Hsu F C, Kleier D A
E.I. DuPont de Nemours and Company, Central Research and Development, Glasgow Business Community, Suite 301, PO Box 6101, Newark, DE 19714-6101, USA.
J Exp Bot. 1996 Aug;47 Spec No:1265-71. doi: 10.1093/jxb/47.Special_Issue.1265.
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 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 permeability and the acid dissociation constant (pK(a)) 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 membrane composition of the sieve tube, permeation characteristics 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 pesticides. 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 harboring a root-specific β-glucuronidase gene to release oxamyl for root-knot nematode control. This propesticide and in situ activation strategy is one way to use the existing body of knowledge for practical purposes. The same principle should be generally applicable to other plant-xenobiotic technologies.
在过去15年里，我们对异生物质韧皮部移动性的理解取得了巨大进展。该主题已从一个了解甚少的现象转变为一个可以通过异生物质的物理化学性质及其移动所经过的维管系统的性质来准确描述的过程。统一数学模型的基本原理是渗透率和酸解离常数（pK(a)）的组合决定韧皮部移动性，这在许多植物系统中的许多化合物上已得到很大程度的验证。然而，由于缺乏关于筛管膜组成、渗透特性和筛管细胞生物化学的必要知识，对该模型进行更精确的测试很困难。此外，将化合物的内在移动性与其在整株植物中的移动性进行定量关联的尝试常常因竞争损失机制而混淆。在实际方面，开发有效的韧皮部移动性农药面临挑战。文中提出了一些考虑因素来解释为什么到目前为止有大量的韧皮部移动性除草剂，而这类杀虫剂和杀菌剂却很少，以及为什么这种情况可能难以改变。韧皮部移动性的知识已经足够完善，能够给出特定的结构规定，使现有农药具有这种移动性。然而，这种结构变化往往会导致农药活性降低。最近，已证明通过将草氨酰葡糖醛酸（一种韧皮部移动性杀线虫原药）与携带根特异性β-葡糖醛酸酶基因的转基因烟草植株相结合，以释放草氨酰来防治根结线虫，可以规避这个问题。这种前体农药和原位激活策略是将现有知识用于实际目的的一种方法。同样的原理通常应适用于其他植物-异生物质技术。
