Orchids occupy a peculiar place in the plant kingdom. With more than 28,000 species across 763 genera, the Orchidaceae is the most diverse family of flowering plants on Earth, yet its members are among the most difficult to propagate. Orchid seeds are dust-like and devoid of nutrient reserves, meaning that in nature, germination depends entirely on symbiotic relationships with fungi that supply the developing embryo with sugars, vitamins, and hormonal cues. Even after germination, orchids exhibit slow growth, prolonged juvenile phases, and limited early photosynthetic capacity. The embryo first develops into a protocorm, a specialized juvenile structure that precedes the differentiation of shoots and roots, and this delicate stage is where most natural propagation fails. As habitat loss and overharvesting push species like Vanda tessellata toward the brink, scientists have increasingly looked to the orchid microbiome for solutions.

Vanda tessellata itself carries considerable stakes. The species is widely used in Ayurvedic and Unani medicine for treating inflammatory, neurological, and respiratory disorders, and its ethnopharmacological value has made it a target of intense collection pressure. Despite prior reports of endophytic fungi from various orchid species, no previous study had systematically investigated the endophytic fungal diversity of V. tessellata or evaluated whether those fungi possess traits useful for plant growth promotion. The Kerala team, led by Parvathi Rajeevan of the Department of Biotechnology under the supervision of Shiburaj Sugathan, set out to close that gap with a study combining field collection, microbiology, molecular taxonomy, and plant physiology.

The researchers collected ten healthy V. tessellata plants from naturally growing populations on roadside trees at Parassuvakkal in Kerala, India, working under a permit from the Kerala Forest Department. From surface-sterilized root, leaf, and flower tissues, 1,050 tissue segments were processed, yielding an impressive 795 endophytic fungal isolates. Six morphologically distinct representatives were then screened for plant growth-promoting traits. The isolation protocol was rigorous: tissues were sterilized with 0.2 percent sodium hypochlorite for thirty minutes, dipped in 75 percent ethanol, rinsed repeatedly in sterile water, and imprinted onto fungal isolation medium to confirm that surface contaminants had been eliminated before incubation at 28 degrees Celsius in darkness.

When the six isolates were tested, one strain stood out decisively. KU/BOT/VT004 produced the highest levels of indole-3-acetic acid, the principal naturally occurring auxin, at 35.05 micrograms per milliliter when grown in Czapek’s Dox broth supplemented with L-tryptophan. The consistent enhancement of IAA production in precursor-supplemented cultures confirmed that these fungi synthesize auxin primarily through the tryptophan-dependent pathway, the same route exploited by many beneficial plant-associated microbes. Auxin matters enormously for orchids because it governs root differentiation, cell elongation, and tissue expansion, precisely the developmental processes that must succeed for a protocorm to become a viable plantlet. The other isolates showed moderate to minimal auxin output, underscoring pronounced strain-specific variation in biosynthetic capacity.

The strain’s metabolic repertoire did not end with auxin. Quantitative assays using Nessler’s reagent showed that KU/BOT/VT004 also produced the most ammonia among the six isolates, at 2.84 micrograms per milliliter, a trait associated with supplying readily assimilable nitrogen to plant hosts. On Chrome Azurol S agar prepared with King’s B medium, the isolate generated the largest siderophore halo, measuring 43.67 millimeters in diameter, indicating vigorous secretion of iron-chelating compounds. Siderophores mobilize ferric iron, an essential cofactor for chlorophyll biosynthesis and photosynthetic electron transport, and their abundance in the fungal secretome offers a mechanistic explanation for the greening effect later observed in treated seedlings. In dual-culture assays, the fungus also suppressed the radial growth of Fusarium oxysporum, a notorious wilt pathogen, by 44.6 percent, hinting at bioprotective value in addition to growth promotion.

