J. Nijsse

Footsteps from insect larvae damage leaf surfaces and initiate rapid responses

Plant resistance to insect herbivory involves gene expression in response to wounding and the detection of insect elicitors in oral secretions (Kessler and Baldwin, 2002, Ann. Rev. Plant/ Biol. 53: 299–328). However, crawling insect larvae stimulate the synthesis of 4-aminobutyrate within minutes and imprints of larval footsteps can be visualized within seconds through superoxide production or transient increases in chlorophyll fluorescence (Bown et al., 2002, Plant Physiol. 129: 1430–1434). Here cryo-scanning electron microscopy was used to demonstrate that larval feet, which are equipped with a perimeter row of hook-like crochets, damage leaf tissue and result in larval footprints. Staining for cell death shows that areas of wounding correspond to footsteps detected through increased chlorophyll fluorescence. Superoxide production in response to footsteps was inhibited by diphenyleneiodonium, an inhibitor of the plasma membrane NADPH oxidase enzyme. Inhibition of superoxide production, however, did not eliminate the detection of cell death. The results demonstrate that larval footsteps damage leaf tissue, and initiate rapid local responses which are not dependent on herbivory or oral secretions. It is proposed that superoxide production at the wound site prevents opportunistic pathogen infection.

Embolism repair in cut flower stems: a physical approach

Abstract The role of xylem anatomical properties on air embolism removal and xylem hydraulic conductance recovery from cut flower stems during the first hours of vase life was studied from a physical point of view. A model based on physical processes was developed and tested using chrysanthemum ( Dendranthema × grandiflorum Tzvelev) cut flowers. The model predicts that the repair process takes place in two major phases. During the first few seconds after replacing in water, initially air-filled vessels at the cut surface partly refill with water. Consequently, reconnections are established between the vase water and the non-cut water-filled xylem vessels just above the cut surface and hydraulic conductance is partly recovered. During the following hours, air partly or completely dissolves into the surrounding water in the stem and hydraulic conductance recovery gradually takes place. The results of the model agreed well with dynamic measurements of hydraulic conductance recovery on chrysanthemum stem segments after aspiration of air. Visual detection of air emboli by cryo-scanning electron microscopy showed that after 1.5 h of repair, air was only present in large-diameter vessels at a position relatively distant from the cut surface of the stem. According to the model, hydraulic conductance repair occurs more readily in stems with smaller diameter vessels. Model calculations and experiments showed that the height of water in the vase influences recovery of water uptake more in stems with large-diameter vessels than in stems with small-diameter vessels. It is concluded that the anatomical structure of the xylem plays an important role in the rehydration capability of cut flowers.

On the Mechanism of Xylem Vessel Length Regulation

The mechanism by which the plant regulates the length of xylem vessels has not yet been elucidated. The length of a xylem vessel depends on the number of fused vessel elements and their individual lengths. In this paper, a straightforward mechanism is postulated to explain how the length of xylem

Nuclear pore dynamics during pollen development and androgenesis in Brassica napus.

Abstract Changes in nuclear pore complex (NPC) densities, NPCs/nucleus and NPCs/µm3, are described using freeze-fractured Brassica napus microspores and pollen in vivo and in vitro. Early stages of microspore- and pollen-derived embryogenic cells were also analysed. The results of in vivo and in vitro pollen development indicate an increase in activity of the vegetative nucleus during maturation of the pollen. At the onset of microspore and pollen culture, NPC density decreased from 15 NPCs/µm2 at the stage of isolation to 9 NPCs/µm2, under both embryogenic and non-embryogenic conditions. This implies that the drop in NPC density might be a result of culturing the microspores and pollen rather than an indication for microspore and pollen embryogenesis in Brassica napus. However, after 1 day in culture under embryogenic conditions, the NPC density increased again and stabilised around 13 NPCs/µm2, whereas under non-embryogenic conditions the NPC density remained about 9 NPCs/µm2. This low density of 9 NPCs/µm2 was also found in the nuclei of sperm cells, in contrast to the 19 NPCs/µm2 found in the vegetative nucleus. It means that, although both the vegetative and sperm nuclei are believed to be metabolically rather inactive in mature pollen, the NPC density of vegetative nucleus is twice as high as the NPC density of the sperm nuclei. In a few cases, embryos formed suspensor-like structures with a NPC density of 9 NPCs/µm2, indicating a lower nucleocytoplasmic exchange of the nuclei of the suspensor cells than with the nuclei in the embryo proper. In addition, observations on NPCs and other organelles, obtained by high resolution cryo-scanning microscopy, are presented.