Iron (Fe) is an essential nutrient for plants, which catalyzes crucial cellular functions such as chlorophyll synthesis, chloroplast development, and antioxidative cell protection (Marschner, 1995). Despite being abundant in soils, Fe mainly exists as the insoluble, nonavailable to plants, ferric [Fe(III)] form (Lucena, 2000). Plants have developed two separate strategies to acquire Fe(III) from soils. In strategy I plants (dicots and nongraminaceous monocots), a reduction step of Fe(III) to the bioavailable ferrous form [Fe(II)] is required before the transmembrane import of iron (Marschner and Römheld, 1994). In strategy II plants (graminaceous species), soil Fe(III) is chelated and transferred by the plant-exudated phytosiderophores without being previously reduced (Marschner and Römheld, 1994). However, the identification of common elements in both strategies has weakened the strict separation between them (Schmidt, 2003).
Iron deficiency induces ferric chelate reduction (FCR) activity in roots in strategy I plant species in conjunction or not with other adaptive responses such as rhizosphere acidification and changes in root hair and transfer cell development (Schmidt, 1999). Nevertheless, in some annual and perennial plants, root FCR activity is induced at the presence of Fe, whereas under deficiency conditions, reduction activity remains at low levels (Bohórquez et al., 2001; Gogorcena et al., 2000, 2004; Ojeda et al., 2003; Zouari et al., 2001).
The photosynthetically active mesophyll cells, and specifically their chloroplasts, are one of the major sinks of Fe in plants (Imsande, 1998; Marschner, 1995). Iron is allocated throughout the plant as Fe(III) chelates, which must be reduced to Fe(II) to enable its transportation in the mesophyll cells (Hell and Stephan, 2003). Ferric reduction capacity of leaf cells has been until now estimated in a few plant species with the use of intact leaves (Kosegarten et al., 1999), excised leaf pieces (Larbi et al., 2001; Nikolic and Römheld, 1999), plasma membrane preparations (Brüggemann et al., 1993; González-Vallejo et al., 1999), and leaf protoplasts (González-Vallejo et al., 2000).
Iron deficiency is a widespread nutritional disorder among the cultivated plant species and particularly in grapevine, which is induced in calcareous and alkaline soils and is known as Fe chlorosis (Mengel, 1995). Impaired Fe nutrition causes severe symptoms in grapevine such as chlorotic leaves and decreased growth and fruit yield (Bavaresco et al., 2003; Bavaresco and Poni, 2003). A wide range of resistance degrees to induced Fe deficiency can be tracked among grapevine species (Bavaresco et al., 1994). Vitis vinifera cultivars grown by their own roots are generally efficient regarding Fe acquisition under Fe deficiency conditions induced in calcareous soils (Galet, 1979). On the contrary, grapevine rootstocks, which are interspecific hybrids used for the control of phylloxera, exhibit diverse behavior under equivalent conditions (Tagliavini and Rombolá, 2001), and the utilization of the most tolerant ones offers the most efficient way for alleviating Fe chlorosis in grapevine. Rootstock genotype affects to a great extent Fe nutrition in the scion cultivars whenever Fe availability is limited as a result of soil conditions (Bavaresco et al., 1993, 2000).
Grapevine is considered to be a strategy I plant because root Fe(III)-reducing capacity and rhizosphere acidification have been demonstrated for several rootstocks and vinifera cultivars under Fe deficiency conditions (Bavaresco et al., 1991; Brancadoro et al., 1995; Jiménez et al., 2007; Nikolic et al., 2000). However, there is insufficient knowledge about the specific mechanisms that are used by grapevine rootstocks for Fe acquisition and plant responses to external Fe supply. Root FCR activity has been studied with time course experiments in the susceptible to Fe deficiency rootstock V. riparia and the tolerant vinifera cultivar ‘Cabernet Sauvignon’ (Jiménez et al., 2007). Furthermore, localization of root FCR activity and contribution of root hairs to Fe acquisition processes have not yet been studied in grapevine. Recently, leaf ferric reduction activity was determined with the use of leaf disks in Concord grapevine (Smith and Cheng, 2007).
The aim of this work was to study the characteristics of FCR activity in roots and leaves of the grapevine rootstock 140 Ruggeri, which is highly tolerant to Fe deficiency stress conditions (Tagliavini and Rombolá, 2001). Root FCR activity was studied in time course experiments after addition or removal of Fe from the nutrient solution, in Fe-deficient and Fe-sufficient micropropagated rootstock plants, respectively. Localization of FCR activity in roots and the contribution of root hairs to Fe(III) acquisition were studied. In parallel, leaf FCR activity was determined with the use of mesophyll protoplasts from in vitro-growing Fe-deficient and Fe-sufficient 140 Ruggeri rootstock plants.
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