Green branches emerging from the column
A tuft of green leafy branches is visible on an attacked stem of Corryocactus brevistylus. They belong to Ligaria cuneifolia, a parasitic plant in the Loranthaceae family. Their base connects with tissue developing inside the cactus. The external part photosynthesises, while the internal part ensures contact with the host. The entire parasite thus retains its own developmental axis, leaves and vascular system while using the body of another plant. [Mauseth et al. 2015, pp. 4–6]
Mauseth and colleagues collected material on 7 June 2004, about 40 km northwest of Arequipa. The detailed sample comprised seven C. brevistylus stems bearing 19 adult plants and 12 seedlings of L. cuneifolia. Different stages of contact were therefore compared, from a young organ penetrating the host to the permanent connection of a developed plant. The material was fixed, thin sections were prepared, and tissues were stained for microscopic observation. [Mauseth et al. 2015, pp. 4]
Water-storage parenchyma has its own bundles
Broad layers of cortex surround the cactus's central stele. In the C. brevistylus stems studied, its thickness reached about 57 mm. Beneath the epidermis lay a hypodermis of 6–8 layers of thick-walled cells. Farther inward was parenchyma containing a network of cortical bundles. These bundles contain xylem and phloem elements and enable transport within extensive tissue distant from the stem centre. [Mauseth et al. 2015, pp. 4–5]
The hypodermis presents a mechanical obstacle, while the broad parenchyma extends the path to the main vascular tissues. The authors consider both features as elements of the cactus's passive resistance to parasitic plants. Ligaria, however, uses connections closer to the surface. Its tissues remain in the outer cortex and contact cortical bundles and strands leading to leaves and buds. [Mauseth et al. 2015, pp. 3, 10–12]
Haustorium and endophyte describe successive stages
The haustorium is an organ enabling a parasite to enter the host's body and establish a connection with it. The authors of the paper discussed use this term for young stages around which the cactus produces wound-healing tissue. They call the later, developed internal portion the endophyte. Endophyte here means tissues of the same flowering plant whose leaves and branches are visible outside. The distinction organises successive phases of contact between the two plants. [Mauseth et al. 2015, pp. 4]
The developed endophyte forms a compact, irregularly lobed area. In the sample, its diameter reached at most about 20 mm, even beneath large external parts. Expansion raised the cactus tissues, forming a collar around the parasite's base. Some host cells were compressed between neighbouring endophyte lobes. Ligaria cells were recognised by features including more strongly stained nuclei and the presence of tannin cells and sclereids. [Mauseth et al. 2015, pp. 6–7]
The same cork-producing tissue has different consequences
Near the seedling, hypodermal cells began dividing and assumed the function of phellogen, the tissue that produces cork. The compact thick-walled layer was replaced by new layers of thin-walled cells interspersed with sclereids. Local restructuring weakened the continuity of the original mechanical barrier. In the authors' interpretation, it facilitated entry into the cactus. [Mauseth et al. 2015, pp. 10, 12]
At the same time, cortical cells could produce wound-healing tissue around the haustorium. In two young stages, this barrier was observed to surround the parasite's organ almost completely. In one, a small endophyte lobe grew beyond the barrier. No analogous cork covering was found around later developed endophytes. The significance of cork therefore depended on where it formed and the developmental stage of the connection. [Mauseth et al. 2015, p. 10, Fig. 4]
The host expands the vascular connection
Near the endophyte, cactus parenchyma cells divided and differentiated into new vascular bundles. Existing cortical bundles and leaf-bud traces could also enlarge. The response was uneven. Some remained small, while others produced substantial secondary xylem and phloem. The connections ended at the endophyte surface, forming a branched host network around it. [Mauseth et al. 2015, p. 11]
In three samples beneath the largest external parts, some leaf-bud traces reached 8 mm in diameter and had hard wood. In the remaining material, connections were usually microscopic. Such strong secondary growth in bundles located within the cortex was unusual for an unattacked stem portion. The authors compare it with a situation in which a nearby areole produces a branch requiring new vascular and mechanical support. [Mauseth et al. 2015, pp. 11–12]
The contact boundary consists mainly of parenchyma
