Fig. 1.3. Scheme of photosynthetic electron transport showing micronutrient components osmoregulatory concentrations of chlorine are, however, of a much higher magnitude than the concentrations at which other micronutrients perform their critical roles.
1.3.5. Secondary Metabolism, Growth Hormones and Signalling Molecules
An important role of micronutrients involves their functioning in biosynthetic pathways. Many enzymes containing micronutrient cofactors catalyze reactions in biosynthesis of secondary metabolites. Precursors for the synthesis of aromatic amino acids are synthesized via shikimate pathway, whose initial reaction involves the condensation of phosphoenolpyruvate and erythrose-4-phosphate to provide 3-deoxy-D-arabinoheptulosonolate-7 phosphate (DAHP), catalyzed by the manganese-activated enzyme DAHP synthase (Herrmann, 1995; Hermann and Weaver, 1999). Production of the lignin precursor p-coumaric acid from trans-cinnamic acid is catalyzed by NADPH-Cytochrome P450 dependent haem enzyme, cinnamate-4-hydroxylase. Zinc-activated cinnamyl dehydrogenase catalyzes the conversion of p-coumaraldehyde and other cinnamic aldehydes to cinnamic alcohol (monolignols), which are subsequently polymerized to lignin.
Biosynthesis of gibberellins, lately recognized as major signalling molecules in plants (Sun and Gubler, 2004), involves the activity of several enzymes activated by micronutrients. Synthesis of the gibberellin precursor ent-kaurene involves catalysis by kaurene synthase, which is activated by Mn2+, Co2+ or Mg2+. The next three steps involving conversion of enf-kaurene to ent-kaurenoic acid are catalyzed by eni-kaurene oxidase, which is a P450 haem monooxygenase (Chappie, 1998). Several 2-oxodependent dioxygenases catalyze the conversion of GA12 aldehyde to gibberellins and the interconversions of the latter, including those leading to production of their bioactive forms (Hedden and Phillips, 2000; Schomburg et al. 2002) (Fig. 1.4).
Three enzymes of jasmonic acid biosynthesis contain iron cofactors. Lipoxygenase is a non-haem iron-containing dioxygenase (Feussner and Wasternack, 2002); fatty acid hydroperoxide lyase and allene oxide synthase are cytochrome P450 dependent monooxygenases (Chappie, 1998). Terminal steps in the biosynthesis of ethylene and abscissic acid are also catalyzed by enzymes having micronutrient cofactors. 1-aminocyclopropane-l-carboxylate oxidase, which catalyzes the synthesis of ethylene, is specifically activated by iron (Prescott and John, 1996). Aldehyde oxidase, which catalyzes the synthesis of abscissic acid from abscissic aldehyde, is a molybdoprotein (Romao et al. 1995). Synthesis of flavones and anthocyanin also involves catalysis by cytochrome P450 haem monooxygenases (Chappie, 1998).
Geranyl geranyl diphosphate ent-Copalyl diphosphate ent-Kaurene synthase (Mn, Mg, Co)
eni-Kaurene ent-Kaurene oxidase (Fe)
GA20, GAi-oxidases (Fe)
Bioactive gibberellins (GA4, GA,)
Fig. 1.4. Steps in gibberellin biosynthesis pathway catalyzed by micronutrient requiring enzymes
Micronutrients are known to influence the generation and detoxification of reactive oxygen species that induce oxidative stress and play an important role in signal transduction (Apel and Hirt, 2004). Enzymes having micronutrient cofactors function as a part of the antioxidant system, offering protection against damage from excessive generation of reactive oxygen species (ROS) during electron transport in mitochondria and chloroplasts (Fridovich, 1986; Elstner, 1991; Asada, 1994). Superoxide dismutases, which have micronutrient cofactors Fe-SOD, Mn-SOD, Cu-Zn SOD, constitute the first line of defense against the ROS (Alscher et al. 2002). Mitochondria have high activities of Mn-SOD and chloroplasts show high activities of Cu-Zn and Fe-SOD. The cytosol and the apoplasm also show Cu-Zn SOD activity. Thus, the superoxide ions (02) are effectively converted to H2Oz within close vicinity of the site of their generation. Accumulation of H202 in toxic concentrations is prevented by its reduction to water in reactions catalyzed by the haem enzymes ascorbate peroxidase (APX) and catalase (CAT). APX, which uses ascorbate as its specific electron donor, reduces H202 to water with concomitant production of monodehydroascorbate, which is spontaneously disproportionated to dehydroascorbate. Working in combination with the ascorbate-glutathione cycle, APX functions as a potent scavenger of H202 in chloroplasts (Asada, 1992, 1997). Catalase, which carries out rapid breakdown of H202, is highly concentrated in peroxisomes, wherein conversion of glycolate to glyoxylate also adds to accumulation of H202. Thus, acting as cofactors of antioxidant enzymes SOD, APX and CAT, the micronutrients (Cu, Zn, Fe, Mn) contribute to defense against oxidative stress. Overexpression of superoxide dismutases in transgenics has been shown to enhance their tolerance to oxidative stress (Gupta et al. 1993; Perl et al. 1993; Slooten et al. 1995; Van Camp et al. 1996).
