Physcomitrium patens Mutants in Auxin Conjugating GH3 Proteins Show Salt Stress Tolerance but Auxin Homeostasis Is Not Involved in Regulation of Oxidative Stress Factors

Abstract

Salt stress is among the most challenging abiotic stress situations that a plant can experience. High salt levels do not only occur in areas with obvious salty water, but also during drought periods where salt accumulates in the soil. The moss Physcomitrium patens became a model for studying abiotic stress in non-vascular plants. Here, we show that high salt concentrations can be tolerated in vitro, and that auxin homeostasis is connected to the performance of P. patens under these stress conditions. The auxin levels can be regulated by conjugating IAA to amino acids by two members of the family of GH3 protein auxin amino acid-synthetases that are present in P. patens. Double GH3 gene knock-out mutants were more tolerant to high salt concentrations. Furthermore, free IAA levels were differentially altered during the time points investigated. Since, among the mutant lines, an increase in IAA on at least one NaCl concentration tested was observed, we treated wild type (WT) plants concomitantly with NaCl and IAA. This experiment showed that the salt tolerance to 100 mM NaCl together with 1 and 10 µM IAA was enhanced during the earlier time points. This is an additional indication that the high IAA levels in the double GH3-KO lines could be responsible for survival in high salt conditions. While the high salt concentrations induced several selected stress metabolites including phenols, flavonoids, and enzymes such as peroxidase and superoxide dismutase, the GH3-KO genotype did not generally participate in this upregulation. While we showed that the GH3 double KO mutants were more tolerant of high (250 mM) NaCl concentrations, the altered auxin homeostasis was not directly involved in the upregulation of stress metabolites.

BoPEP4, a C-Terminally Encoded Plant Elicitor Peptide from Broccoli, Plays a Role in Salinity Stress Tolerance

“…Overexpression of auxin conjugating enzymes encoded by GH3 gene reduces auxin levels in Arabidopsis and causes a dwarfed phenotype [ 41 ]. On the other hand, the GH3 knockout mutant in Physcomitrium patens was tolerant to 250 mM NaCl treatment [ 42 ]. In this study, the expression levels of GH3 family members were reduced in PR/NR, which may contribute to maintenance of auxin homeostasis regulated by BoPep4 under salinity stress ( Figure 5 A and Table S5 ).…”

Molecular and physiological responses to salt stress in salinity-sensitive and tolerant Hibiscus rosa-sinensis cultivars

“…Among the downregulated genes specifically regulated in ‘Porto’ cultivars, two transcripts in SO tissue and one transcript in petals encoding an indole-3-acetic acid-amido synthetase GH3.3 were detected. In the moss Physcomitrium patens , loss of the auxin-conjugating GH3 enzymes results in tolerance to high salt concentrations (Koochak and Ludwig-Muller 2021 ). Similarly, in Arabidopsis the disruption through the multiple knockout of GH3s or GH3-like protein genes might modulate plant tolerance to salt stress by redundant auxin conjugation (Casanova‐Sáez et al, 2022 ).…”