Exogenous GABA application transiently improves the tolerance to root hypoxia on a sensitive genotype of Prunus rootstock

Prunus species are considered to be moderately sensitive to root hypoxia, but with some degree of tolerance to oxygen deficiency. In a previous work on a set of commercial Prunus rootstocks we identified the plum-based rootstock ‘Mariana 2624’ and the cherry-based rootstock ‘Mazzard F12/1’ as the most tolerant and sensitive genotypes, respectively, to root hypoxia by waterlogging. We then sequenced, for both genotypes, the root transcriptome responsive to hypoxic conditions. The early differentially expressed genes included a transcript with high homology to a root-specific Glutamate decarboxylase of Arabidopsis, AtGAD1. A homology search of the peach genome detected four putative GAD sequences. They exhibited a very high identity in their deduced amino acid sequences except in the C-terminal region, where the calmodulin binding domain is located. In addition, these GAD isoforms showed organ-specific expression and different transcriptional responses to root hypoxia stress. GAD is the enzyme responsible for the synthesis of gamma aminobutyric acid (GABA), an amino acid that reaches high levels during stresses. We assessed the effect of exogenous GABA application on molecular, physiological, biochemical and metabolic parameters in ‘Mariana 2624’ and ‘Mazzard F12/1’. The root hypoxia-tolerant genotype was unaffected by GABA supplementation. In contrast, exogenous GABA triggered transient but substantive changes in the root hypoxia-sensitive genotype under waterlogging. This effect was associated with changes in the mRNA levels of some GAD isoforms, an improvement in photosynthetic parameters, a higher CO2 assimilation rate and stomatal conductance, a smaller loss of chlorophyll content, lower levels of H2O2 and increased levels of endogenous GABA. We discuss GABA’s wide spectrum of biological actions and its effect on root hypoxia-sensitive Prunus species’ adaptive response to oxygen deficiency.