Assessment of phloem mobility of xenobiotics in Triticum aestivum and Brachypodium distachyon

Abstract

Due to evolved resistance and environmental regulations, there is a particular need in the agricultural market for a new graminicide. An essential requirement of a novel, foliar applied graminicide is sufficient phloem mobility in the plant to reach meristematic tissues for the expression of activity leading to the desired control of unwanted vegetative growth. A robust and reliable phloem bioassay utilising a monocot species is highly desirable for early stage experimental compounds. Vascular tissues and translocation patterns of organic compounds in purple false brome (Brachypodium distachyon L. P. Beauv.), a model organism for temperate grasses, were studied and compared with those of wheat (Triticum aestivum L.). Microscopic studies with tracer dyes were used to determine if B. distachyon has a xylem discontinuity between the developing seed and the rachilla xylem, the same as found in T. aestivum. Based on 14C-radiolabelled and non-radiolabelled studies using known xylem and phloem mobile pesticidal compounds, there was a significant difference in the amount of the xylem mobile compounds in the chaff and stem as compared with the phloem mobile compounds found in the grain. The findings described in this report show a clear evidence of xylem discontinuity in B. distachyon, and provide a novel system for a rapid screening of phloem mobility of herbicides in monocot species.