Introduction
The productivity of most field crops is sensitive to the availability of N in the soil. Globally, around 85-90 metric tons of nitrogenous fertilizer is added to arable soils each year, an amount predicted to be more than doubled by 2050 (Tilman, 1999). The production of nitrogenous fertilizer is energy and cost demanding in modern crop production systems. Due to the effects of nitrate leaching, denitrification and ammonia volatilization, which have been proved environmentally damaging (Masclaux-Daubresse et al., 2010; Shaviv & Mikkelsen, 1993), it has been estimated that only 30-50% of fertilizer-supplied N is taken up by crop plants, a range which reflects the combined influence of soil type, environment and plant species (Good, Shrawat, & Muench, 2004). Potato is an important source of food and industrial starch. N utilization efficiency is especially poor in potato, on account of its shallow root system (Iwama, 2008) exacerbated by the fact that the crop is frequently grown on highly leachable sandy soils (Cambouris, Zebarth, Nolin, & Laverdiere, 2008). The aim to improve N use efficiency has become important in crop plant breeding research (Chao & Lin, 2015; Garnett, Plett, Heuer, & Okamoto, 2015; Guo, Wang, Fan, Chen, & Cui, 2016). In this respect, progress has been made in sugarcane, maize and rice (Hajari, Snyman, & Watt, 2015; Ju et al., 2015; P. C. Li et al., 2015), but not in potato.
In plants, the interaction with various biotic and abiotic stress agents induces alterations in the protein composition of the PM (Komatsu, 2008). A better understanding of the PM proteome response to various environmental stimuli – and in particular to N deficiency – is seen as providing a route to the development of stress tolerant plants (Cordwell & Thingholm, 2010; Komatsu et al., 2009). The PM acts both, as a selective barrier between the cell and the external environment and as a sensor for the prevailing environmental conditions (Barkla & Pantoja, 2010). It controls a multiplicity of cellular processes such as transport of metabolites and ions, cell differentiation and proliferation, endocytosis, and supports energy generation and signal transduction (Komatsu, 2008; Ray, Kassan, Busija, Rangamani, & Patel, 2016). The transport across membranes, required for acquisition and release of nutrients and signaling molecules, is achieved primarily through integral membrane proteins (Frommer et al., 1994). Members of four transporter families participate in root nitrate uptake, distribution and storage, including proteins from the nitrate transporter 1/peptide transporter family (Leran et al., 2014), the nitrate transporter 2 family (NRT2) (Orsel, Krapp, & Daniel-Vedele, 2002), the chloride channel family (Barbier-Brygoo et al., 2011), and slow anion associated channel homologs (Negi et al., 2008). Ammonium uptake is facilitated by members of the ammonium transporter (AMT) family (Loque & von Wiren, 2004).
Since the PM constitutes a complex platform allowing the translation of external signals into finely tuned appropriate acclimation responses, the ability of plants to exploit several sources of N displays an intricate network in which signaling and regulatory proteins play a key role (Gronnier, Gerbeau-Pissot, Germain, Mongrand, & Simon-Plas, 2018). Getting insights into this complex network through proteomic approaches is hampered, because the PM proteins are underrepresented when proteome analysis is performed. Thus, membrane-specific approaches are necessary to study the roles played by PM proteins. However, the number of studies on root PM proteomes remain scarce (Cheng et al., 2009; Hashimoto, Toorchi, Matsushita, Iwasaki, & Komatsu, 2009; Voothuluru, Anderson, Sharp, & Peck, 2016; Witzel et al., 2018) and are lacking for potato. To unveil key contributors involved in the low N tolerance, we made use of the contrasting response to N deficiency of the two potato cultivars ‘Lambada’ (sensitive) and ‘Topas’ (tolerant). Both cultivars were already characterized and described to differ at the physiological (Schum & Jansen, 2012, 2013, 2014; Schum, Meise, Jansen, Seddig, & Ordon, 2017) and at the whole proteome (Jozefowicz, Hartmann, Matros, Schum, & Mock, 2017; Meise et al., 2017) level. Here, we report on the effect of N deficiency on the root PM proteome composition of the two potato contrasting cultivars and present candidate proteins likely playing a role in the acclimation of the tolerant cultivar to N deficiency.