Abstract
Main ConclusionPermanent glandular trichomes of Robinia viscosa var. hartwigii produce viscous secretion containing several secondary metabolites, as lipids, mucilage, flavonoids, proteins and alkaloids.Robinia viscosa var. hartwigii (Hartweg’s locust) is an ornamental tree with high apicultural value. It can be planted in urban greenery and in degraded areas. The shoots, leaves, and inflorescences of this plant are equipped with numerous persistent glandular trichomes producing sticky secretion. The distribution, origin, development, morphology, anatomy, and ultrastructure of glandular trichomes of Hartweg's locust flowers as well as the localisation and composition of their secretory products were investigated for the first time. To this end, light, scanning, and transmission electron microscopy combined with histochemical and fluorescence techniques were used. The massive glandular trichomes differing in the distribution, length, and stage of development were built of a multicellular and multiseriate stalk and a multicellular head. The secretory cells in the stalk and head had large nuclei with nucleoli, numerous chloroplasts with thylakoids and starch grains, mitochondria, endoplasmic reticulum profiles, Golgi apparatus, vesicles, and multivesicular bodies. Many vacuoles contained phenolic compounds dissolved or forming various condensed deposits. The secretion components were transported through symplast elements, and the granulocrine and eccrine modes of nectar secretion were observed. The secretion was accumulated in the subcuticular space at the trichome apex and released through a pore in the cuticle. Histochemical and fluorescence assays showed that the trichomes and secretion contained lipophilic and polyphenol compounds, polysaccharides, proteins, and alkaloids. We suggest that these metabolites may serve an important function in protection of plants against biotic stress conditions and may also be a source of phytopharmaceuticals in the future.
Highlights
The original results obtained in this study indicate that the glandular trichomes present on the flowers of Hartweg’s locust belong to persistent and repetitive secretion process
Capitate trichomes located on different parts of the inflorescence differ in the distribution, development stage, and length, but have the same microstructure and undoubtedly serve the same protective function
Secretion is transported via the symplast pathway and is collected in the subcuticular space from where it is released through cuticle pores
Summary
Many plants from different botanical families produce diverse external secretory structures, e.g. nectaries, hydath‐ odes, and glandular trichomes releasing secretion with a varying composition.Glandular trichomes are characteristic for representatives of many botanical families, e.g. Asteraceae (Muravnik et al 2016; Xiao et al 2016), Cannabaceae (Mahlberg and Kim 2004), Geraniaceae (Boukhris et al 2013), Lamiaceae (Mau‐ rya et al 2019), Orobanchaceae (Konarska and Chmielewski 2020), Scrophulariaceae (Attar et al 2007), Solanaceae (Bergau et al 2016), Verbenaceae (Silva et al 2016), and Fabaceae (Barros et al 2017a; Vargas et al 2019).1 3 Vol.:(0123456789) 102 Page 2 of 19Planta (2020) 252:102Glandular trichomes produce, store, or secrete mainly lipophilic substances, i.e. fats, waxes, essential oils, and resins (Stojičić et al 2016; Citti et al 2019; Konarska and Chmielewski 2020). It has been found that trichome secretion deters herbivores and acts as a natural pesticide (LoPresti 2015; Murungi et al 2016), inhibits the growth of fungal and bacterial pathogens (Steiner et al 2015; Rodriguez et al 2018), acts as a food or signalling attract‐ ant (Werker 2000; Płachno et al 2019), contributes to the spread of fruits (Heinrich et al 2002), and protects against atmospheric oxidative stress (Li et al 2018). Addition‐ ally, investigations of glandular trichomes and their secretion may contribute greatly to elucidation of the ecology and evolution of the genus (Eiji and Salmaki 2016; Khosroshahi Eyvazadeh and Salmaki 2019) and identification and stand‐ ardisation of medicinal raw materials (Karlygash et al 2016; Livingston et al 2019)
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