Citation: Fiorillo, A.; Manai, M.; Visconti, S.; Camoni, L. The Salt Tolerance–Related Protein (STRP) Is a Positive Regulator of the Response to Salt Stress in Arabidopsis thaliana. Plants 2023, 12, 1704. https:// doi.org/10.3390/plants12081704 Academic Editor: Dayong Zhang Received: 16 March 2023 Revised: 6 April 2023 Accepted: 17 April 2023 Published: 20 April 2023 Copyright: © 2023 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/). plants Article The Salt Tolerance–Related Protein (STRP) Is a Positive Regulator of the Response to Salt Stress in Arabidopsis thaliana Anna Fiorillo 1,† , Michela Manai 1,2,† , Sabina Visconti 1, * and Lorenzo Camoni 1, * 1 Department of Biology, University of Rome Tor Vergata, 00133 Rome, Italy; anna.fiorillo@uniroma2.it (A.F.); michela.manai@uniroma2.it (M.M.) 2 Ph.D. Program in Cellular and Molecular Biology, Department of Biology, University of Rome Tor Vergata, 00133 Rome, Italy * Correspondence: visconti@uniroma2.it (S.V.); camoni@uniroma2.it (L.C.) † These authors contributed equally to this work. Abstract: Salt stress is a major abiotic stress limiting plant survival and crop productivity. Plant adaptation to salt stress involves complex responses, including changes in gene expression, regulation of hormone signaling, and production of stress-responsive proteins. The Salt Tolerance–Related Protein (STRP) has been recently characterized as a Late Embryogenesis Abundant (LEA)–like, intrinsically disordered protein involved in plant responses to cold stress. In addition, STRP has been proposed as a mediator of salt stress response in Arabidopsis thaliana, but its role has still to be fully clarified. Here, we investigated the role of STRP in salt stress responses in A. thaliana. The protein rapidly accumulates under salt stress due to a reduction of proteasome–mediated degradation. Physiological and biochemical responses of the strp mutant and STRP–overexpressing (STRP OE) plants demonstrate that salt stress impairs seed germination and seedling development more markedly in the strp mutant than in A. thaliana wild type (wt). At the same time, the inhibitory effect is significantly reduced in STRP OE plants. Moreover, the strp mutant has a lower ability to counteract oxidative stress, cannot accumulate the osmocompatible solute proline, and does not increase abscisic acid (ABA) levels in response to salinity stress. Accordingly, the opposite effect was observed in STRP OE plants. Overall, obtained results suggest that STRP performs its protective functions by reducing the oxidative burst induced by salt stress, and plays a role in the osmotic adjustment mechanisms required to preserve cellular homeostasis. These findings propose STRP as a critical component of the response mechanisms to saline stress in A. thaliana. Keywords: saline stress; intrinsically disordered proteins; abscisic acid; oxidative stress; abiotic stress 1. Introduction Excessive soil salinity is a dangerous environmental condition for plants [1]. The ionic and osmotic imbalance induced by high salinity impairs water and nutrient uptake and alters seed germination, flowering, and fruiting ability [2,3]. Under salt stress, plants lose their photosynthetic capacity, affecting the availability of resources required to sustain plant growth [4,5]. To overcome the adverse effects of salt stress, plants evolved several mech- anisms, including vacuole compartmentalization of toxic ions, activation of antioxidant defenses, regulation of hormone levels, and accumulation of stress-responsive proteins [4]. The Salt Tolerance–Related Protein (STRP) is a 16 kDa highly hydrophilic protein of Arabidopsis thaliana [6,7]. The amino acid sequence of STRP is 59% identical to ST6– 66, a Thellungiella halophila protein that confers salt tolerance when overexpressed in A. thaliana [6]. Moreover, the strp loss of function mutant is hypersensitive to salt stress; hence, the name Salt Tolerance–Related Protein was proposed [6]. Despite its self-explanatory name, the function played by STRP in salt stress remains largely unknown. In addition to the similarity with ST6–66, the only indication that demonstrates the involvement of STRP in the salt stress response mechanisms is the Plants 2023, 12, 1704. https://doi.org/10.3390/plants12081704 https://www.mdpi.com/journal/plants