A Fresh Look at Celery Collenchyma and Parenchyma Cell Walls Through a Combination of Biochemical, Histochemical, and Transcriptomic Analyses

Celery (<i>Apium graveolens</i>) can be considered as a model plant for studying pectin-enriched primary cell walls. In addition to parenchyma cells with xyloglucan-deficient walls, celery petioles contain collenchyma, a mechanical tissue with thickened cell walls of similar composition. This study presents a comprehensive analysis of these tissues at both early and late developmental stages, integrating data on polysaccharide yield, composition, localization, and transcriptome analysis. Our results reveal that young collenchyma walls possess distinct polysaccharide compositions, including higher levels of rhamnogalacturonan I (RG-I), branched galactans, esterified homogalacturonan, and xyloglucan, compared to parenchyma cells. A significant number of genes encoding proteins involved in pectin methylesterification and acetylation were upregulated in young collenchyma. Different gene isoforms encoding glycosyltransferases involved in RG-I biosynthesis were activated in both collenchyma and parenchyma, suggesting potential variations in RG-I structure and function across different primary cell walls. We identified a set of potential glycosyltransferases involved in RG-I biosynthesis in collenchyma and proposed synthase complexes for heteromannan and heteroxylan. The transcriptome data not only confirmed known biochemical traits of celery cell walls but also provided deeper insights into the peculiarities of cell wall polysaccharide metabolism, thereby helping to narrow down candidate genes for further molecular genetic studies.