Abstract
Cacti have a highly specialized stem that enables survival during extended dry periods. Despite the ornamental value of cacti and the fact that stems represent the main source of explants in tissue culture, there are no studies on their morpho-anatomical and cytological characteristics in Melocactus. The present study seeks to address the occurrence of cells with mixed ploidy level in cacti tissues. Specifically, we aim to understand how Melocactus stem tissue is organized, how mixoploidy is distributed when present, and whether detected patterns of ploidy change after long periods of in vitro culture. To analyze tissue organization, Melocactus glaucescens and Melocactus paucispinus plants that had been germinated and cultivated in vitro were analyzed for stem structure using toluidine blue, Xylidine Ponceau, Periodic Acid Schiff, ruthenium red, and acid floroglucin. To investigate patterns of ploidy, apical, medial, and basal zones of the stem, as well as, periphery, cortex, and stele (vascular tissue and pith) regions of the stem and root apexes from four- and ten-year old cultured in vitro were analyzed by flow cytometry. X-ray micro-computed tomography (XRµCT) was performed with fragments of stems from both species. The scarcity of support elements (i.e., sclereids and fibers) indicates that epidermis, hypodermis, and wide-band tracheids present in cortical vascular bundles and stele, as well as water stored in aquifer parenchyma cells along the cortex, provide mechanical support to the stem. Parenchyma cells increase in volume with a four-fold increase in ploidy. M. glaucescens and M. paucispinus exhibit the same pattern of cell ploidy irrespective of topophysical region or age, but there is a marked difference in ploidy between the stem periphery (epidermis and hypodermis), cortex, stele, and roots. Mixoploidy in Melocactus is not related to the age of the culture, but is a developmental trait, whereby endocycles promote cell differentiation to accumulate valuable water.
Highlights
Cacti are highly adapted to xeric environments, owing to physiological modifications such as the Crassulacean Acid Metabolism (CAM) or morphological adaptations (Anderson, 2001)
We provide new insights on variations in endoreduplication-derived ploidy levels associated with topophysical zones in stems of two endangered Melocactus species; this evidence will improve our understanding on how cacti survive in xeric environments
This work aims to collect cytological, histochemical, and anatomical information to determine whether Melocactus stems are mixoploid, how this mixoploidy is distributed through the stem tissues, and how the pattern of ploidy may be altered by long periods of in vitro culture
Summary
Cacti are highly adapted to xeric environments, owing to physiological modifications such as the Crassulacean Acid Metabolism (CAM) or morphological adaptations (Anderson, 2001) The latter affect the stem, which consists mostly of water-storage aquifer parenchyma tissue and usually has a thick cuticle that limits water loss and a hypodermis (Anderson, 2001; Arruda et al, 2004; Ventura-Aguilar et al, 2017). Some species of Cactaceae such as species of the genus Melocactus do not branch (Machado, 2009) This limits opportunities for vegetative propagation, making them susceptible to overharvesting during the removal of whole specimens from their natural populations for commercialization as ornamental plants (Goettsch et al., 2015; Pérez-Molphe-Balch et al, 2015). Tissue culture techniques represent an attractive form of propagation for this genus, even though their slow growth means that some species require about ten years to reach reproductive age (Machado, 2009; Lema-Rumiń ska and Kulus, 2014; Pérez-Molphe-Balch et al, 2015)
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