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Zinc deficiency-enhanced susceptibility to excess light. Enhancements in chlorosis and necrosis due to increased light intensity are very typical in Zn-deficient source leaves, reflected in a massive accumulation of sucrose and starch (Marschner and Cakmak 1989) causing a high potential for photooxidative damage of chloroplast constituents. In accordance with this suggestion, enhancements in light intensity markedly stimulated appearance of leaf chlorosis under Zn deficiency, but not at adequate Zn supply. Also, partial shading of Zn-deficient leaves prevented or strongly delayed appearance of chlorosis in the shaded areas (Cakmak 2000). Increased severity of leaf chlorosis under high light intensity in Zn-deficient conditions is not caused by lower Zn concentration in leaves but is a consequence of photooxidative damage to chloroplast pigments catalysed by ROS. Photooxidative damage of the chloroplast constituents under Zn deficiency can also be aggravated by reduced activity of enzymes scavenging O2•− and H2O2 in chloroplasts (Hajiboland and Amirazad 2010b).
Zinc deficiency-induced susceptibility to drought stress
It was reported that Zn deficiency is prone to occur in arid and semi-arid regions where soils, particularly top soil, are usually deficient in water (Cakmak et al. 1996). Under drought conditions, Zn mobility in the soil is extremely low, therefore, Zn uptake is usually reduced by low water availability in the substrate. In addition, strongly inhibited root growth in Zn-deficient plants reduces markedly the soil volume exploited by roots and impairs nutrients uptake particularly those are dependent more on spatial availability such as Zn (Marschner 1995). Within plants, there are also some interactions between Zn nutritional status and water relations. It was shown that the ability of plants to cope with water stress during early vegetative stage could be enhanced with adequate Zn supply (Grewal and Williams 2000). Sensitivity to Zn deficiency stress became more pronounced when plants were drought-stressed (Bagci et al. 2007). Impairment of growth in Zn-deficient plants was markedly higher when they were subjected to drought stress and in turn, the effect of drought stress on the inhibition of dry matter production was greater in Zn-deficient compared with Zn-sufficient plants (Hajiboland and Amirazad 2010b).
Because of the effect of Zn deficiency on increasing stomatal limitation (Sharma, Tripathi and Bisht 1995), plants under low Zn supply are more conservative in relation to water economy than sufficient plants when grown under drought stress as indicated by lower water loss, greater water and osmotic potential (Hajiboland and Amirazad 2010b). However, higher growth impairment under combinative effects of Zn deficiency and drought stress is due to damage to photosynthesis apparatus, greater ROS production and remarkable reduction of whole plant photosynthesis following stomatal limitation. On the other hand, under low Zn and drought stress only a small part of Zn taken up by plants is transported into leaves due to significantly lower stomatal opening and transpiration (Hajiboland and Amirazad 2010b).
Reduction of plants resistance to flooding
It was reported that symptoms of Zn deficiency normally appear shortly after flooding (Van Breemen and Castro 1980). Flooding conditions may not only influence Zn availability in soil but also alter plants performance under Zn starvation. In soils, flooded conditions lead to a high concentration of bicarbonate and organic matters which in turn reduce the Zn concentration in soil solution. On the other hand, under flooding conditions concentration of Zn in soil solution decreases through formation of insoluble Zn sulfide (Marschner 1995). Alcohol dehydrogenase (ADH), an enzyme which contains two Zn atoms per molecule, involves in the reaction of acetaldehyde to ethanol. In Zn-deficient plants particularly under anaerobic conditions, ADH activity decreased, which might lead to accumulation of acetaldehyde up to toxic levels (Marschner 1995). Moore and Patrick (1988) reported a decreased root ADH activity in flooded rice plants that was correlated with Zn concentration in the roots and resulted in less ATP production, thereby reducing vital metabolic activities of the roots. Accordingly, a correlation would be expected between susceptibility of plants to Zn deficiency and flooding. In a work with four rice genotypes, however, it was found that ADH activity rather increased in some genotypes under low Zn supply irrespective to their Zn-efficiency trait (Table 3). It could be suggested that, Zn availability at molecular level was not necessarily a limiting factor for ADH holoenzyme in these genotypes. The main cause for negative effect of flooding may be disruption in the function of antioxidant defense system required particularly for Zn-deficient plants under waterlogged conditions. Accordingly, a close correlation has been observed between flooding tolerance and accumulation of ROS in rice genotypes under hypoxic conditions. Content of O2•− and MDA decreased in the lowland rice genotype due to hypoxia while increased significantly in the upland genotype in both low- and adequately-Zn supplied plants (Hajiboland and Beiramzadeh 2008).
