HRGPs represent a major protein component of plant cell walls . They are rich in hydroxyproline but also in serine, lysine, tyrosine, and valine residues, and they contain arabinose and galactose in the attached oligosaccharide chains [31–33]. Extensins represent a subfamily of HRGPs. In contrast to dicotyledonous plant species, the extensin subfamily of monocotyledonous plants is relatively simpler. They are rich in threonine or histidine rather than serine, and hence they are called threonine- or histidine-hydroxyproline-rich glycoproteins (THRGPs or HHRGPs) . Moreover, extensins of dicots are highly glycosylated, contain 50-60% (w/w) of carbohydrate and form a left-handed polyproline II helix while extensins of monocots are less glycosylated and exist in a random coil conformation .
Monoclonal antibodies JIM11 and JIM20 recognize specific arabinosylation patterns of HRPGs such as extensins and Solanaceous lectins but not those of arabinogalactan proteins [20, 21]. Since banana also contain lectins [34, 35] it is possible that JIM11 and JIM20 antibodies recognize except extensins also these lectins. The JIM11 and JIM20 antibodies were used previously to study extensin, extensin-like and HRGP epitopes in diverse dicotyledonous plants [18, 20, 21] but also in green alga  and green seaweed . In monocotyledonous species such as onion, JIM11 and JIM20 extensin epitopes were localized to rhizodermis, exodermis, endodermis, pericycle and phloem of primary root as well as to the root surface (Casero et al. 1998). Here, to our knowledge for the first time, the localization and function of JIM11 and JIM20 HRGP epitopes were studied during somatic embryogenesis of banana, a very important monocot fruit and crop.
In monocotyledonous maize, the mRNA of HRGP accumulates in young organs rich in dividing cells but it decreased in mature tissues . Moreover, it showed a specific pattern of expression in immature embryos . Further study revealed that the accumulation of this mRNA occurred early during cell differentiation and before acquisition of the final cell wall structure . In the present study we showed that ECs of banana contained HRGP epitopes recognized by JIM11 and JIM20 antibodies. Thus, these epitopes might serve as good markers of embryogenic competence in ECs. During embryo development from ECs, the same JIM11 and JIM20 epitopes were abundant at the surface of proembryos and globular embryos. They were likely associated with the proper adhesion and monolayer formation of embryo epidermis. In late-stage embryos, however, the JIM11 and JIM20-positive signal was stronger in inner cortical and vascular tissues. We also showed that developmental distribution and subcellular localization of these surface-located HRGP epitopes were affected by 3,4-DHP treatment, which led to the disintegration of the ECM and disaggregation of the epidermis (resembling callus formation). Particularly important was finding that both immunomodulation with JIM11 antibody as well as treatment with 3,4-DHP negatively affected and reduced embryo formation and germination as well as plant regeneration capacity from banana somatic embryos. Altogether, these data suggest that developmentally regulated HRGP proteins are essential for development, germination and regeneration of banana somatic embryos. Similar results were recently reported by Zhang et al.  on tobacco zygotic embryo development. These authors suggested that extensins reacting to the same antibodies JIM11 and JIM20 play important roles in the cotyledon primordium formation, in the activity of the shoot apical meristem and in vascular differentiation during embryo development.
Although there are many differences between HRGPs and extensins of monocotyledonous and dicotyledonous plant species, there are still some similarities between them. There are few reports about similar localization of extensin epitopes in monocotyledonous and dicotyledonous plant species. For example, in rice (Oryza sativa L.), JIM12 and JIM20 antibodies raised against extensins from dicotyledonous plant species labelled the root tissues in the same pattern as the LM1 antibody  which was derived against extensins from rice . Monocot barley and rice protoplasts contain JIM19 and JIM20 extensin-like epitopes [41, 43], while there were both similarities and differences to the labelling pattern detected in dicot pea . Here we show that JIM11 and JIM20 antibodies prepared against extracts from dicotyledonous plants such as carrot and pea, respectively [20, 21] could recognize HRGPs in banana.
Interestingly, synthetic decapeptide matching the C-terminal sequence of inversion-specific glycoprotein (ISG), a HRGP from algae closely related to the extensins from higher plants, was able to disaggregate alga into individual cells  and this ISG was likely involved in the early processes of ECM biogenesis. Little is known about chemical composition, biogenesis and function of ECM at the surface of somatic embryos [45, 46], especially in monocot plant species. In maize, the ECM contains AGP and pectin epitopes [6, 7]. Here, we found, to our knowledge for the first time, JIM11 and JIM20 HRGP epitopes in the ECM covering outer surface of banana somatic embryos while this ECM was disrupted by treatment with 3,4-DHP.