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    • The gsnor ko plants contain elevated amount of

    2021-09-27

    The gsnor-ko plants contain elevated amount of low and high molecular weight S-nitrosothiols (SNO) indicating that GSNOR activity controls the level of both GSNO and indirectly protein-SNOs [13], [17], [18]. GSNOR deficiency has been shown to cause pleiotropic plant growth defects, impaired plant disease responses, heat sensitivity, and resistance to cell death [17], [18], [19], [20], [21], [22]. Arabidopsis gsnor-ko lines are available as hot5-2 or gsnor1-3 (GABI-Kat 315D11, Col-0) or INRA FLAG_298F11 (Ws). Antisense and overexpression lines have been also generated [6], [12]. Moreover, SAIL_1229_H06.v1 is a promotor insertion line which shows enhanced cmv virus of GSNOR. Additionally, overexpression and antisense lines are available in tomato [23]. All these lines are important tools to analyse GSNOR and SNO function and their regulation. A possible interplay between ROS and GSNOR can be on gene and/or protein level meaning that ROS affect GSNOR expression, GSNOR activity, GSNOR translocation or GSNOR stability/degradation. During the last decade many reports about the function of GSNOR in different physiological context and in different plant species (e. g. Arabidopsis, pepper, tomato, sunflower, tobacco, pea, cauliflower, lettuce) has been published [15], [17], [18], [21], [22], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Moreover, different types of -OMICS studies comparing wild type and gsnor-ko plants gave insight into the regulatory function of GSNOR on transcript, protein and metabolite level [19], [34], [35], [36], [37].
    According to Arabidopsis electronic Fluorescent Pictograph (eFP) browser (http://bar.utoronto.ca/efp/cgi-bin/efpWeb.cgi) the Arabidopsis GSNOR gene is significantly expressed in all organs with the exception of mature pollen. Different abiotic stress condition, which are known to induce ROS production affect GSNOR expression. In tomato for example the expression of GSNOR was significantly affected by alkaline stress [23]. In particular, transcription of GSNOR was inhibited dramatically in response to alkaline stress between 0.5 and 2 d after treatment (P < 0.05). Afterwards, the expression of GSNOR started to increase at 3 d after NaHCO3 treatment, peaked on the sixth day and then declined [23]. In sunflower seedlings exposed to high temperature (38 °C for 4 h), GSNOR gene expression and GSNOR activity have been found to be reduced in hypocotyls with the simultaneous accumulation of SNOs [38]. In Arabidopsis, the GSNOR gene has been shown to be regulated by wounding and salicylic acid, although the activity and SNO content was not analyzed [32]. A similar situation was reported in tobacco leaves, in which after 2 h of mechanical damage both GSNOR mRNA and protein levels decreased [32]. Expression of GSNOR is also reduced by cadmium stress in pea [33]. However, although altered expression of GSNOR under different stress situation has been demonstrated there is no evidence that ROS directly affect expression of GSNOR. A much more important regulatory mechanism to control GSNOR activity seems to be post-translational modification of GSNOR.
    Structure of GSNOR The GSNOR cDNA (1140 pb) of Arabidopsis encodes for a protein of 379 amino acids with a predicted molecular mass of 42.5 kDa. AtGSNOR shows 90% sequence identity with GSNOR from tomato and Zea mays, and is highly homologous to GSNOR sequences of animal or yeast (Fig. 1). Cysteine residues are targets for reversible and irreversible redox-modifications and play an important role in redox signaling. GSNOR is remarkably cysteine rich. In total plant GSNORs have 14–16 cysteine residues (Fig. 1). Thirteen of them are highly conserved within plant GSNORs. Three of them (Cys10, Cys271, Cys370) are located on the protein surface making them directly accessible for post-translational modifications [20]. Moreover, three cysteine residues (Cys47, Cys177, Cys271) are located in the substrate binding site, where Cys47 and Cys177 are coordinating the catalytic Zn2+ together with His69 and a water molecule. The structural Zn2+ is coordinated by Cys99, Cys102, Cys105, and Cys113.