Editorial: On the Diversity of Roles of Organic Acids

Organic acids represent intermediates of major carbon metabolism in plant cells and are, among others, involved in various biochemical pathways, such as glycolysis, the tricarboxylic acid cycle, photorespiration, the glyoxylate cycle, or the photosynthetic C 4 cycle. During the last years, compelling evidence has pointed out that organic acids also display unexpected roles in controlling whole plant cell physiology. These novel roles have become evident by the contribution of a broad spectrum of research laboratories, which have analyzed plants with modified levels of these compounds and found surprising effects (Zell et al. Organic acids are involved in the regulation of a broad range of basic cellular processes, e.g. the modification of cellular pH or the redox state. Therefore, it makes sense that these compounds also play a role in the control of various biochemical and physiological processes in vivo. As extensively analyzed in the case of sugars, a role of organic acids as signaling messengers is emerging (Finkemeier et al., 2013), as well as a role as modulators of the transport across biological membranes (Hedrich and Marten, 1993; De Angeli et al., 2013). Similarly, organic acid metabolism in the cytosol was shown to be involved in abiotic stress responses like cold acclimation (Dyson et al., 2016). Recent evidence even indicates that organic acids are involved in chemical modification of proteins, as for example acetylation or succinylation, with high impact on the in vivo protein activity (Zhang et al., 2011; Weinert et al., 2013). All these novel roles for organic acids have moved them into the spotlight of plant cell biochemistry and whole plant physiology. However, different roles of these compounds still remain to be explored which makes this an intriguing topic in plant sciences. The present Topic contains two original research articles, two reviews and one perspective article, in all of which novel aspects of the metabolism and contribution of organic acids to whole plant physiology are presented. Li et al. examined the effect of gamma-aminobutyric acid (GABA) application on early growth, net photosynthetic rate, gas exchange, osmoregulation, and enzymatic activities in three different maize cultivars. They found that application of GABA improves maize seedling growth, in association with improved net photosynthetic rate and antioxidant enzyme activities. Lehmann et al. analyzed the enhanced CO 2 release of illuminated leaves transferred into darkness (light enhanced dark respiration, LEDR), using species that exhibit different values of LEDR. For this, the authors fed …