Research review paperChilli peppers — A review on tissue culture and transgenesis
Introduction
Capsicum (chilli peppers) is a new world genus from the night shade family originated and domesticated in the American tropics. Capsicum is derived from the Greek word “Kapsimo” meaning to bite (Basu and De, 2003). Chilli terminology is quite confusing; therefore, pepper, chili, chile, chilli, aji, paprika and Capsicum are synonymously used for “chilli pepper” plants. All species in the genus are still not known, since botanists have not been able to explore the areas richest in species diversity due to the critical standard of public security; but there are thought to be 25–30 species of Capsicum, of which five namely C. annuum L, C. frutescens Mill., C. baccatum L., C. chinense and C. pubescens Ruiz and Pavon have been domesticated and currently cultivated (Csilléry, 2006). C. annuum is the most widespread and widely cultivated species in subtropics and temperate countries (Belletti et al., 1998).
Chilli is an indispensable spice used as basic ingredient in a great variety of cuisines all over the world. It is also used as flavourant, colourant and adds tang and taste to the otherwise insipid food. Capsicum species are employed whole or ground and alone or in combination with other flavouring agents, primarily in the pickles, stewed or barbequed (Ravishankar et al., 2003).
The nutritive value of Capsicum is high and it is an excellent source of vitamins C (ascorbic acid), A, B-complex and E along with minerals like molybdenum, manganese, folate, potassium and thiamine. Chilli contains seven times more vitamin C than orange. Beta-carotenoids, and vitamins C and A in chillies are powerful antioxidants that destroy free radicals (Simonne et al., 1997).
The vivid colours exhibited in Capsicum are due to a mixture of esters of capsanthin, capsorubin, zeaxanthine, crytoxanthine and other carotenoids. These extractable colours of chilli pepper fruits are used extensively in the food processing industry to wide range of products such as sausages and meat products, as well as for cheeses, butters, salad dressings, condiment mixtures, gelatine desserts and processed foods (Govindarajan, 1986).
The therapeutic properties and pungency exhibited in Capsicum is attributed to a group of alkaloids known as capsaicinoids (Ochoa-Alejo and Ramírez-Malagón, 2001). The hotness of the chilli is mainly because of capsaicin (C18H27O3N), which is a condensation product of 3-hydroxy, 4-methoxy benzyl amino and decylenic acid. As a medicine it is used as a counter irritant in Lumbago, neuralgia, rheumatic disorders and non-allergic rhinitis. It has a tonic and carminative action. In combination with cinchona it is employed in intermittent and lethargic afflictions and also in atonic gout, dyspepsia accompanied by flatulence, tympanitis and paralysis. Its most valuable application appears in cynanche maligna and scarlatina maligna used either as a gargle or administered internally. The plants have also been used as folk remedies for dropsy, colic, diarrhea, asthma, arthritis, muscle cramps and toothache (Ravishankar et al., 2003).
Vulnerability of the pepper genotypes to a multitude of abiotic and biotic stresses has restricted their potential yield. Abiotic factors that significantly diminish the yield and quality of peppers include extreme of temperature, moisture, light, nutrients and pH among others (Ochoa-Alejo and Ramírez-Malagón, 2001) (Fig. 1). Biotic factors include susceptibility of pepper to various fungi, bacteria and viruses. The phytopathological problem that poses a great threat to cultivated pepper is viral infection (Venkataiah et al., 2003). Chilli peppers are prone to a wide variety of viruses including tobacco mosaic virus, tobacco etch virus, cucumber mosaic virus, potato virus Y, pepper mottle virus, ring spot virus, and tospo virus. Viruses are among the most important group of plant pathogens affecting the Capsicum production worldwide and cause catastrophic economic losses by reducing yield and compromising quality (Suzuki and Mori, 2003). Diseases caused by phytopathogenic fungi (Phytophthora capsici, Fusarium and Rhizoctonia) are also important in yield reduction of chilli peppers (Egea et al., 2002).
Conventional plant breeding combined with improved agricultural practices has contributed to dramatic improvements in Capsicum crops upgrading both quality and use. However, there are certain limitations to this technology due to limited gene pool or the restricted range of organisms between which genes can be transferred. Hence, the plant biotechniques encompassing plant tissue culture and genetic engineering are gradually becoming a functional aspect of classical breeding programs and boost the improvement in Capsicum crop also.
Many important chilli pepper genes with known functions, such as those responsible for taste, colour, or resistance against different pathogens or abiotic factors have been isolated and characterised (Taller, 2006). This knowledge should certainly facilitate chilli pepper improvement or the genetic manipulation of different biosynthetic pathways by genetic engineering (Fig. 1).
A previous review on chilli pepper biotechnology was published by Ochoa-Alejo and Ramírez-Malagón (2001). This review presents the plethora of information of the research work conducted during past years, but describing in more detail the most recent advances reported in tissue culture and transgenesis biotechnology of Capsicum.
Section snippets
Problems associated with Capsicum biotechnology
Preliminary studies have vividly indicated various inherent problems associated with in vitro studies of Capsicum such as severe recalcitrant morphogenic nature, formation of rosette shoots or ill-defined shoot buds, genotypic dependence, which alone or together jeopardize the whole tissue culture efforts and in turn plant improvement through genetic transformation.
Plant regeneration systems
In vitro culture is one of the key tools of plant biotechnology that exploits the totipotent nature of plant cells, a concept proposed by Haberlandt (1902) and unequivocally demonstrated for the first time by Steward et al. (1958). In vitro culture facilitates rapid multiplication of superior clones and is a pre-requisite for improvement of plants via genetic engineering technique. Tissue culture has been exploited to create genetic variability by producing haploids, somaclonal and gametoclonal
Genetic transformation
Conventional plant breeding has contributed significantly to crop improvement over the past fifty years. However, there is intense pressure to produce further improvements in crop quality and quantity as a result of population growth, social demands, health requirements, environmental stress and ecological considerations. Traditional plant breeding is not able to withstand this increasing demand due to the limited gene pool, restricted range of organism between which genes can be transferred
Conclusions and future prospects
Significant progress aimed towards the genetic improvement of Capsicum has been made, but still there is a long way to go in this direction. Important advances have been materialized in pepper regeneration system and genetic transformation; the immergence and establishment of transformation techniques and perfection of the current procedures will be of enormous value as a tool for genetic improvement of Capsicum against various diseases caused by phytopathogenic fungi, bacteria and viruses to
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