PM proteins involved in the tolerance of potato to N deficiency
Proteins functioning in the transport of ions across the membrane were overrepresented in the group of responsive proteins. The abundance of the majority was largely unaffected by N deficiency in ‘Lambada’, while increasing in ‘Topas’. During N deficiency, four NRT2 genes (Krapp et al., 2011; Lezhneva et al., 2014) and six AMT genes were described to be upregulated in Arabidopsis (Yuan et al., 2007). Probably due to the low abundance of the N transporters or to the lack of accessible trypsin cleavage sites, only one protein, namely NRT2.1, was identified in our study. NRT2.1 is a major contributor to the high-affinity transport system operating under low levels of external nitrate (Lezhneva et al., 2014; Orsel, Filleur, Fraisier, & Daniel-Vedele, 2002). The increase in StNRT2.1 protein abundance confirmed that the growth settings used in this study induced the N deficiency response. In Arabidopsis, N deficiency induced expression ofNRT2.1 in the cortex cells of the older part of primary and lateral roots (Wirth et al., 2007). Additionally, it was observed that the expression of AtNRT2.1 increased transiently during short time starvation and decreased to the level slightly higher than before the treatment during prolonged N starvation (Lezhneva et al., 2014). In potato, StNRT2.1 was shown to be higher expressed in the cultivars accumulating more nitrate in the tubers, suggesting that also in potato NRT2.1 is a key factor for nitrate uptake (M’hamdi, Abid, Chikh-Rouhou, Razgallah, & Hassen, 2016). An increase in the StNRT2.1 abundance under N deficiency may likely be linked to a decrease in the tissue nitrate concentration in ‘Topas’, inducing the transcriptional upregulation of this high affinity transport system. In ‘Lambada’ the slight decrease in NRT2.1 abundance might be the consequence of severe N deficiency, as it was previously described that ‘Lambada’ absorbs all the N available in culturing medium faster than ‘Topas’ (Schum & Jansen, 2012). The higher abundance of NRT2.1 is probably one of the key factors determining the more tolerant behavior of the cultivar ‘Topas’ under selected conditions. As the role of NRT2.1 in the nitrate uptake is well characterized (Filleur et al., 2001; Okamoto, Vidmar, & Glass, 2003; Orsel, Filleur, et al., 2002; Wirth et al., 2007), we thus decided to elucidate proteins which have not yet been associated with contrasting N deficiency response.
A few other membrane transporters have been identified to be differentially abundant in ‘Topas’ compared to ‘Lambada’. Amongst these are three PM H+-ATPases (Table 1). These proteins are involved in generating an electrochemical proton gradient across the PM which is the basis for the active transport of various ions and metabolites (Palmgren, 2001). Also, nitrate transport depends on the activity of PM H+-ATPases (Forde, 2000; Meharg & Blatt, 1995). All the H+-ATPases identified in this proteome analysis correlate positively in their abundance with that of NRT2.1 (Pearson R > 0.9, Supplemental Fig. S8), supporting the hypothesis that the sensitive cultivar ‘Lambada’ responds differentially to N deficiency in respect to its transport properties at the PM.
Two putative ABC transporters (one of the A subfamily and one of the B subfamily, ABCB) were present in significantly higher amounts in ‘Topas’ plants exposed to N deficiency (Table 1). In Arabidopsis, the ABCB transporters are known to play a complex role in auxin transport (Blakeslee et al., 2007; Geisler et al., 2005; Hwang et al., 2016) and therefore also in the regulation of root traits (Overvoorde, Fukaki, & Beeckman, 2010). For instance, AtABCB4 participates in the regulation of root hair elongation (Santelia, Fukao, Martinoia, & Geisler, 2006), whereas AtABCB1 impacts on lateral root formation (Geisler et al., 2005). In previous experiments, N deficiency increased the lateral root length in ‘Topas’, but not in ‘Lambada’ (Schum & Jansen, 2013), but so far there is no evidence for the role of ABCB transporters in the auxin transport and in the modification of root architecture in potato.