Lipid metabolism during aging of high-alpha-linolenate-phenotype potato tubers

Strboh A homologue of NADPH oxidase regulates wound-induced oxidative burst and facilitates wound-healing in potato tubers

During 30-months of storage at 4 degrees C, potato (Solanum tuberosum L.) tubers progressively lose the ability to produce superoxide in response to wounding, resist microbial infection, and develop a suberized wound periderm. Using differentially aged tubers, we demonstrate that Strboh A is responsible for the wound-induced oxidative burst in potato and aging attenuates its expression. In vivo superoxide production and NADPH oxidase (NOX) activity from 1-month-old tubers increased to a maximum 18-24 h after wounding and then decreased to barely detectable levels by 72 h. Wounding also induced a 68% increase in microsomal protein within 18 h. These wound-induced responses were lost over a 25- to 30-month storage period. Superoxide production and NOX activity were inhibited by diphenylene iodonium chloride, a specific inhibitor of NOX, which in turn effectively inhibited wound-healing and increased susceptibility to microbial infection and decay in 1-month-old tubers. Wound-induced superoxide production was also inhibited by EGTA-mediated destabilization of membranes. The ability to restore superoxide production to EGTA-treated tissue with Ca(+2) declined with advancing tuber age, likely a consequence of age-related changes in membrane architecture. Of the five homologues of NOX (Strboh A-D and F), wounding induced the expression of Strboh A in 6-month-old tubers but this response was absent in tubers stored for 25-30 months. Strboh A thus mediates the initial burst of superoxide in response to wounding of potato tubers; loss of its expression increases the susceptibility to microbial infection and contributes to the age-induced loss of wound-healing ability.

Clp Protease Complexes from Photosynthetic and Non-photosynthetic Plastids and Mitochondria of Plants, Their Predicted Three-dimensional Structures, and Functional Implications

Tetradecameric Clp protease core complexes in non-photosynthetic plastids of roots, flower petals, and in chloroplasts of leaves of Arabidopsis thaliana were purified based on native mass and isoelectric point and identified by mass spectrometry. The stoichiometry between the subunits was determined. The protease complex consisted of one to three copies of five different serine-type protease Clp proteins (ClpP1,3-6) and four non-proteolytic ClpR proteins (ClpR1-4). Three-dimensional homology modeling showed that the ClpP/R proteins fit well together in a tetradecameric complex and also indicated unique contributions for each protein. Lateral exit gates for proteolysis products are proposed. In addition, ClpS1,2, unique to land plants, tightly interacted with this core complex, with one copy of each per complex. The three-dimensional modeling show that they do fit well on the axial sites of the ClpPR cores. In contrast to plastids, plant mitochondria contained a single approximately 320-kDa homo-tetradecameric ClpP2 complex, without association of ClpR or ClpS proteins. It is surprising that the Clp core composition appears identical in all three plastid types, despite the remarkable differences in plastid proteome composition. This suggests that regulation of plastid proteolysis by the Clp machinery is not through differential regulation of ClpP/R/S gene expression, but rather through substrate recognition mechanisms and regulated interaction of chaperone-like molecules (ClpS1,2 and others) to the ClpP/R core.

Oxidative metabolism and the physiological age of seed potatoes are affected by increased alpha-linolenate content

The effects of high alpha-linolenate content on lipid peroxidation, oxidative stress and loss of plant growth potential during ageing of potato (Solanum tuberosum L.) seed-tubers was examined. Endoplasmic reticulum (FAD3) and plastidal (FAD7) 18:2 fatty acid desaturases were upregulated in potato (cv. Desiree), resulting in a 2-fold average increase in mol percentage 18:3 in the total lipid fraction across all transgenic clones. In double-transformed (FAD3+7) tubers, high alpha-linolenate phenotype effected accelerated ageing, resulting in growth responses characteristic of older seed-tubers. Although respiration rates of wild-type (WT) and FAD3+7 tubers were equal at 7 months of storage, rates had increased by 23% and 50% in WT and FAD3+7 tubers, respectively, by 19 months of storage. Electrolyte leakage of tissue from 19-month-old FAD3+7 tubers was significantly greater than that from WT tubers of the same age, indicating that the high alpha-linolenate phenotype was detrimental to membrane integrity during long-term storage. On average, indices of lipid peroxidation (malondialdehyde, ethane, C-6 aldehydes) were higher in older FAD3+7 tubers, relative to WT tubers. Activities of glucose-6-phosphate dehydrogenase, peroxidase, glutathione reductase, ascorbate peroxidase and monodehydroascorbate reductase increased in tubers with advancing age and were higher, on average, in FAD3+7 tubers. Dehydroascorbate reductase activity decreased with age, with no difference between transgenic and WT lines. Collectively, these results indicate that FAD3+7 tubers underwent a higher degree of oxidative stress during ageing. The age-induced increase in respiration of FAD3+7 tubers was at least partly a response to fuel increased free radical scavenging through the ascorbate-glutathione antioxidant pathway. By affecting the susceptibility of lipids to peroxidation, the degree of fatty acid unsaturation influenced the development of oxidative stress and the overall rate at which growth potential was lost from seed-tubers during ageing. Thus, oxidative stress plays an integral role in modulating the ageing process to affect growth potential from potato seed-tubers.