MITOTIC AND ULTRASTRUCTURAL-CHANGES IN ROOT-MERISTEMS OF GRASS SEEDLINGS TREATED WITH ALANINYL-ALANINE

Alaninyl-alanine (Ala-Ala), is a biologically active dipeptide that in hibits the rooting of germinating seeds. Previous research revealed th at cells of root tips treated with Ala-Ala were void of cellular compo nents, specifically discernible nuclei and mitotic structures, as well as an overall loss of cytoplasmic integrity. Treated root tips had ex tensive cell wall abnormalities, including uneven thickening and shatt ering. Many commonly used herbicides, such as the dinitroanilines, the carbamates, and dithiopyr [S,S-dimethyl l)-6-(trinuoromethyl)-3,5-pyr idinedicarbothioate]. produce effects similar to Ala-Ala in root meris tems. The objectives of these investigations were to use time-course s tudies to observe the mitotic activity of perennial ryegrass (Lolium p erenne L.) seedling roots grown in petri dishes containing Ala-Ala, to describe Ala-Ala-induced changes in cellular ultrastructure in an eff ort to elucidate the mode of action of this biologically active dipept ide through the use of light and transmission electron microscopy, and to compare the results of these studies,vith reported modes of action of other commonly used herbicides. Results showed that Ala-Ala exhibi ted activity on mitosis in root meristems within 4 h of exposure, and by 6 h, reduction in the number of mitotic figures was nearly 100%, re sulting in only interphase cells. Microscopic analysis revealed profou nd treatment effects. By 12 h, dense droplets, presumably membrane lip ids, were visible in vacuoles and intercellular spaces. After 48 h of exposure, epidermal and cortical cells in the treated roots appeared c ompressed with a disruption in cell polarity. Root lateral branching, similar to effects of preemergence herbicides, was also noted after a 48-h exposure time. Root tips showed no gross external abnormalities u ntil after a 96-h exposure to Ala-Ala.