Deficiency on Plants Stress Responses

Molybdenum as component of specific plant enzymes participates in reduction and oxidative reactions. Molybdenum is a component of some bacterial nitrogenases and therefore is especially important for plants that live in symbiosis with N-fixing bacteria (Marschner 1995). Molybdenum deficiency influences plant metabolism at various levels. These responses are related mainly to the requirement of Mo for different types of molyb-doenzymes in plants. Molybdenum is an integral part of an organic pterin complex called the Mo cofactor (MoCo). Plant molybdoenzymes include nitrate reductase (NR), xanthine dehydrogenase/ oxidase (XDH), sulfite oxidase (SO) and those involved in abscisic acid (ABA) and indole-3 acetic acid (IAA) synthesis (aldehyde oxidase; AO). Nitrate reductase and SO contain a dioxo-Mo cofactor, while xanthine dehydrogenase/oxidase and AO have a monoxo-Mo cofactor which requires MoCo insertion and then subsequent sulfuration (Mendel and Haensch 2002). Since Mo is involved in various enzymatic processes and hormone biosynthesis, Mo deficiency can alter vegetative and reproductive growth and susceptibility to biotic (Graham and Stangoulis 2005) and abiotic stress factors. Indeed, Mo plays an important role in plants adaptation to environmental stresses through its effect on the activity of aldehyde oxidase.

9.1 Relationship Between Molybdenum Nutrition and Responsiveness of Plants to Stresses

During last decades there have been numerous reports of the increased susceptibility of Mo-deficient plants to environmental stresses such as salinity, drought and low temperature. These findings could be explained in recent years after elucidation of the role of the gene (LOS5) in the converting desulfo/dioxyo form of MoCo to the sulfide form of MoCo, a cofactor of aldehyde oxidase that catalyzes the last step of abscisic acid (ABA) biosynthesis. The los5 mutant (low expression of osmotically responsive genes) is highly sensitive to drought and salinity as well as cold stress due to the lack of ability for ABA biosynthesis (Xiong et al. 2001). Drought, salinity, and to some extent cold stress cause an increased biosynthesis and accumulation of ABA mainly by the induction of genes coding for ABA bio-synthetic enzymes (Rock 2000). The first gene encodes zeaxanthin epoxidase (ZEP), which converts zeaxanthin to epoxycarotenoid and is defective in the Arabidopsis mutants abal and los6. The cleavage enzyme 9-a's-epoxycarotenoid dioxygenase (NCED) catalyzes the conversion of epoxycarotenoids to xanthoxin. Abscisic acid aldehyde oxidase (AAO) converts ABA aldehyde to ABA and is defective in the Arabidopsis aao3 mutant (Schwartz et al. 2003). Aldehyde oxidase is a MoCo enzyme and requires sulfuration for activation. This step is catalyzed by a MoCo sulfurase (MCSU), which is encoded by the ABA3/ LOS5 locus in Arabidopsis (Fig. 16.5). Mutations in the aldehyde oxidase apoprotein and MoCo biosynthetic enzymes would lead to ABA deficiency in plants (Xiong et al. 2002a) .

9.2 Increased Susceptibility of

Plants to Salt and Drought Stress

Osmotic stress resulting from either high salinity or water deficit induces the expression of numerous stress-responsive genes in plants (Hasegawa et al. 2000). The role of ABA in the signal trans-duction of ionic and nonionic stresses has been extensively studied (Xiong et al. 2002b) . Low ABA levels result in wilty plants with excessive transpiration and without stomatal control, altered seed dormancy, and impaired defense responses (Mendel and Haensch 2002) . The ABA-deficient mutants flacca and aba3/los5, which are disrupted in the MoCo sulfuration step, have wilty pheno-types and increased transpirational water loss (Bittner et al. 2001; Sagi et al. 2002). In turn, one of the distinct responses of Mo-deficient plants is

