Francis C. Hsu

Brassica anther-specific genes : characterization and in situ localization of expression

SummaryA cDNA library of Brassica napus (cv. Westar) was constructed using poly(A)+ RNA isolated from developing anthers of flower buds 2–3 mm in length. Differential hybridization, using cDNA probes complementary to poly(A)+ RNA from developing anthers or seedlings, was used for initial screening. In addition to Southern and Northern blot analyses of selected clones, RNA-PCR assays and in situ hybridization were used to study the temporal and spatial gene regulation in anthers at the transcriptional level. Five independent cDNA clones, showing no cross-hybridization to one another, were characterized, and their expression patterns could be grouped into three distinct categories. Two cDNA clones, BA112 and BA158, are tapetum-specific: the corresponding mRNAs accumulate in young anthers and decline as the tapetum cells degenerate later in anther development. The transcripts represented by BA54 and BA73 accumulate late in anther development and reach a maximum level in mature anthers prior to anthesis; BA54 has been confirmed to be pollen-specific. The third category, represented by BA42, is found to encode a protein sharing 64–67% amino acid similarity with chalcone synthase (CHS) from various plant species; the transcript is localized in the peripheral cells of the vascular bundle, tapetum, and developing microspores.

Phloem mobility of xenobiotics VIII. 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 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 compounds 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.