Small, closely adjoining parenchyma cells dominated the surface of endophyte lobes. Parasite vessels rarely reached this boundary. They often contacted host parenchyma, while direct vessel-to-vessel contact was uncommon. When it occurred, the plants were connected through wall pits. No perforation opening a shared vessel lumen across the host–parasite boundary was observed. [Mauseth et al. 2015, pp. 9–10, Fig. 11–13]
Phloem-like tissue was present inside the endophyte, but no sieve-tube elements or sieve areas were identified in it. Cactus sieve tubes also did not directly reach the endophyte surface. These observations describe the structure of the connection and its limitations. The flow pathway of particular substances and the magnitude of water flux would require physiological measurements, which the paper discussed did not include. [Mauseth et al. 2015, pp. 9, 11]
Wood expansion coexists with barriers
Some cactus cells at the endophyte boundary collapsed and died. Elsewhere, wood rich in sclerotised parenchyma formed, consisting of living cells with strongly thickened, lignified walls. It contained relatively few vessels. The authors consider layers of dead cells and sclerotisation as responses that may impede transfer of water and nutrients to the parasite. They infer their defensive function from anatomy. [Mauseth et al. 2015, pp. 10–12, Fig. 16–17]
Other changes favoured maintenance of the connection. Formation of new bundles, increased xylem area beside the endophyte and expansion of existing strands improved anatomical conditions for its supply. The developing cactus therefore simultaneously produced structures with different consequences. This picture helps explain why a clear host response can coexist with long-term persistence of healthy-looking parasite leaves. [Mauseth et al. 2015, p. 12]
The response extends beyond the parasite's location
Ligaria cells remained in the compact endophyte near the surface. The authors did not detect them in the stele or distant portions of the stem studied. Despite this, the cactus responded beyond the site of direct contact. In the sample described, 130 mm below a large external part, the parenchyma of the rib beneath the parasite was harder than that of the other ribs. On the same side, the stele's wood was thicker. [Mauseth et al. 2015, pp. 10–12]
A separate examination of more than a hundred attacked stems revealed death of the cactus portion above the parasite's location. Dead tissues were replaced by a small concave surface closed by wound-healing tissue. This was not the result of a controlled infection experiment. The authors related the appearance to possible damage to the young apex, while the exact course and cause of dieback remained hypothetical. [Mauseth et al. 2015, pp. 11–12]
Developmental restructuring and the limits of explanation
The main picture of this connection emerges from tissue position. The parasite retains a small endophyte in the cortex, while the host expands the vascular strands supplying it. At the same time, it forms dead layers, wound-healing tissue and sclerotised parenchyma. These are precisely documented directions of developmental change in the studied relationship between Ligaria cuneifolia and Corryocactus brevistylus. [Mauseth et al. 2015, pp. 12–13]
The chemical signal triggering these changes was not identified in the study. Hormone concentrations and expression of genes associated with hormonal signalling were not measured. Describing the parasite's influence on development refers to the relationship visible in histology. The molecular mechanism, cost to water balance and effects on long-term host survival and reproduction remain separate questions for investigation. [Mauseth et al. 2015, Materials and methods, Discussion]
Full text of Mauseth and colleagues' 2015 anatomical study. The material dates from 2004. Histological observations, functional interpretation of barriers and the unidentified signalling mechanism have been separated.
Host Response and Endophyte Structure of the Mistletoe Ligaria cuneifolia (Loranthaceae) Parasitizing Corryocactus brevistylus (Cactaceae)
Mauseth J. D., Benigno S., Cáceres Huamani F., Ostolaza C., Host Response and Endophyte Structure of the Mistletoe Ligaria cuneifolia (Loranthaceae) Parasitizing Corryocactus brevistylus (Cactaceae), Haseltonia, 2015, 21:3-13, doi: 10.2985/026.021.0103
A histological study of Ligaria cuneifolia growing on Corryocactus brevistylus. It compares young haustoria and developed endophytes, hypodermal remodelling, cortical bundle formation, wood growth and barriers at the interface between the plants. Material from near Arequipa was collected in 2004. The authors interpret a defensive function for some changes from anatomy, without experimental identification of a chemical signal.
