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.
Highlights
Salt stress is among the most challenging situations in many areas worldwide for plant growth and development [1] since it results in both osmotic and drought stress conditions for a plant [2]
Studies report that P. patens survives moderate osmotic and salt stress [4,12], and this tolerance is similar to that found for other moss species such as Bryum argenteum and Atrichum undulatum [13], but higher compared to, for example, ferns [13] or the higher plant model Arabidopsis thaliana [4]
Since it is known that indole-3-acetic acid (IAA) homeostasis plays a role in salt stress responses in different plant species such as A. thaliana and P. tremula [17], and the GH3 proteins of P. patens seem to be involved in other stress responses, we investigated whether the previously described double knock-out mutants [7,15,16] show altered phenotypes in comparison to wild type (WT)
Summary
Salt stress is among the most challenging situations in many areas worldwide for plant growth and development [1] since it results in both osmotic and drought stress conditions for a plant [2]. Studies report that P. patens survives moderate osmotic and salt stress [4,12], and this tolerance is similar to that found for other moss species such as Bryum argenteum and Atrichum undulatum [13], but higher compared to, for example, ferns [13] or the higher plant model Arabidopsis thaliana [4]. In P. patens light and high temperatures adversely regulate the response in GH3 double KO mutants [17] During salt stress, this has not yet been studied in P. patens, but a role for auxin conjugates has been reported in Populus tremula [18], Brassica rapa [19], and A. thaliana, and in the latter the overexpression of the P. tremula auxin conjugate hydrolase rendered the plants more tolerant of high salt [20]
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