Effect of iron deficiency on plants stress responses
Iron deficiency is the most intensively studied plant responses to micronutrients deficiencies in the past 30 decades. Due to a known low Fe availability in soils of different types, Fe uptake and homeostasis are tightly regulated in plants ensure a sufficient supply of Fe from the soil. At the root level, Fe deficiency induces various responses aimed at increasing the availability of the metal in the rhizosphere. Strategy I plants (dicotyledonous and nongraminaceous plants) are able to respond to a lack of Fe in the soil by increasing the capacity of root tissues to reduce apoplastic Fe, the acidification of the rhizosphere, accumulation and release of organic acids (mainly citrate) to increase Fe solubility in soil and mobility within plants and the Fe uptake activities in rhizodermal root cells. In Strategy II plants, members of the mugineic acid family of phytosiderophores are secreted into the rhizosphere by the roots, where mugineic acids chelate and help solubilize Fe3+. The Fe3+-mugineic acid complex is then taken up by root cells through the action of Yellow Stripe 1 (YS1) proteins (Curie and Briat 2003). In contrast to strategies at the root level, our knowledge is much more limited on the mechanisms to cope with Fe deficiency acting in the aboveground part of plants.
Iron deficiency-induced oxidative stress
Iron deficiency is expected to reduce activity of CAT and PODs, the ubiquitous haem-containing enzymes. Activity of CAT reduces under conditions of Fe deficiency and therefore is an indicator of Fe nutritional status of plants (Marschner 1995). The high levels of H2O2 in Fe-deficient plants (Marschner 1995) suggest that Fe starvation may induce both a decreased capacity to peroxide detoxification and/or an active production of peroxide with a consequent rise of oxidative cell status. An increase in SOD activity following Fe deficiency and the decreased capacity to detoxify overproduced H2O2 may induce a secondary oxidative stress. Activity of POD isoforms involving in the polymerization of phenols to lignin is depressed in the roots of Fe-deficient plants that leads to the accumulation and/or release of phenolics and probably development of rhizodermal cells (Marschner 1995). In the leaves, however, Fe deficiency affects the activity of different PODs to different extents (Ranieri et al. 2001). Specific APX activity was reduced in the apoplastic fluid as well as at an intracellular level likely because of the high request for Fe from the APX molecule, as it contains, in addition to the haem group, also a non-haem Fe atom. In contrast to APX, neither soluble nor ionically cell wall bound syringaldazine-PODs underwent any change in Fe-deprived plants in comparison to control ones (Ranieri et al. 2001). These results seem to suggest that under Fe-starvation plants try to retain, preferentially, the functionality of the PODs mainly involved in the maintenance of cell wall structure and, in turn, an unchanged cellular homeostasis or turgor. It is well established that APX plays a key role in the removal of H2O2 in the chloroplast and cytosol of higher plants under environmental stress conditions such as drought (Mittler and Zilinskas 1994). Drought stress, as indicated by a decrease in leaf water potential and stomatal closure, resulted in an increase in APX and CuZn-SOD gene expression and in an increase in CAT activity. In addition, plants recovering from drought showed a dramatic increase in APX and CuZn-SOD steady-state transcript levels (Mittler and Zilinskas 19940. Theses results suggest that optimized Fe nutrition could have a significant role in the protection of plants against oxidative stress during the progression of drought and recovery from drought. The regulation of cytosolic (Mittler and Zilinskas 1994) and chloroplastic (Tanaka et al. 1990) APX has been shown at the level of steady-state transcript accumulation and by regulation of protein synthesis. More investigations are needed to understand the functional significance of CAT and POD isoforms under combinative effects of Fe-deficiency and environmental stress factors such as drought and low temperatures.
Iron deficiency-induced damage to photosynthetic apparatus
Iron is important in the synthesis of Chl in higher plants, and leaves suffering from Fe deficiency show damaged chloroplast structure and decreased Chl content. A low Chl leaf not only has a reduced photosynthetic capacity but also absorbs more light per Chl. The light absorbed and not used in photosynthesis could lead potentially to photoinhibitory and photo-oxidative processes (Abadía, Morales and Abadía 1999). This could be especially important under high light intensities found in field conditions (Jiang, Gao and Zou 2001). At high photosynthetic photon flux density (PPFD), the accumulation of excitation energy in the PSII antenna favors the production of triplet Chl that can interact with O2, generating reactive singlet oxygen (1O2). On the other hand, over-reduction of the photosynthetic electron carrier chain would also favor the direct reduction of O2 by PSI, and the subsequent generation of the ROS superoxide, hydrogen peroxide H2O2 and the hydroxyl radical. Plants suffering a PPFD excess may show sustained photoinhibitory damage, which requires de novo protein biosynthesis to be overcome (Aro, Virgin and Andersson 1993).
Molecular and biochemical adaptations of Fe-starved plants to environmental stresses
Under Fe starvation, plants develop mechanisms for improvement their resistance to the second environmental stress factor such as excess light and drought.