Fig. 16.5 The role of Mo in the ABA signaling pathway under environmental stresses. ABA is synthesized from b-carotene via the oxidative cleavage of neoxanthin and a two-step conversion of xanthoxin to ABA via ABA-aldehyde. Environmental stress such as drought, salt and, to a lesser extent, cold stimulates the biosynthesis and accumulation of ABA by activating genes coding for ABA biosynthetic enzymes including ZEP (LOS6/ABA1 in Arabidopsis, codes for zeaxanthin epoxidase), NCED (NCED3 in Arabidopsis, codes for 9-cis-epoxycarotenoid dioxygenase), AAO (AAO3 in Arabidopsis, codes for ABA aldehyde oxidase), and MCSU (LOS5/ABA3 in Arabidopsis, codes for molybdate cofactor sulfurase)

Fig. 16.5 The role of Mo in the ABA signaling pathway under environmental stresses. ABA is synthesized from b-carotene via the oxidative cleavage of neoxanthin and a two-step conversion of xanthoxin to ABA via ABA-aldehyde. Environmental stress such as drought, salt and, to a lesser extent, cold stimulates the biosynthesis and accumulation of ABA by activating genes coding for ABA biosynthetic enzymes including ZEP (LOS6/ABA1 in Arabidopsis, codes for zeaxanthin epoxidase), NCED (NCED3 in Arabidopsis, codes for 9-cis-epoxycarotenoid dioxygenase), AAO (AAO3 in Arabidopsis, codes for ABA aldehyde oxidase), and MCSU (LOS5/ABA3 in Arabidopsis, codes for molybdate cofactor sulfurase)

flaccid and cupped leaves similar with flacca and aba3/los5 mutants (Robinson and Burne 2000).

damages. The impairment of low temperature gene regulation is specific to the los5/aba3 mutation and the LOS5/ABA3 gene is expressed in different plant parts and is a key regulator of ABA biosynthesis in response to stresses (Xiong et al. 2001).

10 Effect of Chlorine Deficiency on Plants Stress Responses

Chlorine is classified as a micronutrient, but it is often taken up by plants at levels comparable to a macronutrient. Supplies of chlorine in nature are often plentiful, and obvious symptoms of deficiency are seldom observed. However, some plant species such as members of Palmaceae and kiwifruit (Actinidia deliciosa) have a much higher chlorine requirement, thus, chlorine deficiency can readily be induced in these species. Chlorine appears to be required for optimal enzyme activity of asparagine synthethase, amylase and ATPase. In photosynthesis, chlorine is an essential cofactor for the activation of the oxygen-evolving enzyme associated with PSII. Chlorine binds to the polypeptides associated with the water-splitting complex of PSII and stabilize the oxidized state of Mn by acting as a bridging ligand (Marschner 1995).

9.3 Increased Frost Damage

10.1 Functions of Chloride in Plants Water and Ion Balance

It has been shown that addition of Mo to acid soils will protect plants against damage caused by low temperature or water logging (Marschner 1995) . Under acidic soil conditions, the molyb-date anion is adsorbed strongly to the surface of Fe and Al oxides by a ligand exchange mechanism and the Mo concentration in the soil solution can be reduced greatly (Hamlin 2007). Molybdenum deficiency is a micronutritional disorder that has been reported frequently in plants grown on acidic soils (Marschner 1995). los5 mutant plants are not only susceptible to salinity and drought but also sensitive to freeze-induced

