Oxidative discolouration in whole-head and cut lettuce: biochemical and environmental influences on a complex phenotype and potential breeding strategies to improve shelf-life

Lettuce discolouration is a key post-harvest trait. The major enzyme controlling oxidative discolouration has long been considered to be polyphenol oxidase (PPO) however, levels of PPO and subsequent development of discolouration symptoms have not always correlated. The predominance of a latent state of the enzyme in plant tissues combined with substrate activation and contemporaneous suicide inactivation mechanisms are considered as potential explanations for this phenomenon. Leaf tissue physical properties have been associated with subsequent discolouration and these may be influenced by variation in nutrient availability, especially excess nitrogen and head maturity at harvest. Mild calcium and irrigation stress has also been associated with a reduction in subsequent discolouration, although excess irrigation has been linked to increased discolouration potentially through leaf physical properties. These environmental factors, including high temperature and UV light intensities, often have impacts on levels of phenolic compounds linking the environmental responses to the biochemistry of the PPO pathway. Breeding strategies targeting the PALand PPOpathway biochemistry and environmental response genes are discussed as a more cost-effective method of mitigating oxidative discolouration then either modified atmosphere packaging or post-harvest treatments, although current understanding of the biochemistry means that such programs are likely to be limited in nature and it is likely that they will need to be deployed alongside other methods for the foreseeable future

Involvement of phospholipase D-related signal transduction in chemical-induced programmed cell death in tomato cell cultures

Phospholipase D (PLD) and its product phosphatidic acid (PA) are incorporated in a complex metabolic network in which the individual PLD isoforms are suggested to regulate specific developmental and stress responses, including plant programmed cell death (PCD). Despite the accumulating knowledge, the mechanisms through which PLD/PA operate during PCD are still poorly understood. In this work, the role of PLD alpha 1 in PCD and the associated caspase-like proteolysis, ethylene and hydrogen peroxide (H2O2) synthesis in tomato suspension cells was studied. Wild-type (WT) and PLD alpha 1-silenced cell lines were exposed to the cell death-inducing chemicals camptothecin (CPT), fumonisin B1 (FB1) and CdSO4. A range of caspase inhibitors effectively suppressed CPT-induced PCD in WT cells, but failed to alleviate cell death in PLD alpha 1-deficient cells. Compared to WT, in CPT-treated PLD alpha 1 mutant cells, reduced cell death and decreased production of H2O2 were observed. Application of ethylene significantly enhanced CPT-induced cell death both in WT and PLD alpha 1 mutants. Treatments with the PA derivative lyso-phosphatidic acid and mastoparan (agonist of PLD/PLC signalling downstream of G proteins) caused severe cell death. Inhibitors, specific to PLD and PLC, remarkably decreased the chemical-induced cell death. Taken together with our previous findings, the results suggest that PLD alpha 1 contributes to caspase-like-dependent cell death possibly communicated through PA, reactive oxygen species and ethylene. The dead cells expressed morphological features of PCD such as protoplast shrinkage and nucleus compaction. The presented findings reveal novel elements of PLD/PA-mediated cell death response and suggest that PLD alpha 1 is an important factor in chemical-induced PCD signal transduction

Inhibition of apoptic cell death induced by Pseudomonas syringae pv. Tabaci and mycotoxin fumonisin B1

The impact of programmed cell death (PCD) inhibitors on lesion formation and biochemical events in transgenic (ttr line) and non-transgenic (Nevrokop 1164) tobacco infected with Pseudomonas syringae pv. tabaci was tested. Programmed cell death in tomato cell culture was induced by Fumonisin B1 (FUM) and effectively abolished by the administration of protease inhibitors and lanthanum. Caspase inhibitors Ac-YVAD-CMK and Z-asp-CH2-DCB, serine protease inhibitor TLCK and LaCl3 were inoculated together with P. syringae pv tabaci in detached tobacco leaves or applied simultaneously with fungal toxin FUM in the tomato cell suspension. The results illustrate that cell death in normal Nevrokop 1164 and ttr transgenic tobacco at sites infected with P. syringae pv tabaci is apoptotic-like and caspase-like proteases and serine proteases are involved in lesion formation. The cell death in the lesions was accompanied with enhanced H2O2 accumulation and MDA production in, the wild type and with reduced levels of both metabolites in the transgenic line. Lanthanum, caspase inhibitors and TLCK reduced the amount of H2O2 and MDA in the affected lesions. FUM-induced cell death in tomato suspension cells was greatly suppressed by the application of protease inhibitors and lanthanum. The results indicated that apoptotic cell death occurs at bacteria inoculation of tobacco leaves, and in fumonisin treated tomato cell