An in Vitro Co-Culture System and Rapid Differential Responses to Fusarium Oxysporum f.Sp. Vasinfectum Race 4 in Domesticated Cotton

Fusarium oxysporum f.sp. vasinfectum Race 4 (FOV4) is a devastating fungal pathogen that affects both domesticated cottons, Gossypium hirsutum (Upland) and Gossypium barbadense (Pima).  Greenhouse and field-based resistance assays can be unreliable due to uneven inoculum load, the presence of endophytes, and uncontrollable environmental factors. An in vitro co-culture method that uses a modified MS medium was designed to support uniform growth of both domesticated cotton species and FOV4 in a scalable format using an inert polyphenolic foam resin to enable host-pathogen investigations in a controlled environment. Roots grown in the aerated foam matrix were morphologically more similar to roots grown in the field when compared to conventional agar-based in vitro systems. Histology results also showed better differentiated xylem in the aerated foam matrix when compared to agar. Inoculation and subsequent co-cultivation of field-verified resistant and susceptible genotypes with FOV4 resulted in an average of 90% diseased or dead tissue within four weeks of inoculation in the susceptible variety, while the highly tolerant checks showed less than 20% symptomatic tissue. FOV4 was isolated from the vascular system of every genotype within 72 hours of infection. This in vitro format offers a scalable system to rapidly screen cotton germplasm for tolerance to fungal pathogens and to facilitate the study of biotic interactions in both domesticated cotton species.