Maize Landraces (Zea mays L.): A New Prospective Source for Secondary Metabolite Production
Virgilio Gavicho Uarrota,
Ricardo Brasil Severino
Maize (Zea mays L.) is a staple food for millions of people in the world and its grain is converted into well-accepted local food products, including gruels, porridges, pastes and infant weaning food. Maize landraces synthesize a myriad of secondary metabolites and these compounds play important roles throughout the plants life cycle as mediators in the interaction of the plant with its environment such as plant-insect, plant-microorganism and plant-plant interaction. Secondary metabolites determine important aspects in human food quality. Moreover, several plant secondary metabolites are used for the production of medicines, dyes, insecticides, flavors and fragrances. The extensive genetic and chemical diversity of maize results from a long domestication process carried out since Pre-Colombian civilizations. More than 250 maize varieties are known worldwide and their origin is linked directly or indirectly to the maize domestication and breeding processes performed by ancient civilizations. One of the properties of maize is the spectacular diversity in morphology among its races, which is seemingly paralleled by an extensive allelic variation as detected by molecular methods. On the other hand, despite the fact that modern farming techniques have greatly increased the yield of maize, the genetic breeding programs adopted have led to a greater genetic uniformity and a meaningful loss of diversity. In fact, nowadays very few of the worlds maize germplasm consist of local and creole varieties (landraces), showing the genetic vulnerability of that species. In this context, small farmers in some regions of the world still cultivate maize landraces that are populations with high genetic variability and represent a valuable source of potentially useful traits such as resistance or tolerance to biotic and abiotic stress factors. In this review, a few of the major issues that should be taken into consideration in approaching maize landraces as source of secondary metabolites of interest for human health and nutrition will be briefly discussed.
Received: December 20, 2010;
Accepted: January 17, 2011;
Published: March 28, 2011
Maize (Zea mays L.) is a widely consumed cereal worldwide and contains
carotenoids, phenolics and anthocyanins which are phytochemicals synthesized
in the plant by secondary metabolite pathways (Lopez-Martinez
et al., 2009). These compounds form a large reservoir of natural
chemical diversity as they have a variety of unique carbon skeletons and functional
group modifications (Kutchan and Dixon, 2005). By definition,
secondary metabolites are not essential for the growth and development of a
plant, but rather are required for the interaction of a plant with its environment.
The multiple levels at which a plant can communicate with other organisms, either
in competition or synergy, are reflected in the functional variety found in
secondary metabolites. The obvious demands of a dynamic environment confer a
natural plasticity to secondary metabolism and drive the evolution of genetic
diversification, which has resulted in an arsenal of natural products (Kutchan
and Dixon, 2005). Because much of secondary metabolism is genus or even
species-specific, as well as many of the confirmed functions of plant secondary
metabolites are quite varied or, at worse, unknown, it is well recognized that
this field has not been fully explored in its potential for medicinal purposes,
for instance (Maraschin and Verpoorte, 1999). This scenario
has been gradually changed with the increase availability of powerful molecular
tools for profiling transcripts and metabolites on an unprecedented scale such
as Nuclear Magnetic Resonance (NMR), Matrix-Assisted Laser Desorption/Ionization
Time-Of-Flight Mass Spectrometry (MALDI-TOF MS), Gas Chromatography (GC) and
High Performance Liquid Chromatography (HPLC)-(Maraschin
et al., 2001), as well as with the realization that secondary metabolism,
although complex, has evolved from primary metabolism in a way that can be understood
and exploited for the creation of novel bioactive molecules. Once an area of
research reserved for organic chemists, the study of plant secondary metabolism
has expanded from the isolation of natural products and the elucidation of their
structures to an analysis of enzymes, genes, regulation and genetic engineering.
