Trypanosomatids of Plants

F.G. Wallace; I. Roitman; E.P. Camargo

I Introduction

In view of the widespread distribution of protozoa, both free living and parasitic, their scarcity as internal parasites of the land spermatophytes is noteworthy. Only certain amoebae (see Nieschultz, 1931) and some of the lower trypanosomatids are known to occur in these plants. In only 12 families in 9 of the 52 orders of Spermatophyta are parasitic protozoa known. These are almost all trypanosomatids and most of the known ones are in latex-containing plants.

From the time of their first discovery (Lafont, 1909), flagellates of laticiferous plants were recognized as being related to Trypanosoma and Leishmania. Lafont was a medical officer in the French army whose interest in latex was prompted by the reputed medicinal qualities of the juice of Euphorbia pilulifera. The abundant starch granules in the latex suggested that it might be a good culture medium, so he asked his assistant, David, to examine the latex of many plants in search of motile organisms. The flagellates he found were named Leptomo nas davidi. Migone (1916), who discovered flagellates in latex of Araujia angustifolia of the milkweed family (Asclepiadaceae) in Paraguay, was a veterinarian. During a field expedition in search of reservoir hosts of Trypanosoma vivax, he looked at the milky juice of Araujia, keeping Lafont’s discovery in mind. After finding flagellates, he injected some of the juice into laboratory animals with negative results.

Much of the early research was stimulated by the vain expectation that plants might be reservoir hosts for human or animal hemoflagellates. The microscopic techniques were those of hematology and the culture media were often those used for trypanosomes. The editor of a botanical journal asked a leading protozoologist of the time to write a review of plant flagellates with the hope of gaining interest from his readers in the subject. There is little evidence that the excellent review that resulted (Mesnil, 1921) had the desired effect.

In the first two decades after their discovery there were numerous records of trypanosomatids from laticiferous plants from many countries, mostly in the tropics, but then there was a long period with little interest in them. In the 1930s Phytomonas was discovered in the sieve tubes of coffee trees in Suriname, causing a disease, but with the decline of coffee culture in that country research on the subject languished.

The modern revival of interest in plant flagellates was due in large part to the late R. B. McGhee who, with his associates, studied the distribution of the milkweed flagellate, traced its life cycle, pointed out the differences between the Phytomonas in its insect host and other trypanosomatids in the same host, and participated in the discovery of pathogenic Phytomonas in palm trees. With the discovery of the palm tree diseases in South and Central America (Parthasarathy et al., 1976; Dollet et al., 1977), an expansion of research on all aspects of trypanosomatids of plants has occurred. Other infections of possible economic significance have been found in manioc, tomatoes, and beans. Wenyon (1926), Nieschulz (1931), and Dollet (1984) have reviewed the subject.

II Taxonomy and Nomenclature

A THE GENERA OF LOWER TRYPANOSOMATIDS

The organisms called “lower trypanosomatids,” to which the plant flagellate parasites belong, do not constitute a formal taxonomic group but include those members of the family that are monoxenous parasites of invertebrate hosts or parasites of plants and insects and comprise the genera Leptomonas, Herpetomonas, Crithidia, Blastocrithidia, Rhynchoidomonas, and Phytomonas. The trypanosomatid parasites of plants are almost all in the genus Phytomonas but, because of the possible confusion of insect stages of that genus with those of other genera and the occasional occurrence of insect parasites as incidental parasites of plants, all of the genera of lower trypanosomatids are defined here. Further information on them may be found in articles by Wallace (1979) and McGhee and Cosgrove (1980). The genera may be defined by using the terms for morphological stages within the family Trypanosomatidae that are characterized as follows (Figure 2.1).

1. Promastigote: Elongated forms with kinetoplast, anterior to the nucleus, flagellum arising near it and emerging from the anterior end.

2. Opisthomastigote: Elongated forms with kinetoplast, posterior to the nucleus, flagellum arising near it, then passing through the body and emerging from its anterior end.

3. Amastigote: Rounded or elongated forms devoid of an external flagellum.

4. Epimastigote: Elongated forms with kinetoplast near the nucleus, flagellum arising near kinetoplast and emerging from the side of the body to run along its surface or along a short undulating membrane to the anterior tip from which it extends as a free flagellum.

5. Trypomastigote: Elongated forms with kinetoplast, posterior to the nucleus, flagellum arising near it and emerging from the side of the body to ran along its surface or along an undulating membrane to the anterior tip from which it may or may not extend as a free flagellum.

6. Choanomastigote: Relatively short, stout, “barleycorn” forms with kinetoplast near and usually anterior to the nucleus. Flagellum emerges from a funnel-shaped reservoir that opens in the wide anterior end.

7. Sphaeromastigote: Rounded forms possessing a free flagellum.

The genera of lower trypanosomatids are characterized as follows: Leptomonas are promastigote parasites of invertebrate animals. Herpetomonas are promastigote parasites of insects that have opisthomastigote stages as well. Blastocrithidia are epimastigote parasites of invertebrate animals. Crithidia are choanomastigote parasites of invertebrate animals. Rhynchoidomonas are trypomastigotes that are tapered to a point at each end; they are parasites of insects. Phytomonas are promastigote parasites of plants and insects. Cysts (spores) are produced by some species of Leptomonas and Blastocrithidia.

The pattern of occurrence of ornithine-arginine cycle enzymes is characteristic of each genus of trypanosomatids (Camargo et al., 1987). Figure 2.2 shows the relationship of the enzymes in the cycle.

Arginine is converted to ornithine and urea by arginase (A). Citrulline hydrolase (CH) converts citrulline to ornithine. The conversion of arginine to citrulline requires arginine deaminase (AD). Ornithine carbamoyl transferase (OCT) converts ornithine to citrulline. The conversion of citrulline to arginine is mediated by arginino-succinate synthetase (ASS) and arginino-succinate lyase (ASL).

In mammals ornithine is derived from glutamate. The conversion ornithine → citrulline → arginine → ornithine is repeated and at each cycle ammonia is taken up by ornithine and by citrulline and converted to urea at the arginine-ornithine step.

In trypanosomatids the arginine cycle amino acids are not derived from glutamate but must come from an exogenous source. The cycle is not repeated, as one or more of the enzymes is always missing. These one-celled organisms can excrete ammonia and have no need to convert it to urea. However, there are usually enzymes to convert arginine or citrulline into ornithine. The polyamines, which are essential for many important reactions related to nucleic acid metabolism, are derived from ornithine.

None of the trypanosomatids has all of the enzymes of the arginine cycle. Trypanosoma has none and Leishmania has arginase. We know the enzyme patterns for three genera of insect trypanosomatids (Figure 2.3). We do not know the enzyme patterns for Blastocrithidia because the only species readily cultured is B. culicis, which has a symbiote that supplies all of the enzymes.

The genera may be separated first by the presence or absence of arginase. Crithidia and Leptomonas have arginase and Herpetomonas and Phytomonas do not. Crithidia and Leptomonas can be distinguished from each other by the absence of the arginino-succinate part of the...