PHLOEM MOBILITY OF XENOBIOTICS .8. A SHORT REVIEW

Great strides have been made in the last 15 years in our understanding of phloem mobility of xenobiotics. The subject has been transformed f rom a poorly understood 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 perm eability and the acid dissociation constant (pK(a)) determines phloem mobility, and this has been largely validated for many compounds in ma ny plant systems. More precise testing of the model is, however, diffi cult due to the lack of requisite knowledge on the membrane compositio n of the sieve tube, permeation characteristics and sieve-cell biochem istry. Furthermore, attempts to relate quantitatively a compound's int rinsic mobility to its whole-plant mobility are often confounded by co mpeting loss mechanisms. On the practical side, there is the challenge of coming up with efficacious phloem-mobile pesticides. Consideration s 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 phlo em mobility is robust enough to allow specific structural prescription s to impart such mobility to existing pesticides. However, such struct ural changes often lead to a reduction of pesticidal activity. Recentl y, it has been demonstrated that this problem can be circumvented by c ombining oxamyl glucuronide (a phloem-mobile pro-nematicide) with a tr ansgenic tobacco plant harboring a root-specific P-glucuronidase gene to release oxamyl for root-knot nematode control. This pro-pesticide a nd in situ activation strategy is one way to use the existing body of knowledge for practical purposes. The same principle should be general ly applicable to other plant-xenobiotic technologies.