Technological advances in analytical chemistry, in particular in the development of high-field nuclear magnetic resonance spectroscopy, mass spectrometry (electrospray and MALDI-MS, e.g.,) and Fourier transform-ion cyclotron mass spectrometry, have facilitated the identification and elucidation of the structures of new compounds present even at low levels within a plant. An enormous quantity of information has been gained on chemical structure of plant secondary metabolites in the past decades, but how can one exploit more this chemo (bio) diversity? This study illustrates modern insights and how useful maize landraces might be as prospected as sources of bioactive compounds.
Mankind has been using plant secondary metabolites for the past few thousand
years for multiple purposes such as dyes, flavors, stimulants, hallucinogens,
vertebrate and human poisons, as well as therapeutic agents. The interest in
secondary metabolites has increased in recent years since many investigations
with respect to human nutrition pointed out that modest and long-term intake
of certain secondary metabolites will have a major impact on preventing incidences
of cancers and many chronic diseases (Kutchan and Dixon,
2005). In this context, maize landraces are thought to be potential sources
of bioactive compounds and could be more intensively exploited since it is known
that those genotypes are rich in chemical diversity (Kuhnen
et al., 2010).
PHARMACOLOGICAL ROLES OF SECONDARY METABOLITES EXTRACTED FROM MAIZE LANDRACES
Maize landraces contain, among others, carotenoids, anthocyanins and phenolic
compounds. These secondary metabolites play important roles in human health.
Epidemiological and in vitro researches suggest an inverse relationship
between consumption of fruits and vegetables and the incidence of various chronic
and degenerative diseases that come with aging such as cancer, cardiovascular
diseases, cataracts and brain and immune dysfunction (Lopez-Martinez
et al., 2009). The health beneficial properties of plant metabolites,
carotenoids and (poly) phenols, for instance, have been related not only to
their high antioxidant and anti-radical activities, but also to several other
biological properties, e.g., anti-mutagenic, pro-estrogenic, anti-angiogenic
and induction of detoxification enzymes such as glutathione transferase and
quinine reductase (Lopez-Martinez et al., 2009;
Kuhnen et al., 2009).
The xanthophylls (oxycarotenoids) lutein and zeaxanthin (Fig.
1) the main carotenoids found in maize grains and derived products, carry
desirable health-related properties, e.g., enhancement of immune function, blockage
of mammary tumor growth and protection against blindness caused by age-related
macular degeneration. These and other human diseases such as psoriasis, diabetes-related
blindness and tumor growth were already referred to angiogenic diseases characterized
by the uncontrolled formation of blood vessels (i.e., neovascularization). Studies
have demonstrated that the xanthophyll-rich seed extract of maize displays anti-vasculo/angiogenic
properties, suggesting a potential role of compounds such as zeaxanthin and
lutein in the prevention of diseases related to uncontrolled process of vessel
formation. The results found shed light on the importance of chemical prospective
approaches as an add-value strategy to raw materials of maize genotypes, since
some of their secondary metabolites (e.g., lutein and zeaxanthin) are quite
valuable compounds. In fact, some ongoing researchers in maize landraces have
been confirming these potentialities (Kuhnen et al.,
NEW INSIGHTS IN MAIZE LANDRACES AND EMERGING OPPORTUNITIES
In recent years, the use of plants as bioreactors has emerged as an exciting
area of research and significant advances have created new opportunities. The
driving forces behind the rapid growth of plant bioreactors include low production
cost, product safety and easy scale up. As the yield and concentration of a
compound is crucial for commercial viability, several strategies have been developed
to boost up the biosynthesis of some target secondary metabolites (Maraschin
et al., 2002; 2000a).
||Chemical structures of carotenoids found in maize landraces
seeds, silks and flowers. Retinoic acid is an oxidized and more stable form
of retinol, i.e., vitamin A
Maize landraces usually present a well adapted phenotype in their sites of
cultivation resulting from a peculiar genetic variability positively interacting
with (a) biotic factors, e.g., soil, climate, pests and diseases. For that,
an expressive number of secondary metabolites are thought to be involved regulating
the ecological relationships of a given maize landrace population and its environment.
