Plant trichomes are simple hair-like structures that extend from the epidermal surface of plants and are present on most terrestrial plants (Johnson, 1975). Trichomes increase the ability of the epidermal layer to moderate surface temperatures and reduce transpiration as the pubescence traps air, which effectively thickens the barrier between the epidermis and the environment (Schuepp, 1993). In addition to abiotic stresses, trichomes are known to protect plants against herbivores, insects, pathogens, and mechanical injuries (Barton, 2014; Halpern et al., 2007; Szymanski et al., 2000). Trichomes vary greatly in their morphology, but can be classified into two types: simple trichomes (nonsecreting) and glandular (secreting) trichomes (Wagner et al., 2004). In Arabidopsis (Arabidopsis thaliana), unicellular simple trichomes have proven to be great models for understanding cell fate and differentiation (Breuer et al., 2009; Marks et al., 2009; Wagner, 1991). Glandular or secreting trichomes are present in many vascular plants and are multicellular structures with secreting glands at the tip of the stalk, which often produce and store terpenoids, phenylpropanoid oils, and other secondary metabolites (Mahmoud and Croteau, 2002; Schilmiller et al., 2008; Wagner et al., 2004). Other epidermal outgrowths include thorns, prickles, and spines. Although these terms refer to botanically different structures with different features, all are commonly referred to as “thorns”; however, botanically, thorns are modified branches and spines are modified leaves, both containing adjoining vascular tissue. In contrast, prickles are formed by multiple cellular divisions of the epidermis and do not contain internal vascular tissue (Bieniek and Millington, 1967; Blaser, 1956; Coyner et al., 2005; Esau, 1977; Posluszny and Fisher, 2000). Most bramble research uses the term thorns to refer to these multicellular epidermal outgrowths, but for clarity and as correct botanic terminology, the term prickle for the presence of epidermal appendages and prickle-free for the absence is used in this manuscript.
The genus Rubus is an excellent member of the family Rosaceae to understand prickle initiation and development. Fruits of the genus Rubus, also known as caneberries or brambles, are an important source of cellulose, vitamin E, natural pigments, antioxidants, and phenolic compounds (Beattie et al., 2005). Prickles are an unappealing trait in red and black raspberry and blackberry (Clark et al., 2007), as well as in many wild Rubus species. Prickles complicate both fruit harvesting and field management (pruning, training). Because of this, there have been many efforts to use the prickle-free phenotype in breeding, thus resulting in the prickle-free cultivars such as ‘Joan J’ and ‘Glen Ample’ red raspberry (R. idaeus L.) (Finn et al., 2008), and ‘Natchez’, ‘Chester’ (Clark and Moore, 2008), ‘Apache’, and ‘Triple Crown’ blackberry (R. hybrid), among others. The prickle-free phenotype has long been sought after in blackberry sports of popular cultivars; however, combining the prickle-free trait with other important traits through traditional breeding approaches can be time-consuming and difficult because of polyploidy and/or high heterozygosity among the germplasm. Because the genus has both prickled and prickle-free cultivars with similar genetics, comparisons at both the morphological and molecular levels is possible in understanding prickle development. This knowledge is the first step toward the development of prickle-free versions of economically important cultivars using genome-editing techniques.
An analysis (Coyner et al., 2005) of prickle development using a blackberry prickle chimera provided a significant step in understanding how different embryonic layers (LI, LII, LIII) communicate for prickle formation. In the study, the chimeric blackberry, which produced prickles in only a few regions of the plant, was characterized. The results provided evidence of communication between epidermal and cortical cells for prickle development in blackberry. On the removal of outermost layer (LI-derived epidermal layer) of the prickle-free area, prickles were produced from the layer underneath (LII-derived cambial layer), suggesting the presence of molecular cues between the two layers. There have been additional studies in understanding morphological structures, histochemical features, origin, and genetic patterns of prickles in Rosaceous species (Asano et al., 2008; Kellogg et al., 2011; Rajapakse et al., 2001). Although prickles of red raspberry consist of only epidermal tissues, prickles of blackberry comprise both epidermal and cortical tissues but still lack vasculature (Kellogg, 2009). There is limited knowledge regarding the molecular pathways contributing to the prickle-free trait, which has hindered the application of modern molecular approaches to studying prickle development. To identify the molecular pathways controlling prickle development, a thorough understanding of prickle morphology in the genus is required to serve as the foundation for these additional studies.
A previous morphological study on understanding prickle development in Rubus suggested that prickles are modified glandular trichomes (Kellogg et al., 2011). In the study, the prickled red raspberry cultivar Heritage and the low prickle cultivar Canby, the prickle-free blackberry cultivar Arapaho and the fully prickled cultivar Prime Jim, and the prickled ‘Radtko’ rose cultivar (Rosa hybrida L.) were examined. However, a genetically prickle-free red raspberry cultivar was not included for comparison. Without this comparison, it could not be determined if glandular trichomes were present in the prickle-free genotype/phenotype or if the development was halted prematurely as in the case of ‘Canby’, which is often described as prickle-free but in reality is functionally prickle-free on most of the mature tissue. ‘Canby’ produces glandular trichomes that mature into prickles only in some instances.
In this study, morphological differences between prickled and prickle-free genotypes/phenotypes in different Rubus species were compared to better understand prickle development within the genus. To better understand the process of prickle development in the genus, scanning electron micrographs of different members of the genus were examined, including prickled red raspberry, prickle-free red raspberry, prickled blackberry, prickle-free blackberry, prickled black raspberry (R. occidentalis L.), prickle-free black raspberry, and a prickled complex hybrid with purple fruit (purple hybrid) (R. occidentalis × R. idaeus). The prickled wineberry (R. phoenicolasius L.), which also produces long, dense hairs, was included to examine the contrast between prickle and hair development.
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