A decrease in the activity of the antioxidant enzymes due to limited availability of the metal cofactors weakens the antioxidant defense mechanism and exposes the plants to greater damage from the ROS. Cakmak (2000) has provided a comprehensive update on the role of zinc in protection of plants against oxidative stress.
Another example of the protective role of micronutrients is the role of iron as a cofactor of choline monooxygenase (CMO) in glycine betaine accumulating plants. CMO catalyzes the first step in the biosynthesis of glycine betaine which acts as osmoprotectant, stabilizing the quarternary structure of proteins and maintaining the structural integrity of cellular membranes under stressful conditions imposed by high salinity and temperature (Graham, 1995).
Not only are micronutrients involved in providing protection against abiotic stresses, they also provide protection against pathogenic infections (Graham, 1983; Graham and Webb, 1991; Huber and Graham, 1999). This, micronutrients may do in two ways. Manganese, copper and boron are involved in the metabolism of phenolic compounds and their polymerization to lignins and suberins, which strengthens the plant cell walls, making them less susceptible to penetration by pathogens (Mehdy, 1994). Another way that the micronutrients offer protection against pathogenic infections is through enhanced production of H202, which functions as a cytotoxic compound against the pathogens and also activates the defense mechanism of the host (Lamb and Dixon, 1997; Orozeo-Cardinas et al. 2001). Copper, as a constituent of diamine oxidases, plays an important role in generation and regulation of H202 concentration in response to attack by pathogens (Fedrico and Angelina, 1991, Rea et al. 1998).
Functions of micronutrients are not localized to a particular plant part, cell or cell compartment. Therefore, they have to be available at all the functional sites in concentrations commensurate with their biochemical requirement. This necessitates a mechanism for their homeostasis. This may involve induction of high-affinity transport systems in response to deficiency and a mechanism for their subcellular sequestration under conditions of overload. High-affinity uptake mechanisms for uptake of iron are upregulated on sensing iron deficiency (Marschner and Romheld, 1994). The level of iron supply may also determine the tissue and cellular mobilization of iron (Bereczky et al. 2003, Thomine et al. 2003). The active process of boron uptake is induced in response to boron deficiency (Dannel et al. 2002). Zinc regulates the uptake of phosphorus by regulating the genes encoding high-affinity phosphate transport (Huang et al. 2000). Several TF 111 A-type zinc finger proteins such as SUPERMAN, AtZFPl, PetSPL3 and Be ZFP1 are implicated in the developmental regulation of various floral and vegetative organs (Takatsuji, 1999). Zinc is also reported to be involved in the prevention of cell death in plants (Dietrich et al. 1997). In Ambidopsis, a zinc finger protein functions as a negative regulator in plant cell death.
So far, the predominant sporophytic generation of higher plants has been the main focus of investigations on the roles and deficiency responses of micronutrients. It is being increasingly realized that poor reproductive yield of plants subjected to deficiency of micronutrients cannot be entirely attributed to their effect on photosynthetic efficiency and/or partitioning of assimilates. Reproductive development of plants may be severely limited even when deficiency is induced, nearing onset of reproductive phase, by which time plants have accumulated enough photosynthates. Many studies on the reproductive development following deprivation of micronutrient supply to plants point to a more direct involvement of micronutrients in reproductive development (Graham, 1975; Dell, 1981; Sharma et al. 1990; Sharma et al. 1991; Pandey et al. 2006). Development of different floral organs in Arabidopsis, petunia and Chinese cabbage has been suggested to involves TF III A type zinc finger control of cell division and/or expansion of particular cell types (Takatsuji, 1999) There are also reports suggesting that the role of zinc finger polycomb group proteins in development of female gametophyte and seed development (Grossniklaus et al. 1998; Brive et al. 2001). Reproductive development may also be inhibited because of poor delivery of micronutrients in the reproductive tissues due to some morphological limitations or poor transport (Brown et al. 2002; Takahashi et al. 2003). Studies using transgenic tobacco plants (naat transgenics) and crosses between the naat transgenics and the wild type (Takahashi et al. 2003) show severe limitation in delivery of Zn, Cu and Mn into the reproductive parts due to lack of nicotianamine, which has a role in chelation and long-distance transport of the cationic micronutrients (Welch, 1995; Von Wiren et al. 1999).
There is a quantitative requirement of the micronutrients for optimal performance of their roles. Deviations from the optimum are reflected in aberrations in structure, function, and developmental and adaptive responses of plants. These issues are addressed in the following chapters of Part I.
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