Identification of a caleosin associated with hazelnut (Corylus avellana L.) oil bodies

Time Domain NMR Approach in the Chemical and Physical Characterization of Hazelnuts (Corylus avellana L.)

'Tonda Gentile Romana' and 'Tonda di Giffoni' (Corylus avellana L.) are two Italian hazelnut cultivars, recognized under the quality labels "Protected Designation of Origin" (PDO) and "Protected Geographical Indication" (PGI), respectively. Hazelnut seeds are characterized by a complex microstructure and the presence of different physical compartments. This peculiarity has been studied and evidenced by Time Domain (TD) Nuclear Magnetic Resonance (NMR) experiments. This technique allowed the assessment of the presence of different diffusion compartments, or domains, by evaluating the distribution of the spin-spin relaxation time (T₂).The aim of this research was to develop a method based on ¹H NMR relaxometry to study the mobility in fresh hazelnut seeds ('Tonda di Giffoni' and 'Tonda Gentile Romana'), in order to determine differences in seed structure and matrix mobility between the two cultivars. TD-NMR measurements were performed from 8 to 55 °C in order to mimic post-harvest processing as well the microscopic textural properties of hazelnut. The Carr-Purcell-Meiboom-Gill (CPMG) experiments showed five components for 'Tonda Gentile Romana' and four components for 'Tonda di Giffoni' relaxation times. The two slowest components of relaxation (T_2,a about 30-40% of the NMR signal, and T_2,b about 50% of the NMR signal) were attributed to the protons of the lipid molecules organized in the organelles (oleosomes), both for the 'Tonda Gentile Romana' and for the 'Tonda di Giffoni' samples. The component of relaxation T_2,c was assigned to cytoplasmic water molecules, and showed a T₂ value dominated by diffusive exchange with a reduced value compared to that of pure water at the same temperature. This can be attributed to the water molecules affected by the relaxation effect of the cell walls. The experiments carried out as a function of temperature showed, for 'Tonda Gentile Romana', an unexpected trend between 30 and 45 °C, indicating a phase transition in its oil component. This study provides information that could be used to strengthen the specifications underlying the definitions of "Protected Designation of Origin" (PDO) and "Protected Geographical Indication" (PGI).

Identification of Key Lipogenesis Stages and Proteins Involved in Walnut Kernel Development

Walnuts are abundant in oil content, especially for polyunsaturated fatty acids, but the understanding of their formation is limited. We collected walnut (Juglans regia L.) kernels at 60, 74, 88, 102, 116, 130, and 144 days after pollination (designated S1-S7). The ultrastructure and accumulation of oil bodies (OBs) were observed using transmission electron microscopy (TEM), and the oil content, fatty acid composition, and proteomic changes in walnut kernels were determined. The oil content and OB accumulation increased during the development and rose sharply from S1 to S3 stages, which are considered the key lipogenesis stage. A total of 5442 proteins were identified and determined as differentially expressed proteins (DEPs) using label-free proteomic analysis. Fatty acid desaturases (FAD) 2, FAD3, oleosin, and caleosin were essential and upregulated from the S1 to S3 stages. Furthermore, the highly expressed oleosin gene JrOLE14.7 from walnuts was cloned and overexpressed in transgenic Brassica napus. The overexpression of JrOLE14.7 increased the oil content, diameter, hundred weight of seeds and changed the fatty acid composition and OB size of Brassica napus seeds. These findings provide insights into the molecular mechanism of oil biosynthesis and the basis for the genetic improvement of walnuts.

A computational study on the structure-function relationships of plant caleosins

Plant cells store energy in oil bodies constructed by structural proteins such as oleosins and caleosins. Although oil bodies usually accumulate in the seed and pollen of plants, caleosins are present in various organs and organelles. This issue, coupled with the diverse activities of caleosins, complicates the description of these oleo-proteins. Therefore, the current article proposes a new classification based on the bioinformatics analysis of the transmembrane topology of caleosins. Accordingly, the non-membrane class are the most abundant and diverse caleosins, especially in lower plants. Comparing the results with other reports suggests a stress response capacity for these caleosins. However, other classes play a more specific role in germination and pollination. A phylogenetic study also revealed two main clades that were significantly different in terms of caleosin type, expression profile, molecular weight, and isoelectric point (P < 0.01). In addition to the biochemical significance of the findings, predicting the structure of caleosins is necessary for constructing oil bodies used in the food and pharmaceutical industries.

OsClo5 functions as a transcriptional co-repressor by interacting with OsDi19-5 to negatively affect salt stress tolerance in rice seedlings

Caleosins constitute a small protein family with one calcium-binding EF-hand motif. They are involved in the regulation of development and response to abiotic stress in plants. Nevertheless, how they impact salt stress tolerance in rice is largely unknown. Thereby, biochemical and molecular genetic experiments were carried out, and the results revealed that OsClo5 was able to bind calcium and phospholipids in vitro and localized in the nucleus and endoplasmic reticulum in rice protoplasts. At the germination and early seedlings stages, overexpression transgenic lines and T-DNA mutant lines exhibited reduced and increased tolerance to salt stress, respectively, compared with the wild-type. Yeast two-hybrid, bimolecular fluorescence complementation and in vitro pull-down assays demonstrated that the EF-hand motif of OsClo5 was essential for the interactions with itself and OsDi19-5. Yeast one-hybrid, electrophoretic migration shift and dual-luciferase reporter assays identified OsDi19-5 as a transcriptional repressor via the TACART cis-element in the promoters of two salt stress-related target genes, OsUSP and OsMST. In addition, OsClo5 enhanced the inhibitory effect of OsDi19-5 in the tobacco transient system, which was confirmed by qRT-PCR analysis in rice seedlings under salt stress. The collective results deepen the understanding of the molecular mechanism underlying the roles of caleosin in the salt stress response. These findings will also inform efforts to improve salt tolerance of rice.