Foliar Application of Proline and GABA Modulates the Chaperone System Response to Drought in Arabidopsis thaliana

The proteinogenic amino acid proline and the non‐protein amino acid γ‐aminobutyric acid (GABA) are metabolically related, accumulate in plant cells under adverse conditions, and play various roles in cellular protection. Treatment of plants with these bioactive molecules increases drought tolerance. Proline can function as a chaperone, but the central role in maintaining cellular proteostasis belongs to heat shock proteins (HSPs). Using Arabidopsis thaliana , we investigated how the foliar application of proline and GABA, separately and combined, affects the level and dynamics of the chaperone response to soil drying and rehydration. Using biochemical and transcriptomic analysis, we found that both exogenous amino acids increased endogenous proline accumulation with different kinetics and specifically altered the expression of genes encoding HSPs and heat shock transcription factors (HSF). Proline treatment reduced drought‐dependent induction of AtHSP70‐4 and AtHSFB2a , but upregulated AtHSP18.2, AtHSP101, AtHSFA2 and AtHSFA7a . After GABA treatment, the reduced induction of AtHSP70‐4 , AtHSP17.4, AtHSFB1 and AtHSFB2a was associated with an upregulation of HSFA2 and HSFA7a . In contrast, both amino acids did not affect HSFA3 and HSFC1 expression. The combined use of proline and GABA showed a complex relationship between their metabolic and functional pathways. It was less effective in improving drought tolerance and altered gene expression compared to the individual uses of the amino acids: the expression patterns of AtHSP70‐4 and AtHSP101 were close to the effect of proline, and the expression patterns of other genes were close to the effect of GABA. These results suggest that exogenous proline and GABA may improve plant protein homeostasis under water deficit by specifically modulating the chaperone system response.