Piqueria trinervia is one of ca. 20 species of this genus belonging to the Com-positae with a wide distribution in Mexico, Central America, and the Antilla. It is a nonhardy herbaceous annual, 70-100 cm tall, and has oblong, lanceolate, slightly toothed leaves, three-nerved and large, loose corymbose panicles of small creamy white flowers. This species grows in open, moist, or dry rocky hillsides and in grasslands at an elevation of 1200-3300 m. The plant dies after the rainy season in Mexico. It has several vernacular names, e.g., tabardillo herb, St. Nicholas herb, or in an ancient native Mexican language, Yoloxiltic. It is often used in domestic medicine as a febrifuge and antipyretic, and against malaria, biliary calculus, typhus, and rheumatism.
This species is of interest as an ornamental. It is grown for the clusters of tiny white flowers, used in flower arrangements. Interestingly, this plant is apparently not eaten by cattle even if they are pressed by hunger. The plants usually grow in relatively isolated, pure populations of different sizes, suggesting an allelopathic effect.
Piqueria trinervia is found as a pioneer of secondary succession in abandoned agricultural or ruderal fields. It is frequently destroyed by farmers because of its presence as a weed, however, it has also been cultivated as an ornamental.
It has a high natural propagation due to the abundance of seeds that are produced by the thousands and having a high germination rate (up to 95%). However, horticulturists prefer, as a conventional practice for its propagation, to make cuttings from the lower part of the plant after flowering, since old plants produce them in abundance.
Since the end of the last century, Piqueria trinervia has often been the object of study. Gonzalez (1890) isolated “one resin, a stain, an essential oil, chlorophyll, a gum and other undetermined principles including an alkaloid”. After this very early work, Bohlmann and Zdero (1968) isolated (-) santalal, a terpenoid of low molecular weight, and Bohlmann and Suwita (1978) reported the presence of terpene compounds.
Romo et al. (1970) in a systematic analysis of the compounds of Piqueria trinervia were able to isolate several substances, from which two monoterpene diaster-oisomers, piquerol A and B, were subsequently the object of investigations of their biological properties.
The conspicuous allelopathic effect in natural populations was confirmed in bioassays with piquerol A and B. Testing several species, Gonzalez de la Parra et al. (1981) demonstrated an allelopathic and acaricidal action of these compounds. Although both, piquerol A and B showed allelopathic and acaricidal action, the former induced a stronger effect. This fact, coupled with the lower yield of piquerol B in natural populations, led to the research mostly with piquerol A.
The potential applications of this substance include medical as well as agrochemical uses, which may have an interesting impact on the world market, particularly due to its effects and lack of toxicity. However, it is necessary to develop strategies that help to maintain the natural populations and do not destroy this genetic resource. Plant biotechnology can play an important role in the production of useful secondary metabolites such as piquerol A.
Conclusions and Prospects
From the investigations on Piqueria trinervia Cav., it is clear that the yield from in vitro raw material is much higher than that obtained in vivo. (However, the comparison between in vitro and in vivo must be made with caution; see below.) Thus continued research is very desirable in order to optimize the system and to improve the yield of the potentially valuable piquerol A. Another conclusion is that the factors that play a role in the in vitro production of piquerol A can be manipulated through the use of growth regulators, environmental conditions, and medium components. In particular, the modulation of the in vitro response of the various explants is needed in order to generate hairy root-like structures that have been shown to produce the highest yield of the desired secondary metabolite.
Some of our results suggest the prospect of inducing hairy roots through genetic transformation in order to optimizate the system.
In preliminary experiments, the content of piquerol A in the intact seedlings grown in vitro was compared with whole adult plants taken from the wild. Table 3 presents these results. However, this comparison must be made with caution, because of the relatively scarce in vitro raw material. Moreover, different methods of quantifying piquerol A were used for in vitro and in vivo material. For the former, gas chromatography was utilized, and piquerol A appeared together with a mixture of components from in vitro biomass. On the other hand, a purification process was carried out before quantification in the in vivo material. Consequently, during the purification process, lower values from the in vitro material can result, even though the observed differences presented in Table 3 are strong enough to keep in mind for future research.
According to the results, roots from seedlings (2 months old) cultured in vitro show the highest content piquerol A. Young leaves and stem from seedlings grown in vitro also contain abundant metabolite. The values detected in the complete in vivo adult plants were lower by several orders of magnitude than those in vitro. These low values are probably due to a loss of piquerol A during the metabolic activity of the plant.
In the in vitro seedlings, roots present almost twice the quantity of piquerol A than leaves plus stem, implying probably that the biosynthesis of the compound takes place more actively in the roots and that a process of trans-location occurs thereafter.
These results, coupled with the fact that hairy root-like structures were the most active producers of piquerol A, show that the generation of roots and transformed roots can increase the yield of the compound in vitro.
Farnsworth and Soejarto (1985), in a very interesting paper, suggested that potential medicinal plants that will be extinct by the year 2000 in the USA alone can have a total value of approx. 3.248 billion dollars. P. trinervia Cav. is part of the rich reservoir of world plant biodiversity that we are obliged to preserve for future generations. On the other hand, this plant also presents potentialities that should be directed toward improving the quality of life for mankind now and in the future.
Selections from the book: Medicinal and Aromatic Plants VIII (1995).