Contrastive analysis of trichome distribution, morphology, structure, and associated gene expression reveals the formation factors of different trichome types in two commercial Rosa species
Introduction
Trichomes are present on the epidermal surfaces of the leaves, stems, sarcocarps, fruits, seeds, and other parts of many terrestrial plants (Chen et al., 2014). They originate from epidermal cells and are characterized by their morphology, including whether they are glandular or non-glandular, unicellular or multicellular, branched or non-branched (Werker, 2000). These structures can perform a range of functions for ecological interactions, such as protection against physical or biological stress (Karabourniotis et al., 2020). Furthermore, trichome structure has been used traditionally as an important indicator in plant classification (Ma et al., 2016; Sajna and Sunojkumar, 2018).
The mechanisms of trichome initiation and development for unicellular trichomes have been extensively studied in Arabidopsis (Hülskamp, 2004; Schellmann and Hülskamp, 2005; Machado et al., 2009). Numerous studies have identified a set of putative transcriptional regulators (comprising both positive and negative transcription factors) that control the process of epidermal cell development. The positive regulators include R2-R3 type MYB transcription factors GLABRA1 (GL1), the basic helix-loop-helix (bHLH) protein GLABRA3 (GL3), and the WD-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1) (Walker et al., 1999; Zhang et al., 2003). These three positive regulatory proteins together activate trichome differentiation by enhancing the expression of GLABRA2 (GL2) (Serna, 2004). Single-repeat R3 MYB transcription factors CAPRICE (CPC), TRIPTYCHON (TRY), and transcriptional enhancers of TRY and CPC, such as ETC1, ETC2, and ETC3, competitively bind to bHLH factors when they move to the neighboring cells (Gan et al., 2011; Pesch and Hülskamp, 2011; Tominaga-Wada and Wada, 2017). However, unlike unicellular trichomes, relatively little is known about the development and regulatory networks of multicellular trichomes in plants.
Rosa roxburghii Tratt. (RR) is widely distributed in southwest and northwest China and possesses fruits that are valued for their high levels of ascorbic acid (AsA), superoxide dismutase activity, and cancer-preventing effects. Similarly, R. sterilis S. D. Shi (RS) exhibits numerous nutritious and antioxidant activities. RR and RS fruits are being increasingly utilized as functional foods in the food industry to produce preserved foods and desserts, as well as in the beverage industry to produce wine, beer, and soft drinks (He et al., 2016; Li et al., 2016). RR and RS belong to the Rosaceae family and have been found to be very closely related based on Random Amplification of Polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) molecular markers (Wen and Deng, 2003; Wen et al., 2004). However, RR and RS differ in their appearance, the most significant features of which include seed abortion and differences in fruit shape, size, and anatomy.
Trichomes are present on the buds, stems, pedicels, and fruits of RR and RS, which affects the appearance and perceived quality of the fruits. However, to date, little is known about the formation or regulation of trichomes in these plants. A detailed analysis of trichome types and distribution in the vegetative parts of Rosa is lacking from previous studies (Caissard et al., 2006; Asano et al., 2008; Feng et al., 2015). He et al. (1994) and Kellogg et al. (2010) explored the morphological diversity and anatomy in Rosa trichomes, and we also previously reported the types of trichomes in two germplasms of R. roxburghii (Wang et al., 2019). However, the lack of structural studies and exploration of the molecular mechanisms of the development of trichomes has limited the elucidation of the associated molecular pathways. As the current understanding of trichome distribution and development in RR and RS is insufficient, here we analyze the various forms of those structures on the stems, sepals, fruits, and pedicels in relation to the morphology, distribution, ultrastructure, and histochemistry of the trichomes. Furthermore, changes in the relative expression levels of trichome-related genes (TRY, CPC, GL1, GL2, GL3, TTG1, PDF2, and PDF2-like) in both developing buds and in vitro organs with different types of trichomes were studied to facilitate the improvement of Rosa germplasms through genetic engineering.
Section snippets
Plant materials
Ten-year-old trees of R. roxburghii (RR) and R. sterilis (RS) were obtained from the fruit germplasm repository of Guizhou University, Guiyang, China (26°42.408′N, 106°67.353′E). Samples were collected at random and immediately frozen in liquid nitrogen and stored at −80 °C until processing.
Stereomicroscopy
The floral buds, stems, leaves, pedicels, sepals, and fruits of RR and RS were examined using a stereomicroscope (Zeiss Stemi 508) to characterize the types and distribution of the trichomes. Samples of
Morphology and structure of the trichomes
Trichomes were abundant on the surfaces of both the vegetative and reproductive organs of RR and RS, including the floral buds, stems, leaves, sepals, and fruits (Fig. 2). However, there were differences in the types of trichomes present in these organs. Broadly, the trichomes present on the surfaces of RR and RS could be split into two main categories: multicellular glandular trichomes and multicellular non-glandular trichomes. In RR, the glandular trichomes were only observed on the abaxial
Discussion
Trichomes are hair-like structures that are common on the surfaces of branches, roots, leaves, and sepals (Chen et al., 2014;Haratym et al., 2015), and indeed some fruits are also thickly covered with trichomes. The surface of the fruits of RR and RS is one such example, being densely covered with trichomes, as observed in many members of the Rosa family. These trichomes have implications for fruit quality, cultivation, and processing (Singh et al., 2020). Here, two morphologically distinct
CRediT author statement
Wen-Tao Ma: Performed the experiments, analyzed the data, prepared figures and/or tables,writing- original draft preparation. Min Lu: Analyzed the data, approved the final draft. Richard A. Ludlow: Reviewed drafts of the paper, approved the final draft. Dao-Jing Wang and Jing-Wen Zeng: Contributed reagents/materials/analysis tools. Hua-Ming An: Conceived and designed the experiments, approved the final draft. All authors read and approved the final manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by grants from the Joint Fund of the National Natural Science Foundation of China and the Karst Science Research Center of Guizhou Province (Grant No. U1812401), National Natural Science Foundation of China (31660549), the Talent Project of Guizhou Province (Project No. 20164016) and the Construction Program of Biology First-class Discipline in Guizhou (GNYL2017009).
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