**Abstract**

The most extreme manifestation of parasitism occurs in holoparasites, plants that are totally achlorophyllous. Among them, the genus *Lophophytum* (Balanophoraceae) is characterized by an aberrant vegetative body called a tuber, devoid of stems and leaves. The genus is exclusively South American, comprising five taxa, which parasitize the roots of trees or shrubs. This review focuses on the Argentine species of the genus*: L. leandri* and *L. mirabile* subsp. *bolivianum*. Topics covered include: morphology and anatomy of the vegetative body and host–parasite connection; structure, anatomy and development of the staminate and pistillate flowers; sporogenesis and gametogenesis, embryo sac inversion; endospermogenesis, embryogenesis and fruit development. The evolutionary trend in the gynoecium and embryo sac of the Balanophoraceae is also discussed to reflect the variability. Finally, observations were made on the synchronization of the life cycles of the parasites and hosts to infer possible ways by which parasitism has evolved, until now unknown.

**Keywords:** embryology, embryo sac inversion, holoparasitism, host–parasite connection, legume, tuber

#### **1. Introduction**

Most vascular plants (Pteridophytes and Spermatophytes) are autotrophic, producing their food through photosynthesis. However, a significant number of plants have adopted a heterotrophic mode of life, obtaining part, or all, of their requirements from other organisms [1–4]. These can be divided into myco-heterotrophs (living in symbiosis with fungi through which they feed on decaying organic matter and the so-called parasitic plants, that grow on other plants and establish an organic union or haustorium by which they derive food directly from the host [4, 5].

There are two basic types of parasitic plants: hemiparasites and holoparasites [6]. The former possess chlorophyll and are capable of photosynthesis (at least during some phase of their cycle) and they obtain only water and mineral salts through haustoria with the host. The most extreme manifestation of parasitism occurs in holoparasites, which are totally achlorophyllous, obtaining all their nutrients from the host, on which they are totally dependent [7]. Most holoparasites are found parasitizing the roots of their hosts.

Worldwide, many of the parasitic plants represent major losses to agriculture, especially in Africa, where root holoparasites cause serious damage to cereals and legumes [8, 9]. Conversely, others are on the red lists of endangered plants, such as the Balanophoraceae [10, 11].

According to Kuijt [1] and Musselman & Press [12] there are about 3,000 species of parasitic plants, representing approximately 1% of flowering plants. Other recent studies put this number at 292 genera and ca. 4750 species [3, 4, 6, 13]. According to Heide-Jørgensen [3] the parasitic plants are distributed in 280 genera and 20 families, 90% (4,100 ssp.) are hemiparasites and only 10% (390 ssp.) are holoparasites. About 60% are root parasites and 40% are stem parasites. Holoparasites are represented in the families Orobanchaceae, Cynomoriaceae, Lennoaceae, Apodanthaceae, Cytinaceae, Raflesiaceae, Hydnoraceae and Balanophoraceae [3, 4]. Parasitism evolved independently in different groups of Angiosperms and there are thirteen lineages where at least one species is parasitic [14, 15].

It is agreed that these modifications respond to a phenomenon of evolutionary convergence [1, 3]. In this sense, Westwood (2010) emphasizes that the study of the structure of parasitic plants provides the conceptual framework for understanding the "specialization" of plants in general.

Among the more specialized holoparasites are the species of the family Balanophoraceae L. C. Richard et A. Richard, which are devoid of chlorophyll and parasitize the roots of trees and shrubs. The best summary of the known characteristics of the family Balanophoraceae can be found in Kuijt & Hansen's work [16]. These plants develop a vegetative body called a tuber, which is partially or totally underground, of variable shape and color, from whitish-yellowish to yellow, orange to reddish-orange or brownish, or even purplish. It lacks the structures of the typical cormophytic organization, as the body is not differentiated into root, stem, and leaves [1, 3, 17–21].

A peculiarity of holoparasites is the tendency to acquire foreign genes from their host plants. It has recently been demonstrated that *L. mirabile* not only harbors in its mitochondria a majority of genes from its host, but also depends on them to carry out cellular respiration. Twenty-three of the 35 protein genes were obtained from Leguminosae. But what is most interesting is that these genes have replaced the native genetic material [22–24].

The family Balanophoraceae is distributed in tropical and subtropical areas. It has 17 genera and 42 species [3, 4, 16]. The genus *Lophophytum*, which is exclusively South American [17, 25–30], comprises five taxa:


This contribution is based on the results of years of research on the genus *Lophophytum*, focusing on the Argentine species: *L. leandri* and *L. mirabile* subsp. *bolivianum* (hereafter *L. mirabile*). The bibliography used is Sato's

*Anatomy, Embryology and Life Cycle of* Lophophytum*, a Root-Holoparasitic Plant DOI: http://dx.doi.org/10.5772/intechopen.99981*

doctoral thesis [31] and the numerous papers derived from it [32–35]. The existing bibliography on the other species of the genus *Lophophytum* is scarce, mainly reduced to taxonomic works.

