig. 6). This result suggests CcNST1 is a high hierarchical activator of lignin biosynthesis during fruit development. Future work using the Camellia-based genetic transformation systems is now required to uncover the downstream genes regulated by CcNST1.
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Yan C, Nie Z, Hu Z, Huang H, Ma X, et al. 2022. Tissue-specific transcriptomics reveals a central role of CcNST1 in regulating the fruit lignification pattern in Camellia chekiangoleosa, a woody oil-crop. Forestry Research 2:10 doi: 10.48130/FR-2022-0010
Tissue-specific transcriptomics reveals a central role of CcNST1 in regulating the fruit lignification pattern in Camellia chekiangoleosa, a woody oil-crop
- Received Date: 16 May 2022
- Accepted Date: 15 July 2022
- Published Online: 03 August 2022
Abstract: Fruit lignification is of significant economic importance because it affects the quality of fruit and the production of seed oil. The specified lignification pattern in Camellia chekiangoleosa fruits plays critical roles in its seed oil yield, but little is known about how this lignification process is regulated. Here, we report on a comprehensive tissue-specific transcriptomics analysis conducted for C. chekiangoleosa fruit. By mining the differentially expressed genes, we found that lignin biosynthesis and transcriptional regulation pathways were significantly enriched in the lignified tissues. The homolog of NST-like transcription factor, CcNST1, was highly expressed in lignified seed coat and endocarp tissues; transgenic analyses of CcNST1 in Arabidopsis and hybrid poplar revealed the enhanced lignification levels of various tissues. Gene expression analysis of the transgenic lines uncovered potential downstream genes involved in the regulation of lignin biosynthesis. This work provides a valuable gene expression resource and identified the pivotal role of CcNST1 in regulating the lignin biosynthesis underlying fruit lignification.