γ-Aminobutyrate (GABA) is a ubiquitous four-C, nonprotein, amino acid that has been linked to stress, signaling, and storage in plants. In this paper, we discuss analytical, enzyme-linked, and colorimetric methods for analyzing GABA and related metabolites, and review tracer evidence for the derivation of GABA from glutamate and its subsequent catabolism to succinic semialdehyde and either succinate or γ-hydroxybutyrate. Also, we describe biochemical, complementation, bioinformatic, recombinant, and modelling strategies for identifying genes and investigating properties of the encoded proteins responsible for transport and metabolism of GABA. For Arabidopsis, evidence supports the involvement of a plasma membrane GABA transporter, a mitochondrial GABA permease, a cytosolic Ca2+/calmodulin- and pH-regulated cytosolic glutamate decarboxylase, a pyruvate- and glyoxylate-regulated mitochondrial GABA transaminase, a redox-regulated mitochondrial succinic semialdehyde dehydrogenase, and redox-regulated glyoxylate/succinic semialdehyde reductases located in both cytosol and plastid, respectively. This simple biochemical model does not account for species and tissue differences in the isoform complement of GABA pathway enzymes or transcriptional control of the pathway. In a companion review, we provide a more integrated view of GABA metabolism and function.

γ-Aminobutyrate (GABA) is a ubiquitous nonprotein amino acid that accumulates in plants in response to abiotic and biotic stresses. In a companion paper, we discussed the origin of GABA from glutamate and subsequent catabolism to succinic semialdehyde and either succinate or γ-hydroxybutyrate (GHB), and the characteristics of genes and proteins responsible for GABA permease, glutamate decarboxylase, GABA transaminase, succinic semialdehyde dehydrogenase, and succinic semialdehyde reductase activities. In this paper, we explore gene expression and transcript–metabolite relationships during the response to abiotic stress, and describe phenotypes of genetic mutants and relationships of GABA metabolism to other plant functions. Evidence indicates that both gene-dependent and -independent processes are involved in the response of the GABA pathway to abiotic stresses. Study of stress-specific responses and their interplay with the C/N network and various signalling pathways would be more informative if circadian rhythms and light–dark transitions upon imposition of the stress were always taken into account, and relevant genes and metabolites simultaneously profiled in wild-type plants or genetic mutants.

MacDonald, W. N., Blom, T. J., Tsujita, M. J. and Shelp, B. J. 2013. Review: Improving nitrogen use efficiency of potted chrysanthemum: Strategies and benefits. Can. J. Plant Sci. 93: 1009–1016. Floricultural greenhouse operations can pose significant environmental risk due to the leaching and runoff of nutrients such as [Formula: see text]. To face this challenge, Ontario growers have adopted recirculating systems, such as the subirrigated “ebb and flow” system, on benches, troughs and concrete floors. Chrysanthemum (Chrysanthemum morifolium Ramat.) is the most commonly grown floricultural greenhouse crop species in the world and the potted type is well adapted to subirrigation. Reduction of N fertilization towards the end of the crop cycle is generally recommended to improve the shelf life of cut and potted plants, but it is uncertain how this practice influences the N status of the plant and the electrical conductivity of the growing medium. In this review, we discuss N use efficiency and strategies for managing the [Formula: see text] status of plants, and then propose that this knowledge can help to improve the N use efficiency of potted chrysanthemum grown with subirrigation under greenhouse conditions.

MacDonald, W. N., Tsujita, M. J., Blom, T. J. and Shelp, B. J. 2014. Impact of various combinations of nitrate and chloride on nitrogen remobilization in potted chrysanthemum grown in a subirrigation system. Can. J. Plant Sci. 94: 643–657. Subirrigation is gaining popularity as an environmentally friendly strategy for managing the nutrition of potted chrysanthemum (Chrysanthemum morifolium Ramat.), as well as many other popular potted ornamental crops grown in the greenhouse. Subirrigation systems recycle the nutrient solution, but unlike the common practice of overhead irrigation, salts are more likely to accumulate in the top of the growing medium. Thus, further research is required to optimize the technology for the timing and application rates of soluble nitrogen in order to benefit from the high mobility of this nutrient within plants. This study tested the hypothesis that N use efficiency of subirrigated potted chrysanthemum can be improved by managing the nitrate status of the plant. Replacement of the nitrate supply with water 1 wk prior to inflorescence emergence (the midpoint of the experimental 10-wk crop cycle) was more effective in reducing the nitrate contents of above-ground tissues, particularly the stems and petioles, than the use of nitrate or various combinations of nitrate and chloride, thereby resulting in an improved N usage index (i.e., shoot dry mass×(shoot dry mass/shoot N content)). Shoot or inflorescence dry mass and N content were unaffected at the conclusion of the crop cycle by this treatment, whereas the electrical conductivity of the pot medium was reduced. Thus, elimination of nitrate prior to inflorescence emergence can be an effective strategy for improving N use efficiency in subirrigated potted chrysanthemums without sacrificing quality.

Deyman, K. L., Chiu, G., Liu, J., Brikis, C. J., Trobacher, C. P., DeEll, J. R., Shelp, B. J. and Bozzo, G. G. 2014. Effects of elevated CO2 and 1-methylcyclopropene on storage-related disorders of Ontario-grown Empire apples. Can. J. Plant Sci. 94: 857–865. The impact of 1-methylcyclopropene (1-MCP) application on CO2-induced physiological injury in Empire apple fruit during controlled atmosphere storage was assessed over a 3-yr period using an experimental design involving multiple treatment replicates. Fruit harvested at optimal maturity from one or two orchards were treated with or without 1 µL L−1 1-MCP, then chilled at 0 or 3°C under various CO2 partial pressures (5, 2.5 or 0.03 kPa CO2) in the presence of 2.5 kPa O2 for up to 46 wk using a split-plot design. Fruit were sampled periodically for assessment of flesh browning and external peel injury. The maximal incidence of external CO2 injury varied from 15 to 100% over the 3 yr, and the most rapid development of this disorder was evident at 5 kPa CO2. The incidence of external CO2 injury as a function of storage time was influenced by orchard location and storage temperature. Moreover, the incidence of flesh browning at 0°C and 5 kPa CO2 was influenced slightly by orchard; this disorder was never higher than 30%, and the impact of elevated CO2 was inconsistent across years. Notably, there was no evidence for negative effects of 1-MCP on the incidence of storage-related disorders.