Mitotic and Ultrastructural Changes in Root Meristems of Grass Seedlings Treated with Alaninyl‐Alanine

Abstract

Alaninyl‐alanine (Ala‐Ala), is a biologically active dipeptide that inhibits the rooting of germinating seeds. Previous research revealed that cells of root tips treated with Ala‐Ala were void of cellular components, specifically discernible nuclei and mitotic structures, as well as an overall loss of cytoplasmic integrity. Treated root tips had extensive cell wall abnormalities, including uneven thickening and shattering. Many commonly used herbicides, such as the dinitroanilines, the carbamates, and dithiopyr [S,S ‐dimethyl 2‐(difluoromethyl)‐4‐(2‐methylpropyl)‐6‐(trifluoromethyl)‐3,5‐pyridinedicarbothioate], produce effects similar to Ala‐Ala in root meristems. The objectives of these investigations were to use time‐course studies to observe the mitotic activity of perennial ryegrass (Lolium perenne L.) seedling roots grown in petri dishes containing Ala‐Ala, to describe Ala‐Alainduced changes in cellular ultrastructure in an effort to elucidate the mode of action of this biologically active dipeptide through the use of light and transmission electron microscopy, and to compare the results of these studies with reported modes of action of other commonly used herbicides. Results showed that Ala‐Ala exhibited 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%, resulting in only interphase cells. Microscopic analysis revealed profound treatment effects. By 12 h, dense droplets, presumably membrane lipids, were visible in vacuoles and intercellular spaces. After 48 h of exposure, epidermal and cortical cells in the treated roots appeared compressed 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 until after a 96‐h exposure to Ala‐Ala.