Agata Leszczuk

Structural network of arabinogalactan proteins (AGPs) and pectins in apple fruit during ripening and senescence processes

The cell wall is an essential framework determining the overall form of the plant cell. Our study was focused on the distribution of arabinogalactan proteins (AGPs), arabinan, and homogalacturonan in fruit cells during ripening and storage with emphasis on quantitative analysis of their presence in particular regions of the cell wall - plasma membrane. The localization of the examined compounds was determined with immunohistochemistry techniques and immunogold labelling. Spatio-temporal colocalization between AGPs epitopes - [βGlcA(1→3)-αGalA(1→2)Rha] recognized by JIM13 and MAC207 antibodies, and arabinan labelled by the LM16 antibody was detected in the inner cell wall layer, in association with the plasma membrane. The specific arrangement of AGP and arabinan epitopes differentiated them from homogalacturonan epitopes, consisting of GalA residues recognized by LM19 and LM20 antibodies in all the examined fruit maturity stages. The disruption of cell wall - plasma membrane continuum, observed during ripening-associated softening process, was associated with both the substantial decrease of AGPs, pectins content and with remodeling of their arrangement. The results indicate that the textural properties of fruit during growth and postharvest storage, an attribute of fruit quality becoming selection criteria for consumers, depend on the existence of dynamic network organizing polysaccharides and glycoproteins in the extracellular matrix.

Distribution of arabinogalactan proteins and pectins in the cells of apple (Malus × domestica) fruit during post-harvest storage

Background and Aims Changes in the arrangement of cell wall components determine cell wall properties (integrity, stiffness), thereby affecting the macro-scale properties of fruits, which are important for consumers and industry. Arabinogalactan proteins (AGPs) are ubiquitous components of the plant cell, in which they have various functions. Currently, AGPs are considered to be one of the less well-known, enigmatic proteoglycans, a consequence of their heterogeneous structure and unclear mechanism of activity. Methods An immunocytochemical study was conducted to elucidate the distribution of AGPs and pectic polysaccharides contained in apple (Malus × domestica) fruit during senescence. De-esterified homogalacturonan (LM19), methyl-esterified homogalacturonan (LM20), processed arabinan (LM16) and three AGP epitopes (JIM13, JIM15, MAC207) were identified in the fruit at three stages: fresh fruit, and fruit at 1 and 3 months of post-harvest storage. Key Results Microscopy revealed spatio-temporal changes in the localization of all examined epitopes. Changes of fruit cell wall assembly and its degradation were confirmed by determination of the galacturonic acid content in the WSP (water soluble pectins), CSP (chelator soluble pectins) and DASP (dilute alkali soluble pectins) fractions. Conclusions The results revealed dependencies between AGPs, arabinan and homogalacturonan distribution in apple fruit, which are correlated with changes in microstructure during senescence. We propose that AGPs are involved in establishment of the cell wall - plasma membrane continuum.

Specific ultrastructure of the leaf mesophyll cells of Deschampsia antarctica Desv. (Poaceae)

The ultrastucture of mesophyll cells of Deschampsia antarctica Desv. (Poaceae) leaves was investigated using the standard method of preparing material for examination in transmission electron microscopy (TEM). The investigated leaves were collected from the Antarctic hairgrass growing in a tundra microhabitat and representing xermorphic morphological and anatomical features. The general anatomical features of mesophyll cells are similar to those in cells of another grass leaves. The observations of the ultrastructure of mesophyll cells have shown that the organelles are located close to each other in a relatively small amount of the cytoplasm or closely adhere to each other. Organelles such as mitochondria, peroxisomes, and Golgi apparatus, as well as osmiophilic materials are gathered close to the chloroplasts. The chloroplast of the mesophyll cells of the D. antarctica leaf can form concavities filled with the cytoplasm. Such behaviour and ultrastructure of organelles facilitate exchange/flow of different substances engaged in the metabolic activity of the cell between cooperating organelles.