Ectopic divisions in vascular and ground tissues of Arabidopsis thaliana result in distinct leaf venation defects.

Leaf venation patterns vary considerably between species and between leaves within a species. A mechanism based on canalization of auxin transport has been suggested as the means by which plastic yet organized venation patterns are generated. This study assessed the plasticity of Arabidopsis thaliana leaf venation in response to ectopic ground or procambial cell divisions and auxin transport inhibition (ATI). Ectopic ground cell divisions resulted in vascular fragments between major veins, whereas ectopic procambial cell divisions resulted in additional, abnormal vessels along major veins, with more severely perturbed lines forming incomplete secondary and higher-order venation. These responses imply limited vascular plasticity in response to unscheduled cell divisions. Surprisingly, a combination of ectopic ground cell divisions and ATI resulted in massive vascular overgrowth. It is hypothesized that the vascular overproduction in auxin transport-inhibited wild-type leaves is limited by simultaneous differentiation of ground cells into mesophyll cells. Ectopic ground cell divisions may negate this effect by providing undifferentiated ground cells that respond to accumulated auxin by differentiation into vascular cells.

AFLP fingerprinting reveals pattern differences between template DNA extracted from different plant organs.

AFLP (amplified fragment length polymorphism) fingerprinting of cultivars of bread wheat (Triticum aestivum) and some of its wild relatives has allowed the efficient detection of large numbers of polymorphic amplified fragments. While the reproducibility of fingerprints in repeated experiments is high, pattern differences were observed between fingerprints obtained from seed and leaf DNA template from the same wheat accession. These distinct organ specific amplified DNA fragments were shown to be due neither to genotypic mixtures nor to pathogen contamination. They are likely a result of differences in DNA methylation between organs. Even greater numbers of organ specific amplified fragments were observed when fingerprints obtained from the root and shoot of individual seedlings of the wheat relatives Aegilops mutica and Aegilops speltoides were compared. This phenomenon underlines the importance of ensuring that DNA is extracted from physiologically uniform tissue in phylogenetic studies based on AFLP fingerprints. For this purpose, mature seed is a convenient source.

The inheritance of B chromosomes in Allium schoenoprasum L.

The inheritance of B chromosomes has been investigated. In Allium schoenoprasum from the River Wye, Powys; controlled crosses between plants of known B chromosome constitution were carried out, and the numbers of Bs present in the progenies scored. There was considerable heterogeneity in B chromosome transmission rates across the range of B chromosome numbers, through both the male and female lines, and in crosses involving odd and even numbers of Bs. The mean B number of the progeny was lower than the Mendelian expectation in 41 of the 57 crosses, significantly so in about a third of cases. No progeny had a mean B number significantly higher than expected. The mean transmission rate per B was not significantly different between parents with different numbers of Bs, in either the male or female line, or between plants carrying odd or even numbers of Bs. Transmission through the male line (0.43) was higher than through the female line (0.35), but not significantly so. The overall mean transmission rate per B (0.4), estimated from all crosses, was significantly lower than the expected rate of 0.5. This study provides conclusive evidence that B chromosomes in A. schoenoprasum from the River Wye lack an effective accumulation mechanism and, furthermore, show significant losses during transmission to the progeny of controlled crosses.