Boron is a metalloid that is mostly found in biological systems as boric acid. It can play vital physiological and structural roles in many mechanisms [1], [2]. Both boric acid and borate can form reversible interactions with biomolecules that contain adjacent cis-hydroxyl groups, such as riboflavin, adenosine monophosphate, pyrimidine nucleotides, ribose, apiose, and polysaccharides [3], [4], [5].

Microorganisms require boron for their physiology and metabolism [6]. Boron is present in the structure of certain antibiotics produced by various bacterial species. Additionally, bacteria can regulate population behavior by using chemical signals, known as quorum sensing. Interestingly, autoinducer-2 (AI-2) contains boron and has been identified as one of the critical signals involved in communication between bacterial communities [7]. As in nature, the diversity of bacteria in the gut of animals and humans is also high. It has been discovered that many gut species can secrete and sense AI-2. While people consume boron-containing compounds in their diets, the targets of boron in the body were unclear until recently. However, recent studies have revealed that boron is essential for humans and animals and required for the symbiotic relationship between host organisms, which are humans or animals, and their microbiomes [8]. Furthermore, autoinducer-2-borate (AI-2B) has been identified as a signaling molecule that protects its host from pathogens [9]. These two molecules play critical roles in some gut disorders such as dysbiosis [8], [10]. The mTOR pathway drives many crucial cellular processes such as translation, cell growth, and autophagy. In addition to these functions, the TOR pathway has been found to have contributions to gastrointestinal disorders. Recent research has shown that there is a complex crosstalk between probiotics and the TOR pathway [11].

In animals, boron affects various mechanisms which include carbohydrate, mineral metabolism, energy consumption, regulation of several enzyme activities and embryonic development. The essentiality of boron in humans has not been reported so far but it has beneficial effects. It has roles in steroid hormone metabolism, healthy bone development, and cell membrane maintenance [2], [12]. A primary function of boron at the molecular level is that it cross-links pectins in plant cell walls [6], [13], [14].

When present at high concentrations, boron can also have toxic effects. Both boron deficiency and toxicity may have negative outcomes for plants and animals [15], [16]. However, the physiological basis of boron toxicity is not clear [15]. Many plant genes, which are involved in boron transport and tolerance, have been found so far [17], [18]. Yeast has been used as a model organism for the characterization of many plant boron-tolerant genes. ATR1 was found as one of the boron tolerance genes in yeast [19]. Bioinformatics analyses showed that YMR279C and YOR378W were two paralogs of the ATR1 gene. The expression of YMR279C decreased the intracellular boron levels and provided remarkable boron resistance to the yeast cells [20].

There is a relationship between the expression of the ATR1 gene, which is the major boron efflux pump, and the expression of amino acid biosynthesis genes [19]. This mechanism is strictly regulated by the transcription factor Gcn4 in response to boron stress. In regulating protein synthesis, eIF2 plays a central role [21]. Boron has also been shown to induce the phosphorylation of eIF2α in a Gcn2 protein kinase-dependent manner, interfering with the initiation of translation and inhibiting protein synthesis [22]. Additional genes that confer boron resistance or sensitivity to yeast cells have been identified by screening a haploid yeast deletion collection [23]. Six mutants (elp1∆, elp3∆, elp6∆, ncs2∆, ncs6∆, and kti12∆) are boron-resistant, and all of them carry deficiencies in Wobble base modifications in tRNA. These mutants constitutively activate the Gcn4 transcription factor and thus upregulate ATR1 expression [24]. Additionally, cells lacking GCN4 are unable to induce ATR1 expression in response to boron stress [22]. All these studies indicate that Gcn4 plays a central role in the boron stress response.

In this study, we aimed to further characterize the roles of GCN4 in the boron stress response by analyzing the transcription factors and cellular pathways that converge on the Gcn4 transcription factor. To do so, we first investigated the role of Gcn1 in boron stress and found that the Gcn2 protein kinase requires the GCN1 gene for its function in the boron stress response. We then examined the TOR, SNF, and PKA pathways for their involvement in boron stress response. Our results showed that Gcn4 activity was reduced in the absence of the TOR1 gene in response to boric acid treatment. Likewise, ATR1 mRNA expression levels were found to be lower in tor1Δ and gln3Δ mutants. Therefore, the TOR1 and GLN3 genes are considered to be important players in the boron stress response mechanism upstream of Gcn4.