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.