## 1 Introduction
The increased amount of alarmones in response to stress conditions leads to the immediate arrest of rRNA, tRNA, and ribosomal protein gene expression, followed by the induction of expression of genes encoding proteins involved in adaptation to unfavourable conditions [14; 15]. The metabolism of (p)ppGpp in _E. coli_ is regulated by RelA and SpoT enzymes encoded by paralogous genes. RelA is a (p)ppGpp synthetase, whereas SpoT is mainly a (p)ppGpp hydrolase, however, in certain conditions it exhibits low activity of alarmone synthetase. Most bacteria possess only one bifunctional Rel enzyme [16; 17; 18; 19]. The presence of (p)ppGpp in photosynthetic _Eucaryota_ was first confirmed in the alga _Chlamydomonas reinhardtii_, where the accumulation of alarmones in response to amino acid starvation was observed [20]. Homologs of the bacterial genes \(RelA/SpoT\) called \(RSH\) (\(RelA/SpoT\) Homologs) were first identified in _A. thaliana_[21] and, in subsequent years, \(RSH\) genes have been identified in other plant species [22; 23; 24; 25]. RSH proteins have been divided into three groups, i.e., \(RSH1\), \(RSH2/3\), and \(CRSH\), based on their primary structure and domain structure [26]. In _A. thaliana_, there are four genes encoding RSH proteins, namely \(RSH1\), \(RSH2\), \(RSH3\), and \(CRSH\) (Ca\({}^{2+}\)-activated \(RSH\)). RSH1 exhibits only (p)ppGpp hydrolytic activity due to the substitution, critical for (p)ppGpp synthase activity, of glycine by serine in the RSD domain. Proteins belonging to the RSH2/3 group (AtRSH2 and AtRSH3) can both synthesize and hydrolase alarmones, whereas CRSH proteins do not possess a functional hydrolytic domain (HD domain) and are (p)ppGpp synthases [26; 27; 30]. Members of the RSH1 group possess a TGS domain which has been proposed to play a regulatory role in ligand binding [27], and a role in establishing the RSH-ribosome interaction in chloroplasts [28; 29; 30]. Moreover, RSH1 as the only group of plant RSH proteins that possess the ACT domain [30], recently described as an RNA recognition motif (RRM) domain [28]. CRSH group proteins also contain the EF-hand motif at the C-terminus of the protein. Interestingly, this Ca\({}^{2+}\)-binding motif has not been identified in any bacterial or plant homolog [26; 31]. It was confirmed in vitro that, for (p)ppGpp synthase activity, \(CRSH\) requires Ca\({}^{2+}\)[32]. The plant stringent response hasbeen implicated in the stress response, flowering, seed development, photosynthesis, plant senescence, and nutrient remobilization [27]. In animals, homologs of bacterial SpoT have been identified (Mesh1) with alarmone hydrolysing activity [33]. However, until quite recently, the existence of (p)ppGpp in metazoo has been questioned. Last year the presence of ppGpp in _Drosophila_ and human cells was shown [34], opening a new chapter in the discussion about the origin and functions of alarmones. In the present study, we attempt to answer the question about the complexity of the plant RSH groups in representatives of the _Brassicaceae_ family via the in silico analysis of _RSH_ genes and RSH proteins from selected species of this plant family. Inspired by the postulated role of RSH in the plant response to varied abiotic and biotic factors, we also examined _B. napus RSH_ gene expression in response to salinity. Moreover, we analysed the expression of _BnRSHs_ in the presence of _Seratia liquefaciens_, _S. plymuthica_, and _Massilia timonae_, PGPR bacteria for which the ability to promote the growth of rape has been confirmed. To pinpoint other potential regulators of _RSH_ gene expression, we revealed the presence of multiple putative regulatory _cis_-elements in the promoter regions of _BnRSH_ genes.