Most of the chlorine in plants is not incorporated into organic molecules or dry matter, but remains in solution as the monovalent ion chloride (Cl-). Chlorine concentrations required for biochemical functions are relatively low in comparison to concentrations required for osmoregulation. Chlorine concentration in plants exceed the critical deficiency level (~6 mM) by two orders of magnitude, therefore, is important in osmotic adjustment and plant water relations including role in xylem volume flow and root pressure, phloem loading and unloading (Marschner 1995).The accumulation of chloride in plant cells increases tissue hydration and turgor pressure. This osmotic function of Cl- works closely with K+ to facilitate cell elongation and growth. Relative differences in the uptake of cations and anions by plants require the maintenance of electroneutrality in plant cells as well as in the external soil solution. As an anion, chloride serves to balance charges from cations (Heckman and Strick 1996). However, it seems likely that specified function of chlorine in osmoregulation is mainly restricted to distinct organs (e.g., extension zones, the stigma of grasses, pulvini of Mimosa pudica during seis-monastic leaf movement) or cells (e.g., guard cells) (Marschner 1995). This osmoregulatory function in specific tissues requires also concentrations of chloride that are not typical of a micro-nutrient (Flowers 1988) . In rapidly expanding tissues such as elongating cells of roots and shoots and expanding leaves, chloride accumulates in the tonoplast, to function as an osmotically active solute. Chloride is essential for stomatal functioning in some plant species such as onion (Allium cepa L.), which lacks functional chloroplasts for malate synthesis (Schnabl and Raschke 1980). Members of the Palmaceae such as coconut (Cocus nucifera L.) and oil palm (Elaeis guineen-sis Jacq.) also need Cl- for stomatal functioning (Marschner 1995).

10.2 Drought Tolerance

The most commonly described symptom of chlorine deficiency is wilting of leaves, especially at the margins. As the deficiency becomes more severe, the leaves may exhibit curling, shriveling, and necrosis. Roots of chlorine-deficient plants become stubby with club tips. In chlorine-deficient wheat, the symptoms are expressed as chlo-rotic or necrotic lesions on leaf tissue (Engel et al. 2001). In coconut palm, the symptoms are exhibited as wilting and premature senescence of leaves, frond fracture, and stem cracking and bleeding (Marschner 1995). Coconut palm is of great economic importance in the tropics and subtropics and drought is one of the main environmental factors that limit coconut productivity.

In this plant, chloride is an importance factor in the mechanisms governing stomatal opening and closure and is also important for stomatal regulation, particularly during the dry season. Moreover, its high concentration in coconut leaf tissues means that it acts as an osmoticum in maintaining tissue turgor during drought (Braconnier and Bonneau 1998). Differences in the gas exchanges in coconut during the dry and the rainy seasons confirmed the important role of chloride in this palm. In the dry season, chlorine deficiency has a depressive effect on gas exchanges right from the morning, which worsens as the day wears on. This results in reduction of stomatal conductance and net photosynthesis. Under moderate drought, coconut palms not suffering from a chlorine deficiency respond to higher evaporative demand by increasing their stomatal conductance and transpiration, and by maintaining a reasonable level of net photosynthesis. Under the same conditions, deficient palms react by reducing their sto-matal conductance and net photosynthesis, hence expressing a state of stress. The chloride therefore enables coconut palms to withstand the dry season, by maintaining a relatively high level of leaf gas exchanges (Braconnier and Bonneau 1998) .

10.3 Resistance Against Pathogens

Addition of chlorine has been reported to reduce the severity of at least 15 different foliar and root diseases on 11 different crops (Heckman 2007). Several possible mechanisms may explain the effects of chlorine nutrition on disease suppression and host resistance. In acid soils, chloride inhibits nitrification (Rosenberg et al. 1986). Keeping N in the ammonium form can lower rhizosphere pH and influence microbial populations and nutrient availability in the rhizosphere (Heckman and Strick 19962• Competition between chloride and nitrate for uptake also tends to reduce nitrate concentrations in plant tissues. When plants take up more ammonium and less nitrate, it usually causes rhizosphere acidification, which in turn, may enhance Mn availability (Thompson et al. 1995).

Chlorine can also enhance Mn availability by promoting Mn-reducing microorganisms in soil. Factors which increase Mn availability have been associated with improved host resistance to diseases in grain crops (Huber 1989). Higher concentrations of chlorine in plant tissues can also enhance water retention and turgor when roots have been attacked by pathogens. The amount of organic acids, such as malate, in plant tissues and exuded from roots, decreases with chlorine supply. This action deprives pathogens of an organic substrate (Goos et al. 1987).

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