Identifying the fungus required converging lines of evidence. Microscopic examination with lactophenol cotton blue staining and a modified slide culture technique revealed hyaline, septate, branched hyphae bearing smooth-walled, cylindrical conidia measuring on average 30.3 micrometers in length and 4.9 micrometers in width, hallmarks of the genus Colletotrichum in the family Glomerellaceae. Scanning electron microscopy after glutaraldehyde fixation and gold sputter coating confirmed dense conidial production with well-defined morphology. Colony appearance shifted across five different culture media, from cottony white-grey growth on malt extract agar to compact colonies with strong reverse pink pigmentation on Sabouraud dextrose agar. Molecular identification relied on amplification and Sanger sequencing of the internal transcribed spacer region of ribosomal DNA using the universal primers ITS1 and ITS4. BLASTn comparison against GenBank and a Maximum Likelihood phylogenetic analysis in MEGA 11, employing the Jukes-Cantor model with 1,000 bootstrap replicates, placed the isolate closest to the type strain of C. cf. cobbittiense. Because ITS sequences alone do not always resolve species boundaries within the notoriously complex genus Colletotrichum, the authors conservatively appended the abbreviation “cf.” to the species name. The sequence was deposited in GenBank under accession PX861570, and the living culture was preserved in the National Fungal Culture Collection of India as NFCCI-6297.

The most striking results came from the functional plant assays. The team grew asymbiotically germinated V. tessellata protocorms on half-strength Murashige and Skoog medium supplemented with 10 percent cell-free fungal culture filtrate, obtained by passing ten-day-old cultures through a 0.22-micrometer filter to ensure that any observed effects derived from soluble metabolites rather than living fungus. After sixty days, filtrate-treated protocorms reached a fresh weight of 1.07 grams, nearly double the 0.57 grams recorded for protocorms on basal medium and ahead of a peptone-supplemented nutritional control at 0.96 grams. By ninety days, treated protocorms weighed 2.93 grams, more than twice the peptone control, and at 120 days they still led at 2.53 grams versus 2.00 grams. Equally telling was what happened to the controls: protocorms on basal half-strength medium showed no further biomass gain after sixty days, a clear sign of nutrient limitation, whereas filtrate-treated cultures kept growing. Chlorophyll analysis at sixty days, performed by acetone extraction and spectrophotometric readings at 645 and 663 nanometers, showed total chlorophyll of 9.42 milligrams per liter in treated plantlets against 8.32 in the peptone control and just 5.23 in basal medium, indicating enhanced pigment biosynthesis and photosynthetic potential. All comparisons were statistically significant, analyzed by analysis of variance followed by Tukey’s multiple comparison tests.

The implications of these findings extend well beyond a single orchid species. The genus Colletotrichum has long carried a reputation as a group of destructive plant pathogens, responsible for anthracnose diseases in countless crops, yet research in recent years has unveiled a spectrum of lifestyles within the genus ranging from pathogenic to mutualistic. The landmark example is Colletotrichum tofieldiae, shown in 2016 to confer phosphate-dependent fitness benefits on Arabidopsis roots. KU/BOT/VT004 adds to a growing roster of beneficial Colletotrichum endophytes and demonstrates that secreted fungal metabolites alone, without physical colonization, are sufficient to elicit substantial growth responses in orchid tissue. The authors are appropriately careful in their interpretations, noting that the study quantified metabolite production in culture rather than nitrogen assimilation or iron uptake in plant tissues, and that direct colonization dynamics remain unexplored. They also acknowledge that an uninoculated medium filtrate control was not included and that the specific bioactive compounds were not chemically characterized, leaving metabolomic profiling, HPLC analysis, colonization studies, and greenhouse validation as essential next steps before any practical deployment.

Even with those caveats, the work opens a tangible pathway toward conservation biotechnology. If the metabolites driving protocorm growth can be identified, reproduced, and standardized, they could be incorporated into tissue culture protocols that currently rely on synthetic hormones and nutrient cocktails, potentially improving the survival and vigor of laboratory-raised orchids destined for reintroduction into the wild. For a species like Vanda tessellata, threatened simultaneously by habitat destruction and medicinal overharvesting, a fungal ally capable of accelerating propagation could make the difference between continued decline and meaningful recovery. More broadly, the study reinforces a theme running through modern plant science: that the organisms living cryptically within plant tissues, often invisible and unnamed, may hold some of the most practical answers to the challenges of biodiversity conservation and sustainable cultivation. The quiet fungus inside a roadside orchid in Kerala, it turns out, has plenty to say.