Callus was obtained from hypocotyls of Mesembryanthemum crystallinum seedlings cultured on two types of medium-germination medium (GM) and callus induction medium (CIM). Following subculture on shoot induction medium SIM1, the callus formed on CIM medium regenerated roots or somatic embryos, while that obtained on GM medium was non-regenerative. The activities of CuZn-superoxidase dismutase (SOD) were comparable in all calli, but the activities of FeSOD and MnSOD varied according to the activity of photosystem II and the regenerative potential of the tissues. Catalase (CAT) activity was related to H2O2 concentration and affected by both the culture conditions and the morphogenic potential of the calli. The possible role of CAT, SODs and H2O2 in the regeneration of M. crystallinum from callus is discussed.

The formation of an extracellular matrix surface network (ECMSN), and associated changes in the distribution of arabinogalactan-protein and pectin epitopes, have been studied during somatic embryogenesis (SE) and callogenesis of Trifolium nigrescens Viv. Scanning electron microscopy observations revealed the occurrence of an ECMSN on the surface of cotyledonary-staged somatic embryos as well as on the peripheral, non-regenerating callus cells. The occurrence of six AGP (JIM4, JIM8, JIM13, JIM16, LM2, MAC207) and four pectin (JIM5, JIM7, LM5, LM6) epitopes was analysed during early stages of SE, in cotyledonary-staged somatic embryos and in non-embryogenic callus using monoclonal antibodies. The JIM5 low methyl-esterified homogalacturonan (HG) epitope localized to ECMSN on the callus surface but none of the epitopes studied were found to localize to ECMSN over mature somatic embryos. The LM2 AGP epitope was detected during the development of somatic embryos and was also observed in the cell walls of meristematic cells from which SE was initiated. The pectic epitopes JIM5, JIM7, LM5 and LM6 were temporally regulated during SE. The LM6 arabinan epitope, carried by side chains of rhamnogalacturonan-I (RG-I), was detected predominantly in cells of embryogenic swellings, whilst the LM5 galactan epitope of RG-I was uniformly distributed throughout the ground tissue of cotyledonary-staged embryoids but not detected at the early stages of SE. Differences in the distribution patterns of low and high methyl-esterified HG were detected: low ester HG (JIM5 epitope) was most abundant during the early steps of embryo formation and highly methyl-esterified form of HG (JIM7 epitope) became prevalent during embryoid maturation.

Steviol glycosides (SvGls) are plant secondary metabolites belonging to a class of chemical compounds known as diterpenes. SvGls have been discovered only in a few plant species, including in the leaves of Stevia rebaudiana Bertoni. Over the last few decades, SvGls have been extensively researched for their extraordinary sweetness. As a result, the nutritional and pharmacological benefits of these secondary metabolites have grown increasingly apparent. In the near future, SvGls may become a basic, low-calorie, and potent sweetener in the growing natural foods market, and a natural anti-diabetic remedy, a highly competitive alternative to commercially available synthetic drugs. Commercial cultivation of stevia plants and the technologies of SvGls extraction and purification from plant material have already been introduced in many countries. However, new conventional and biotechnological solutions are still being sought to increase the level of SvGls in plants. Since many aspects related to the biochemistry and metabolism of SvGls in vivo, as well as their relationship to the overall physiology of S. rebaudiana are not yet understood, there is also a great need for in-depth scientific research on this topic. Such research may have positive impact on optimization of the profile and SvGls concentration in plants and thus lead to obtaining desired yield. This research summarizes the latest approaches and developments in SvGls production. Key points • Steviol glycosides (SvGls) are found in nature in S. rebaudiana plants. • They exhibit nutraceutical properties. • This review provides an insight on different approaches to produce SvGls. • The areas of research that still need to be explored have been identified.

Androgenic plant regeneration from wheat anther callus was accompanied by the formation of a conspicuous extracellular matrix surface network (ECMSN) around the induced callus cells and young embryo-like structures. Microscopic observations at the onset of regeneration revealed the presence of two distinct types of cells on the callus surface: large, loosely attached parenchymatous cells and small tightly packed meristematic cells arranged in multicellular clusters. Parenchyma cells of the callus had smooth surface, while on the surface and between the cells of multicellular clusters numerous fine fibrils of ECMSN were observed. The structural arrangement of the ECMSN changed during culture. On the surface of globular embryo-like structures, before protoderm formation, the ECMSN was the most abundant and arranged as a compact layer of secretion with wide strands visible at the cell junctions. Further development of globular embryos was disturbed, giving rise to branched structures outlined by continuous epidermis. The development of such regenerants was accompanied by gradual degradation of the extracellular network and finally its complete disappearance. Digestion with protease did not destroy the network. Treatment of the calluses with chloroform and washing with ether-methanol led to partial destruction of the network, while digestion with pectinase removed the network completely and resulted in the collapse of surface embryo cells.

The distribution of several arabinogalactan protein and pectic epitopes were studied during organogenesis in androgenic callus of wheat. In cell wall of mature and degenerating parenchyma cells, the arabinogalactan epitopes JIM4, JIM14, JIM16 or LM2 were expressed differently according to the cells location. LM2 was observed also in meristematic cells of regenerated shoot buds and leaves. Anti-pectin JIM7 labelled the wall of meristematic cells but fluorescence was strongest in outer walls of surface cells of callus and shoot buds coated by extracellular matrix surface network (ECMSN). During leaves growth the ECMSN disappeared, and JIM7 fluorescence decreased. JIM5 epitope was abundant in the cell walls lining the intercellular spaces of callus parenchyma and in tricellular junctions within regenerated buds and leaves.