Abstract
Browning rhizome (also known as “browning tuber disease”) in Indian lotus (Nelumbo nucifera) caused by the nematode Hirschmanniella diversa is an emerging agronomic problem. In this study, the authors documented the early infection processes of H. diversa in the apices of young rhizomes of Indian lotus by electron microscopy analysis using an artificial inoculation method. Nematodes were attracted to young rhizome apices, invading them via narrow indentations by 4.5 hr after inoculation. Host cells adjacent to the cavity around the invading nematodes were absent and appeared to have disintegrated during infestation. Following contact with the nematodes, host cell walls and cellular contents became electron-dense and less defined, likely due to digestive enzymes secreted by the invading nematodes. Nematodes invaded to a depth of about 1 mm by 24 hr after inoculation, but did not penetrate the plant any further vertically, similar to the observation of browning in mature rhizomes in the field. The authors propose that the invasion sites of young rhizome apices become blackish-brown blotches as rhizomes mature in the field due to oxidation.
Highlights
Indian lotus (Nelumbo nucifera Gaertn), cultivated in Japan since ancient times, includes both the flowering lotus, with high ornamental value, and the edible lotus, the rhizomes of which are consumed as a vegetable (Minamikawa and Tanaka, 1959). Mihira and Nagai (1996) reported that nematodes cause blackish-brown blotches on the surface of edible lotus rhizomes
Ultrastructure of Hirschmanniella diversa in browning rhizomes: Uematsu et al Many cytological studies have investigated the attraction, parasitic behavior, and feeding behavior of H. oryzae and H. imamuri (Babatola and Bridge, 1980; Bilgrami et al, 1985; Goto, 1969; Mihira and Nagai, 1996; Vecht and Bergman, 1952), but little is known about these properties in H. diversa (Liu and Yu, 2002; Uematsu et al, 2015, 2016)
We aimed to clarify the early infection processes of H. diversa invading the apex of young lotus rhizomes and the resulting changes in host cells using light and electron microscopy
Summary
Indian lotus (Nelumbo nucifera Gaertn), cultivated in Japan since ancient times, includes both the flowering lotus, with high ornamental value, and the edible lotus, the rhizomes of which are consumed as a vegetable (Minamikawa and Tanaka, 1959). Mihira and Nagai (1996) reported that nematodes cause blackish-brown blotches on the surface of edible lotus rhizomes. A pathogen isolated from lotus showing the same symptoms was identified as Hirschmanniella diversa Sher, 1968 (Sher, 1968) by Mizukubo (2002) Both H. diversa and H. imamuri are considered the major species causing browning rhizome disease in Japan, with occurrences of H. diversa expanding countrywide (Koyama et al, 2013; Mizukubo, 2015). Three species of Hirschmanniella have been reported in Japan: Hirschmanniella oryzae (van Breda de Haan, 1902) Luc and Goodey, 1964, H. imamuri, and H. diversa (Mizukubo, 2002) Both H. oryzae and H. imamuri parasitize rice (Oryza sativa L.) roots and cause root rot (Vecht and Bergman, 1952; Babatola and Bridge, 1980; Bilgrami et al, 1985). We froze specimens using liquid nitrogen to prevent nematode movement and immersed the frozen specimens in chemical fixative to examine the ultrastructural interfaces
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