agriculture Article Free Amino Acids and Methylglyoxal as Players in the Radiation Hormesis Effect after Low-Dose γ-Irradiation of Barley Seeds Ivan Pishenin 1 , Irina Gorbatova 1 , Elizaveta Kazakova 1 , Marina Podobed 1 , Anastasiya Mitsenyk 1 , Ekaterina Shesterikova 1 , Alexandra Dontsova 2 , Dmitriy Dontsov 2 and Polina Volkova 1, *   Citation: Pishenin, I.; Gorbatova, I.; Kazakova, E.; Podobed, M.; Mitsenyk, A.; Shesterikova, E.; Dontsova, A.; Dontsov, D.; Volkova, P. Free Amino Acids and Methylglyoxal as Players in the Radiation Hormesis Effect after Low-Dose γ-Irradiation of Barley Seeds. Agriculture 2021, 11, 918. https://doi.org/10.3390/ agriculture11100918 Academic Editor: Michelle Wirthensohn Received: 2 September 2021 Accepted: 22 September 2021 Published: 24 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Laboratory of Cellular and Molecular Radiobiology, Russian Institute of Radiology and Agroecology, 249032 Obninsk, Russia; pishenin.Ivan@gmail.com (I.P.); gorbatova.irina.96@mail.ru (I.G.); elisabethafeb19@gmail.com (E.K.); podobedmyu@gmail.com (M.P.); micenyk-anastasi@mail.ru (A.M.); EShesterikova89@gmail.com (E.S.) 2 Barley Breeding and Seed Production Department, Agricultural Research Center “Donskoy”, 347740 Zernograd, Russia; doncova601@mail.ru (A.D.); dontsova601@gmail.com (D.D.) * Correspondence: volkova.obninsk@gmail.com Abstract: Low-dose γ-irradiation can stimulate plant growth and development; however, the knowl- edge on the molecular mechanisms of such stimulation is yet fragmented. Irradiation of seeds leads to the mobilisation of endosperm resources and reallocation of available nitrogen to facili- tate development. Based on the metabolomic analysis, several metabolites possibly involved in radiation stimulation were studied using the HPLC approach in barley cultivars after γ-irradiation of seeds. The comparison of changes in metabolite concentrations and changes in morphological traits after irradiation revealed seven metabolites that may be involved in the growth stimulation after γ-irradiation of barley seeds. Among them are free amino acids, such as γ-aminobutyric acid, β-alanine, arginine, lysine, glutamine, methionine, and a signalling compound methylglyoxal. Keywords: radiation hormesis; GABA; β-alanine; arginine; lysine; glutamine; methionine; methylglyoxal; HPLC; morphological stimulation 1. Introduction The world will require a dramatic increase in food production in the next 30 years because of the world population growth and the effects of climate change including, but not limited to, increased temperature, changing patterns of rainfall, and elevated levels of CO 2 and ozone [1]. Improving the overall tolerance of plants to environmental stressors is a major focus of agrobiology, while the study of changes in metabolic pathways in response to stressors may be crucial for developing more tolerant plant cultivars [2]. Low-dose γ-irradiation of seeds often leads to a range of positive growth effects, such as increased biomass, accelerated germination and development, and improved immune responses and tolerance to stressors—the phenomenon known as the radiation hormesis [2–5]. A disruption of cellular homeostasis that ultimately leads to the unfolded protein response (UPR) [6] and heat shock response (HSR) [7] is among possible triggers of radiation hormesis. The UPR signalling pathway operates in the endoplasmic reticulum increasing the folding and clearance capacity of a cell by downregulation of the protein synthesis and upregulation of the synthesis of chaperones and components for proteasome degradation [7]. Similar processes in the nucleus and cytoplasm are controlled by the HSR pathway [7]; however, evidence suggests an interconnection between the UPR and HSR pathways in plants [8]. The positive effect on growth and development of low-dose irradiated plants can also be related to the modulation of reactive oxygen species (ROS) levels [5] and phytohormonal balance [9]. ROS-related responses might be connected to changes in phytohormonal signalling pathways since evidence supports ROS as being Agriculture 2021, 11, 918. https://doi.org/10.3390/agriculture11100918 https://www.mdpi.com/journal/agriculture