Bean and Pea Plastoglobules Change in Response to Chilling Stress.

Plastoglobules (PGs) might be characterised as microdomains of the thylakoid membrane that serve as a platform to recruit proteins and metabolites in their spatial proximity in order to facilitate metabolic channelling or signal transduction. This study provides new insight into changes in PGs isolated from two plant species with different responses to chilling stress, namely chilling-tolerant pea (Pisum sativum) and chilling-sensitive bean (Phaseolus coccineus). Using multiple analytical methods, such as high-performance liquid chromatography and visualisation techniques including transmission electron microscopy and atomic force microscopy, we determined changes in PGs' biochemical and biophysical characteristics as a function of chilling stress. Some of the observed alterations occurred in both studied plant species, such as increased particle size and plastoquinone-9 content, while others were more typical of a particular type of response to chilling stress. Additionally, PGs of first green leaves were examined to highlight differences at this stage of development. Observed changes appear to be a dynamic response to the demands of photosynthetic membranes under stress conditions.

Galactolipid deficiency disturbs spatial arrangement of the thylakoid network in Arabidopsis thaliana plants.

The chloroplast thylakoid network is a dynamic structure which, through possible rearrangements, plays a crucial role in regulation of photosynthesis. Although the importance of the main components of the thylakoid membrane matrix, galactolipids, in the formation of the network of internal plastid membrane was found before, the structural role of monogalactosyldiacylglycerol (MGDG) and digalactosylidacylglycerol (DGDG) is still largely unknown. We elucidated detailed structural modifications of the thylakoid membrane system in Arabidopsis thaliana MGDG- and DGDG-deficient mutants. An altered MGDG/DGDG ratio was structurally reflected by formation of smaller grana, local changes in grana stacking repeat distance, and significant changes in the spatial organization of the thylakoid network compared with wild-type plants. The decrease of the MGDG level impaired the formation of the typical helical grana structure and resulted in a 'helical-dichotomic' arrangement. DGDG deficiency did not affect spatial grana organization but changed the shape of the thylakoid membrane network in situ from lens like into a flattened shape. Such structural disturbances were accompanied by altered composition of carotenoid and chlorophyll-protein complexes, which eventually led to the decreased photosynthetic efficiency of MGDG- and DGDG-deficient plants.

Spatial Nano-Morphology of the Prolamellar Body in Etiolated Arabidopsis thaliana Plants With Disturbed Pigment and Polyprenol Composition.

The prolamellar body (PLB) is a periodic bicontinuous membrane structure based on tubular tetrahedral units. PLBs are present in plant etioplasts and, upon illumination, directly transform into the lamellar thylakoid networks within chloroplasts. Efficient tubular-lamellar rearrangement and later formation of the photosynthetically active thylakoid membranes are crucial steps in the development of plant autotrophy. PLB membranes are mainly composed of galactolipids, carotenoids, and protochlorophyllide (Pchlide), the chlorophyll precursor, bound in a complex with NADPH and Pchlide oxidoreductase. Although the PLB structure has been studied for over 50 years, the direct role of particular membrane components in the formation of the PLB paracrystalline network remains elusive. Moreover, despite the numerous literature data regarding the PLB geometry, their reliable comparative analysis is complicated due to variable experimental conditions. Therefore, we performed comprehensive ultrastructural and low-temperature fluorescence analysis of wild type Arabidopsis thaliana (Arabidopsis) seedlings grown in different conditions typical for studies on etiolated seedlings. We established that the addition of sucrose to the growing media significantly affected the size and compactness of the PLB. The etiolation period was also an important factor influencing the PLB structural parameters and the ratio of free to complex-bound Pchlide. Thus, a reliable PLB structural and spectral analysis requires particular attention to the applied experimental conditions. We investigated the influence of the pigment and polyprenol components of the etioplast membranes on the formation of the PLB spatial structure. The PLB 3D structure in several Arabidopsis mutants (ccr1-1, lut5-1, szl1-1npq1-2, aba1-6, pif1, cpt7) with disturbed levels of particular pigments and polyprenols using electron tomography technique was studied. We found that the PLB nano-morphology was mainly affected in the pif1 and aba1-6 mutants. An increased level of Pchlide (pif1) resulted in the substantial shift of the structural balance between outer and inner PLB water channels and overall PLB compactness compared to wild type plants. The decrease in the relative content of ß-branch xanthophylls in aba1-6 plants was manifested by local disturbances in the paracrystalline structure of the PLB network. Therefore, proper levels of particular etioplast pigments are essential for the formation of stable and regular PLB structure.