This way, a better understanding of the effects of ecological factors on the
adaptation response of those genotypes seems to be relevant as one aims at to
improve the yield of a target bioactive compound in populations of Z. mays
landraces. Additionally, such an approach has increased in importance since
maize landraces are claimed to be low-energy input crops as they allow obtaining
a certain yield with minimum ecological impact (i.e., eco-friendly). On the
other hand, despite the great potential as source of already known or new active
biomolecules, studies on the chemical diversity of maize landraces are lacking.
In a second approach, Ethnopharmacological studies from various regions of
the world on the use of maize landraces have been published showing pharmacological
activities that include among others mild diuretic, tonic and urinary demulcent,
antispasmodic, anti inflammatory and antioxidant properties. Aqueous extracts
of maize female flowers, i.e., stigma and styles, have also been used to pass
kidney stones, to cure bladder ailments, gout, benign prostatic hyperplasia,
edema (water retention), lowering blood pressure and even helping rheumatism
symptoms or hypertensions (Kuhnen et al., 2010).
Discovery of a single new pharmaceutical agent originated from plant material
might be a time-cost and extremely expensive process. In a random collection
for pharmacological screening, most part of the plant extracts tested (95% or
more) is inactive and great part of those active extracts contain already known
compounds. If Ethnopharmacological or Ethnobotanical research is used to provide
initial information, success in screening is significantly improved; with 20-60%
of plant extracts tested showing some pharmacological activity (Soejarto
et al., 2005). One valuable strategy used to improve the identification
of bioactive compounds from plants was known as ethno-directed screening
and encompasses field working with traditional healers (curanderos).
Taking into account that maize landraces are well known genotypes in their native
regions, one can find information on the traditional knowledge associated to
their usage for both nutritional and human health treatments. However, despite
the considerable research activity in natural products identification and activity
analysis, this field has not been enough explored and this is also true for
ECOLOGICAL FACTORS OF INFLUENCE ON THE CONTENT OF SECONDARY METABOLITES
Since secondary metabolites represent a chemical interface between plants and
surrounding environment, their syntheses are frequently affected by environmental
conditions. Thus, variations in the total content and/or of the relative proportions
of secondary metabolites in plants can take place. The main environmental factors
that can streamline or alter the production or concentration of secondary metabolites
in plants are seasonality, circadian rhythm, developmental stage and age, temperature,
water stress, UV radiation, soil nutrients, altitude, atmospheric composition,
tissue damage (Gobbo-Neto and Lopes, 2007) and phosphorus
nutrition (Uarrota, 2010). Figure 3
shows some examples of secondary metabolites found in maize landraces tissues
(Kuhnen et al., 2010) that are affected by the
In recent years, efforts are underway to improve the carotenoid levels in staple
food crops to overcome vitamin A deficiency in areas with limited access to
animal products, fruits and vegetables. An increasing number of studies have
been performed showing the bioactive potential of secondary metabolites (Maraschin
et al., 2000b, 2003). Maize landraces are
sources of three carotenoids, namely β-carotene, β-cryptoxanthin and
α-carotene that are pro-vitamin A compounds. Besides, maize is also a good
source of nonprovitamin A carotenoids, including lutein and zeaxanthin, which
play beneficial roles in human health (Menkir et al.,
2008). The consumption of carotenoid-rich foods is associated with reduced
risks of developing cardiovascular diseases, enhanced immune responses, improved
vision and prevention of night blindness, as well as the maintenance of healthy
skin and gastrointestinal and respiratory systems. Increased dietary intake
