|Bell, Alois - Al|
Submitted to: Journal of Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2005
Publication Date: 9/30/2005
Citation: Agudelo, P., Robbins, R., Stewart, J.M., Bell, A.A., Robinson, A.F. 2005. Histological observations of Rotylenchulus reniformis on Gossypium longicalyx and interspecific cotton hybrids. Journal of Nematology. 37(4):444-447. Interpretive Summary: There are many kinds of microscopic worms called nematodes that feed on the roots of cotton and other crops, debilitating the plants and markedly decreasing yields. The direct losses to U.S. cotton farmers caused by nematodes each year are estimated by the National Cotton Council to exceed $300,000,000. The reniform nematode is one of the most important nematodes on cotton in the U.S. Some varieties of crop plants are resistant to one or more kinds of nematodes, allowing them to be grown successfully in fields where those nematodes occur. Unfortunately, no varieties of cotton are resistant to the reniform nematode and all suffer considerable damage when grown in soil where the nematode is present in high numbers. However, highly resistant hybrids have been made by crossing the kind of cotton that farmers grow with a wild plant from Africa that is immune to the reniform nematode and these hybrids are now being used by plant breeders to develop resistant varieties. This process takes several years and involves the production and evaluation of many seeds through several generations of plants. It is important to understand what makes plants resistant so that resistant plants can be identified correctly. In this study the invasion of hybrid roots by microscopic reniform nematodes was investigated by embedding roots in plastic and then slicing and staining the roots and nematode different colors with special dyes to allow the nematodes and the subtle changes they cause in roots to be studied. We found that immature nematodes invade roots of the wild plants in large numbers just as though they do in regular cotton plants, but the nematodes do not develop into adults and the root cells surrounding them die, depriving them of food. In hybrid plants, the situation for most nematodes is similar to what you see in the wild plant, but in some cases the plant cells do not die and normal nematode development occurs. This tells us that in plants with an intermediate level of resistance, some nematodes will develop normally and some will not, due to unknown factors related to the position of the root in the root system and the level of development of the root when it is invaded. This information will be valuable in interpreting intermediate levels of resistance in plants that are descended from the hybrids during the variety development process.
Technical Abstract: The development and histopathology of reniform nematode (Rotylenchulus reniformis) on roots of Gossypium longicalyx, G. hirsutum, and two interspecific hybrids derived from them was studied by light microscopy. Gossypium longicalyx is reported to be immune to reniform nematode, but the mechanisms for resistance are not known. In G. longicalyx roots, penetration by female nematodes was confirmed, and incipient swelling of the females, indicating initiation of maturation of the reproductive system, was observed. Maturation up to the formation of a single embryo inside the female body was frequent, but no maturation beyond this was observed. In both hybrids, development was inhibited but progressed farther than in the immune parent. Reactions ranged from highly compatible, with the formation of active syncytia and full development of females, to incompatible with little or no development of the female. Compatible plants showed characteristic hypertrophied cells, enlarged nuclei, dense cytoplasm, and partial dissolution of cell walls, whereas incompatible plant reactions included lignification of the cells adjacent to the nematode head, or the complete collapse and necrosis of the cells involved. The need to characterize reactions and to carefully select among the plants descended from the hybrids during the introgression process, as well as the importance of combining the results of reproduction tests with histological observation of the plant-nematode interactions is discussed.