An efficient thermal dissipation in Fe-deficient leaves
To avoid photodamage, the excess radiant energy must be dissipated properly in Fe-starved plants particularly under higher light intensities and drought stress. The excess of light over than that can be used in photosynthesis could be especially important in field conditions, where PPFD is as high as 2200 µmol m-2 s-1 (Abadía, Morales and Abadía 1999). Non-photochemical quenching is one of the mechanisms that prevent or alleviate damage to the photosynthetic apparatus. In this mechanism, excess radiation energy is dissipated as heat in the light harvesting antenna of PSII (Müller, Li and Niyogi 2001). The excessive light energy due to reduced photosynthetic carbon metabolism imposed by lower stomatal opening and/or lower demand because of impaired growth, could be dissipated as heat through non-photochemical quenching. The xanthophyll cycle is an important dissipation mechanism and it may play an important role in Fe-deficient leaves. Compared with control, the ratio of xanthophyll to Chl in Fe-deficient leaves is 1.2 times higher. Higher degree of deepoxidation and higher carotenoid content was clearly a protective response to excess irradiance caused by lower photosynthsis in Fe-deficient leaves. On the other hand, energy dissipation depending upon D1 protein turnover is probably very important in Fe-deficient leaves and there is a close relationship between xanthophyll cycle and D1 protein turnover (Jiang, Gao and Zou 2001). The constitutive level of thermal dissipation in the leaves is approximately 20% but increased up to 70-74% of the absorbed light under moderate water stress (Morales et al. 2000). Accordingly, Fe-deficient leaves could remain without apparent damage in the field for months, which indicates that they have very efficient protective mechanisms (Fig. 2).
An efficient protection of plants against reactive oxygen species
The H2O2 content undergoes a significant increase following the Fe starvation treatment. Although H2O2 is not particularly detrimental to cell metabolism, it can be reduced to extremely reactive •OH radicals through the Fenton reaction in the presence of Fe in the free form or bound to small molecules, such as amino acids, nucleotides and organic acids (Ranieri et al. 2001). In Fe-deficient plants the content of catalytic Fe is extremely low, and consequently, the Fenton reaction was probably unlike to occur. In fact, experimental evidences indicated the absence of oxidative damages to lipids and proteins and suggest that Fe-deficient plants are still sufficiently protected against oxidative stress (Ranieri et al. 2001). In addition, some of the enzymes involved in the detoxification of ROS found to be overexpressed in the roots under Fe-limiting conditions (Zaharieva and Abadía 2003). An improved protection of membranes in Fe-deficient plants is another strategy for survival of plants under prolonged Fe deficiency. Thylakoids are very sensitive targets for photo-destruction by ROS, because of their unique lipid composition containing highly unsaturated (C18:3) fatty acids. Iron deficiency alters such lipid composition, decreasing the concentration of unsaturated (C18:3) and increasing those of saturated fatty acids (C18:0 and C16:0) (Abadía et al. 1988). This makes thylakoids from Fe-deficient plants less susceptible to be degraded by ROS. In fact, oxidatively damaged lipids (and proteins) do not accumulate in Fe-deficient leaves (Iturbe-Ormaetxe et al. 1995).
Remodeling of the major light-harvesting antenna protein of PSII
Recent molecular and biological analyses revealed that several photosynthetic organisms remodel their photosynthetic apparatus under prolonged Fe deficiency. These structural changes minimize photooxidative stress to the thylakoid membrane of algae grown under Fe-deficient conditions (Moseley et al. 2002). In higher plants, photosynthetic apparatus is not damaged in Fe-deficient leaves after prolonged high irradiation suggesting also a mechanism for a long-term acclimation to Fe-deficient conditions. In the mechanism of thermal dissipation of excess light energy, the LHCII protein plays a critical role in plant chloroplasts. The LHCII genes of higher plants are divided into at least six classes, referred to as Lhcb1-Lhcb6, and the genomes of higher plants contain multiple copies of some LHCII genes. Under excess sunlight, LHCII is rapidly and reversibly switched into a photoprotective quenched state in which excess light energy is dissipated as heat, resulting in non-photochemical quenching of Chl fluorescence. Lhcb1-mediated thermal dissipation of excess light energy is possibly regulated by other components i.e. Lhcb2, Lhcb4, that regulate the stabilization, migration and supramolecular organization of Lhcb1 proteins and contribute to the photoprotective mechanism (Saito et al. 2010).