of lutein and zeaxanthin has been associated with lowering of the risk of cataracts,
age-related macular degeneration and other degenerative diseases (Menkir
et al., 2008). Since the various carotenoids have distinct roles
in human metabolism, further enhancement of their levels in staple food crops
may have a positive health impact in areas where maize is consumed. Of interest,
maize is a staple food crop of utmost importance in less favored countries where,
no rare, chronic vitamin A deficiency in childhood is a major concern. The stimulus
to low-energy input crops cultivation (maize landrace, for instance) in such
countries is claimed to be relevant for obvious economical, social and ecological
MAJOR IMPACT OF CARBON, NITROGEN AND SULFUR RESOURCES ON THE PRODUCTION OF SECONDARY METABOLITES
Nutrient supply plays a major role regulating biosynthesis, accumulation and
degradation pathways in plants. For example, the content of the metabolically
linked carbon and nitrogen compounds has a major quantitative impact on the
extent of production of secondary metabolites in plants. The carbon-nitrogen
balance hypothesis claims that under conditions of limited nitrogen availability
as usually found in production systems of maize landraces, secondary metabolism
is directed toward producing carbon-rich metabolites and vice versa (Coley
et al., 1985). Yet, this idea seems to be correct for some cases
but not all. This is most probably due to the fact that changes in carbon and
nitrogen availability (such as photosynthesis and nitrogen fertilization) do
not always lead to analogous changes in the relative levels of carbon-rich and
nitrogen-rich primary metabolites, but rather the opposite (Matt
et al., 2002). On the other hand, the relative endogenous pools of
carbon-rich or nitrogen-rich primary metabolites seem to have a strong correlation
with the levels of carbon-rich or nitrogen-rich secondary metabolites (Fritz
et al., 2006). Matt et al. (2002)
showed that an increase in the sugar/amino acid ratio resulted in the elevation
of carbon-rich phenylpropanoids and a decline in the levels of the nitrogen-containing
alkaloid nicotine. Sulfur (S) is also an essential primary compound for plants,
taking part in enzyme functions (e.g., formation of disulfide bonds and S-containing
cofactors), redox regulation (e.g., glutathione and Fe-S clusters), donation
of methyl groups in numerous reactions, including DNA methylation and mRNA capping
through S-adenosylmethionine (i.e. SAM), as well as in the synthesis of polyamines
(Rausch and Wachter, 2005). However, S is also incorporated
into an array of secondary metabolites (Hirai and Saito,
2008). Catabolic recycling of S-containing secondary metabolites occurring
upon plant growth in S-deficient environments is crucial for the response of
the plants to various stresses. In cruciferous plants, for example, including
the model plant Arabidopsis, glucosinolates are the major S-containing secondary
metabolites that are required for plant defense against pathogens (Sonderby
et al., 2010). Indeed, the activation of sulfate acquisition and
repression of glucosinolates production occur in parallel under S limitation.
Under S-limiting conditions, plants generally enhance S assimilation through
activating S transporters, while at the same time also activating a putative
thioglucosidase, thought to be involved in the hydrolytic degradation of glucosinolates
for catabolic sulfur recycling, as well as in negatively regulating glucosinolates
biosynthesis genes (Aharoni and Galili, 2010).
Expanding this subject to maize landraces, it is worth mentioning that those genotypes usually show a good adaptation to poor soils, even guaranteeing interesting grain yields under limiting soil conditions for hybrid varieties. Such a phenotype is thought to be in connection with the synthesis and accumulation of secondary metabolites, i.e., an ecological strategy of survival of Z. mays landraces populations, with eventual meaningful production of valuable compounds for human health. Unfortunately, such an issue seem not to be enough understood, avoiding a more intensive and rational exploitation of the maize landraces potential.
