Journal of Experimental Botany

Great strides have been made in the last 15 years in our understanding of phloem mobility of xenobiotics. The subject has been transformed from a poorly under-stood phenomenon to a process that can be accurately described by the physicochemical properties of the xenobiotic and the nature of the vascular system through which it moves. The basic tenet of the unified mathematical model is that the combination of the permeability and the acid dissociation constant (pKa) determines phloem mobility, and this has been largely validated for many compounds in many plant systems. More precise testing of the model is, however, difficult due to the lack of requisite knowledge on the mem-brane composition of the sieve tube, permeation char-acteristics and sieve-cell biochemistry. Furthermore, attempts to relate quantitatively a compound's intrinsic mobility to its whole-plant mobility are often confounded by competing loss mechanisms. On the practical side, there is the challenge of coming up with efficacious phloem-mobile pesticides. Considerations are forwarded to explain why so far there are numerous phloem-mobile herbicides and yet precious few such insecticides and fungicides, and why the situation might be difficult to change. The knowledge of phloem mobility is robust enough to allow specific structural prescriptions to impart such mobility to existing pesti-cides. However, such structural changes often lead to a reduction of pesticidal activity. Recently, it has been demonstrated that this problem can be circumvented by combining oxamyl glucuronide (a phloem-mobile pro-nematicide) with a transgenic tobacco plant harb-oring a root-specific p-glucuronidase gene to release oxamyl for root-knot nematode control. This pro