Effect of manganese deficiency on plants stress responses
Manganese deficiency caused development of characteristic visual leaf symptoms such as intravenous chlorosis and subsequently the development of necrotic spots, which are supposed to be related to disorganization of the thylakoid system and loss of PSII reaction centers (Papadakis et al. 2007). Perturbations in the photosynthetic apparatus clearly involved a markedly reduced efficiency of PSII under Mn deficiency due to loss of the PSII core protein (PsbA) (Husted et al. 2009). Even mild Mn deficiency without any distinct leaf symptoms may induce damages to PSII and consequently limit harvest yields, adaptability, and survivability of plants under field conditions. Regarding the fact that Mn is a key component of PSII, the relationship between differential Mn efficiency of plant genotypes and the resistance of the photosynthetic apparatus to perturbations induced by Mn deficiency have been reported (Husted et al. 2009).
Increased production of ROS
The improper function under Mn deficiency not only of the water splitting reaction but also of the photosynthetic electron transport chain increases the probability of oxidative stress for leaf chloroplasts. Under such stress, molecular O2 operates as an alternative acceptor for non-utilized electrons and photon energy, resulting thus in the generation of ROS (Papadakis et al. 2007). On the other hand, Mn is a component of Mn-SOD, and reduction of ROS scavenging due to reduced Mn-SOD activity is expected. However, reports on the change of Mn-SOD activity in Mn-deficient plants are contradictory, reduction (Yu, Osborne and Rengel 1999), no change or even increase (Shenker, Plessner and Tel-Or 2004) of Mn-SOD activity have been reported. Nevertheless, oxidative stress caused by Mn deficiency has been reported in various species (Yu, Osborne and Rengel 1998). Although Mn is not a structural component of Chl molecule and is not directly involve in the biochemical pathways of biosynthesis of Chl, leaves with low Mn content are chlorotic because of Chl losses (Henriques 2003). The ability of ROS to cause photooxidative damages to membranes, organic molecules including Chl under Mn deficiency explains this phenomenon.
Increased susceptibility to higher light intensity
The values of electron transport rate and effective quantum yield of PSII are considerably lower in Mn-deficient than in the Mn-sufficient plants (Papadakis et al. 2007). Under low or intermediate irradiances high percentage of excess photon energy of PSII produced in Mn-deficient leaves is dissipated as heat via the xanthophyll cycle (Papadakis et al. 2007). Accordingly, reduction of effective quantum yield of PSII and increase of non-photochemical quenching are associated with the xanthophyll pigment cycle that provides photoprotection of photosystem by the dissipation of excess absorbed photon energy (Müller, Li and Niyogi 2001). In contrast, under higher light intensities, the reduced values of effective quantum yield of PSII observed in Mn-deficient plants, could be ascribed to the increase of the percentage of close-reduced PSII reaction centers (decreased values of photochemical quenching) due to photoinhibition (Papadakis et al. 2007). Xanthophyll cycle is not fully stimulated in Mn-starved leaves and large amount of excess photons is not successfully dissipated (Jiang, Gao and Zou 2002). It was suggested that the deficiency in trans-tylakoid pH gradient is responsible for the decrease of the xanthophyll cycle-dependent non-radiative dissipation in Mn-starved leaves. Therefore, under high photon flux density much more active oxygen is produced in Mn-starved leaves and PSII reaction centers are damaged by active oxygen in Mn-starved leaves, which resulted in serious photoinhibition (Jiang, Gao and Zou 2002).
Increased susceptibility to drought and high temperatures
Very limited information is currently available on how Mn deficiency affects plant water relations. Although some authors reported lower transpiration rates in some plant species with visible symptoms of Mn deficiency (Singh, Misra and Srivastava 2001), other reports demonstrated that Mn deficiency leads to a marked increase in transpiration (Hebbern et al. 2009) (Fig. 3). Elevated transpiration and lower water use efficiency in Mn-starved plants can be related to more open stomata during the daytime and imperfect nocturnal closure of stomata. Ion leakage from guard cells observed in Mn-deficient leaves will inevitably lead to an imperfect stomatal closure during both night and daytime, resulting in poor water use efficiency (Hebbern et al. 2009). These evidences implied that drought will put additional stress on Mn-deficient plants that are already suffering from disturbances in key metabolic processes. Thus, drought and the poor water use efficiency will attenuate Mn deficiency because Mn-deficient plants exhibit a reduction in root/shoot ratio, which further restricts plants from exploring the soil for available water resources and for plant available Mn.
In turn, water availability in the soil affects Mn uptake by plants (Maschner 1995). Under flooded conditions, soils show low redox potentials, thus Mn deficiency is virtually unknown in plants adapted to waterlogged conditions such as lowland rice. However, when common lowland-rice varieties are cultivated under non-flooded conditions, higher soil redox potential leads to strong reduction of plant Mn availability (Snyder, Jones and Coale 1990). It was shown that Mn uptake of lowland rice was affected by reduced soil water content under non-flooded conditions. In contrast, soil moisture had little effects on P, Fe, Zn, and Cu nutrition (Tao et al. 2007).