Plant Cell Cultures as Model Systems to Study Programmed Cell Death

Sorting the Wheat From the Chaff: Programmed Cell Death as a Marker of Stress Tolerance in Agriculturally Important Cereals

Conventional methods for screening for stress-tolerant cereal varieties rely on expensive, labour-intensive field testing and molecular biology techniques. Here, we use the root hair assay (RHA) as a rapid screening tool to identify stress-tolerant varieties at the early seedling stage. Wheat and barley seedlings had stress applied, and the response quantified in terms of programmed cell death (PCD), viability and necrosis. Heat shock experiments of seven barley varieties showed that winter and spring barley varieties could be partitioned into their two distinct seasonal groups based on their PCD susceptibility, allowing quick data-driven evaluation of their thermotolerance at an early seedling stage. In addition, evaluating the response of eight wheat varieties to heat and salt stress allowed identification of their PCD inflection points (35°C and 150 mM NaCl), where the largest differences in PCD levels arise. Using the PCD inflection points as a reference, we compared different stress effects and found that heat-susceptible wheat varieties displayed similar vulnerabilities to salt stress. Stress-induced PCD levels also facilitated the assessment of the basal, induced and cross-stress tolerance of wheat varieties using single, combined and multiple individual stress exposures by applying concurrent heat and salt stress in a time-course experiment. Two stress-susceptible varieties were found to have low constitutive resistance as illustrated by their high PCD levels in response to single and combined stress exposure. However, both varieties had a fast, adaptive response as PCD levels declined at the other time-points, showing that even with low constitutive resistance, the initial stress cue primes cross-stress tolerance adaptations for enhanced resistance even to a second, different stress type. Here, we demonstrate the RHA's suitability for high-throughput analysis (∼4 days from germination to data collection) of multiple cereal varieties and stress treatments. We also showed the versatility of using stress-induced PCD levels to investigate the role of constitutive and adaptive resistance by exploring the temporal progression of cross-stress tolerance. Our results show that by identifying suboptimal PCD levels in vivo in a laboratory setting, we can preliminarily identify stress-susceptible cereal varieties and this information can guide further, more efficiently targeted, field-scale experimental testing.

Mass Spectrometric Approaches to Study the Metabolism of Jasmonates: Biotransformation of Exogenously Supplemented Methyl Jasmonate by Cell Suspension Cultures of Moringa oleifera

Jasmonic acid (JA) and derivatives play a crucial role in plant adaptation to environmental stress. The exogenous application of methyl jasmonate (MeJA) results in activation of stress-related genes and subsequent production of secondary metabolites. This implies the biotransformation of the hormone into a more active form. In this study, Moringa oleifera cell suspension cultures were treated with 100 μM MeJA. Methanolic cell extracts were analyzed with reverse phase ultrahigh performance liquid chromatography (UHPLC) coupled to a quadrupole time-of-flight high definition mass spectrometer (QTOF-HDMS, with MS^E data acquisition). Using a targeted approach for jasmonate profiling, extracted ion chromatograms were generated. MS fragmentation patterns were subsequently derived and molecular formulae calculated from the MS data of each ion. Furthermore, triple quadrupole (QqQ) MS, operated in selected reaction monitoring (SRM) mode, was employed to target masses of interest. In addition to MeJA and JA, three jasmonoyl-amino acids were annotated: jasmonoyl-valine (JA-Val), jasmonoyl-isoleucine/leucine (JA-Ile/Leu), and jasmonoyl-phenylalanine (JA-Phe), based on characteristic precursor and product ions. Furthermore, JA conjugated to a hexose was observed, as well as hydroxylated and carboxylated derivatives of JA-amino acid conjugates. The data point to active metabolism of the externally added MeJA by the M. oleifera cells through biotransformation and bioconversion reactions that can be investigated in depth using advanced mass spectrometric analyses.

H2O2 Signature and Innate Antioxidative Profile Make the Difference Between Sensitivity and Tolerance to Salt in Rice Cells

Salt tolerance is a complex trait that varies between and within species. H₂O₂ profiles as well as antioxidative systems have been investigated in the cultured cells of rice obtained from Italian rice varieties with different salt tolerance. Salt stress highlighted differences in extracellular and intracellular H₂O₂ profiles in the two cell cultures. The tolerant variety had innate reactive oxygen species (ROS) scavenging systems that enabled ROS, in particular H₂O₂, to act as a signal molecule rather than a damaging one. Different intracellular H₂O₂ profiles were also observed: in tolerant cells, an early and narrow peak was detected at 5 min; while in sensitive cells, a large peak was associated with cell death. Likewise, the transcription factor salt-responsive ethylene responsive factor 1 (TF SERF1), which is known for being regulated by H₂O₂, showed a different expression profile in the two cell lines. Notably, similar H₂O₂ profiles and cell fates were also obtained when exogenous H₂O₂ was produced by glucose/glucose oxidase (GOX) treatment. Under salt stress, the tolerant variety also exhibited rapid upregulation of K⁺ transporter genes in order to deal with K⁺/Na⁺ impairment. This upregulation was not detected in the presence of oxidative stress alone. The importance of the innate antioxidative profile was confirmed by the protective effect of experimentally increased glutathione in salt-treated sensitive cells. Overall, these results underline the importance of specific H₂O₂ signatures and innate antioxidative systems in modulating ionic and redox homeostasis for salt stress tolerance.