Among the topics included are: i) morphology and anatomy of the vegetative body, including the host/parasite interphase; ii) structure, anatomy and ontogeny of unisexual flowers, iii) description of embryological processes, from gamete formation, iv) morphology and anatomy of fruit and seed, v) taxonomic value of floral characteristics, vi) observations on dissemination, germination and the establishment of the parasitic relationship with the host, vii) the evolutionary trend in the gynoecium and embryo sac of the Balanophoraceae, and viii) synchronization of parasite and host life cycles.

#### **2. Morphology and anatomy or vegetative organs**

*Lophophytum* plants are formed of an underground vegetative body or tuber, spheroidal or slightly flattened, and 4-(9.5)-15 (38) x 3-(6.5)-12 cm in size [20, 21]. The tubers are connected to the roots of the host tree, close to the trunk. The tubers have no apex and no regions that resemble shoot or root apical meristems; there are no scales, leaves, branches, runners, or roots emerging from the tubers (**Figure 1A**–**C**). The host/parasite interface attachment point is a "woodrose", no larger than 5 cm in diameter; this region has a "coralloid" design in which the host wood is intermingled with the host tissue development (**Figure 1B**). Externally the tubers are dark brown to black and the surface is covered by polygonal or hexagonal "warts" of variable sizes between 0.4 to 1.2 cm.

Anatomically the tuber consists of an outermost black warty surface zone, and an interior body, white in *L. mirabile*, and pink in *L. leandri* (**Figure 1D** and **E**). The warty zone lacks an epidermis, stomata and trichomes. It is composed of a variable number of compact parenchyma cells without any intercellular spaces, with thin, cellulose walls and a completely tanniniferous cytoplasm. The outer cells are progressively detached, as the tuber grows. Solitary or clustered brachysclereids are dispersed between the parenchyma cells of the surface zone (**Figure 1F**).

The interior body is composed of storage parenchyma and abundant collateral bundles that are randomly distributed (**Figure 1F** and **G**). The cells of the peripheral zone showed a positive reaction for tannin by the ferrous sulfate method [36], while the parenchyma cells of the central region have abundant amyloplasts stained with IKI (confirmed by polarized light and the presence of a hilum) and other spherical wax or fat bodies (stained with Sudan IV, not rotated by polarized light, and no hilum) [36–38]. The brachysclereids occur occasionally in the outer region. Vascular bundles are dispersed in the interior body, not organized in a eustele; many of them are continuous from the interior body to the warty zone. The xylem of the vascular bundles is remarkable because the vessel elements have scalariform pitting with ingrowths (**Figure 1H**).

Inflorescences are the only aerial part of the plant and their peculiarity is their endogenous origin (in relation to their own tissues), a characteristic unique to Angiosperms [3, 20, 21, 31–33]. Each tuber usually has one inflorescence, however up to six inflorescences may be produced per plant (**Figure 1A**, **B** and **D**). The inflorescences are monoecious, consisting of one main axis or primary rachis of 2-(21)-40 cm tall, which rises above the soil surface. Short secondary rachises carrying unisexual flowers are inserted in the axil of each bract of the primary rachis; the proximal ones with pistillate flowers and the distal ones with staminate flowers.

#### **Figure 1.**

*A, B, E–H: L. leandri; C, D, I, J: L. mirabile. A: Hypertrophied root (ro) of P. rigida with tuber (tu) and immature inflorescence (in) fully covered by scales. B: Tuber showing the warty surface (ws) and the woodrose host/parasite interface (wr). C: Tuber with fully developed inflorescence showing the pistillate (pf) and staminate flowers (sf), the arrow indicates the site where the tuber broke away from the host root. D and E: Longitudinal section through unfixed small tubers showing the warty surface zone, natural color of internal body and the primordium of inflorescence. F: Warty surface zone (wz) showing a group of brachysclereids (arrow) and parenchyma cells of the internal body (ib). G: Transection of vascular bundle. H: Detail of vessels with wall ingrowths (arrow). I and J: Host wood (hw) intermixed with parasite cells (pa). Scales, A,L: 2 cm; B,D: 1 cm; E: 0.5 cm; F-G,I-J: 50 μm; H: 10 μm.*

Immature inflorescences are completely covered by black scales (**Figure 1A**). The scales are shed at flower maturity, starting in the medium region where the staminate flowers are first exposed (**Figure 1C**).

*Anatomy, Embryology and Life Cycle of* Lophophytum*, a Root-Holoparasitic Plant DOI: http://dx.doi.org/10.5772/intechopen.99981*

#### **2.1 Tubers / host Interface**

The root where the parasite is installed stops its growth and elongation after infection, forming a woody gall (**Figure 1A**) [20, 21]. Tuber development is always observed in woody roots, larger than 1 cm in diameter. Infections are focused on the cambium, where the parasite cells divide intensely producing a strong undulation of the cambial zone (**Figure 1I**). One of the main consequences of the infection is the alteration of the axial and radial systems typical of secondary wood (**Figure 1J**).

In the affected xylem, the vessels are narrow and abundant, oriented in concentric rings. The fibers between the vessels are replaced by lignified parenchyma cells, with the same circular distribution of vessels. In the phloem, the tangential bands of the normal wood are almost completely replaced by parenchyma cells, very few fibers, and cells with crystals can be observed, disorganized and dispersed, but no sieve tubes elements are detected. This interaction of the host tissues (both xylem and phloem) with those of the parasite was the origin of the choraloid design of the interphase.
