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.