METABOLIC PATHWAYS OF SECONDARY METABOLITES
Although in the past 10 years quite some research has been done aiming at to
increase secondary metabolites production, a major constraint has been the poor
characterization of plant secondary metabolic pathways at the level of biosynthetic
intermediates and enzymes. Consequently, very few genes from plant secondary
metabolism are available, hampering the improvement of yield of a given compound
of interest through genetic manipulation, for example. The best studied pathway
at the genetic level seems to be the one leading to the formation of flavonoids
and anthocyanins. Because of their antioxidant activity, high levels of anthocyanins
in foods, for example, are an interesting trait and this is especially relevant
for some maize landraces where prominent amounts of anthocyanins and their aglycone
forms (e.g., cyanidin, delphinidin and malvidin) are found in their grains and
female flowers (Kuhnen et al., 2010). Figure
2 shows an overview of the biosynthetic pathways of plant secondary metabolites
and their main intermediary compounds.
More often it has been aimed to increase the production of certain compounds
in the normal producing plant species by epigenetic manipulation (Maraschin
and Verpoorte, 1999). However, when the results are unsatisfactory, techniques
of genetic manipulation have been used to transfer (part of) a pathway to other
plant species (i.e., heterologous transformation) or microorganisms (Verpoorte
and Memelink, 2002). For that, from a metabolic engineering point of view,
two general approaches have been followed to increase the production of target
compounds. Firstly, methods have been employed to change the expression of one
or few genes, thereby overcoming specific rate limiting steps in the pathway,
to shutdown competitive pathways and to decrease catabolism of the product of
interest. Secondly, attempts have been made to change the expression of regulatory
genes that control multiple biosynthesis genes (Verpoorte
and Memelink, 2002). The tight link between metabolic fluxes of primary
metabolites and the accumulation of secondary metabolites renders the engineering
of the latter compounds quite complex as it demands the consideration of the
entire metabolic network in order to redirect primary metabolites into secondary
metabolites with respect to the primary-secondary interface (Aharoni
and Galili, 2010). Indeed, it seems to be consensus that for a successful
genetic manipulation of secondary metabolites biosynthetic pathways of plant
species, deeper studies are urgent regarding the dynamic interactions between
primary and secondary metabolic pathways and the effect of intrinsic and ecological
regulator factors, as one envisage technological applications thereof (Maraschin
and Verpoorte, 1999). On this subject, maize landraces, thanks to their
high genetic variability, have been claimed as important sources of genes for
genetic breeding programs aiming at to improve nutritional and health traits
of Z. mays grains and derived products.
||The production of secondary metabolites is tightly associated
with pathways of primary/central metabolism, such as glycolysis, shikimate
and production of AAA and aliphatic amino acids
||Secondary metabolites whose occurrence is affected by seasonality,
circadian rhythm, water availability and tissue damage in maize landraces
In past few years, an increase number of studies have been revealing maize landraces as useful sources of raw materials for secondary metabolite production such as carotenoids, anthocyanins and phenolics compounds which are thought to play important roles in human health. Thus, maize landraces constitute a prospective germplasm for improving or increasing the production of those compounds with direct application in food, pharmaceutical and cosmetical industries. In fact, there seem to be a wide range of secondary metabolite profiles among individuals and varieties analyzed under the same growing condition, allowing the exploitation of a prominent chemical diversity of those genotypes. Besides, within this framework of ideas, heirloom maize landraces varieties are a still important resource against genetic erosion and as livelihood for uncountable small farmers in several countries. Otherwise, in comparison with the genetic diversity of maize landraces available world wide, studies focusing on the chemical characterization of those genotypes are still scarce in the literature. In fact, more studies are needed to understand the regulation and integration of secondary metabolism pathways with primary ones as well as the (a) biotic factors affecting the synthesis of health-valuable compounds and how to optimize it in maize landraces. Finally, the extinguishment of such germplasm resulting from the adoption of modern agricultural techniques, i.e., hybrid and transgenic maize varieties, is a continuous concern and its potential as source of valuable bioactive molecules might shed new insights into its usage, corroborating to preserve that invaluable genetic resource.
This study was carried out under CNPq-Brazil/MCT-Mozambique Postgraduate fellowship program. The authors wish to thank all anonymous reviewers.
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