Indole-3-Acetic Acid Controls Cambial Growth in Scots Pine by Positional Signaling1

The vascular cambium produces secondary xylem and phloem in plants and is responsible for wood formation in forest trees. In this study we used a microscale mass-spectrometry technique coupled with cryosectioning to visualize the radial concentration gradient of endogenous indole-3-acetic acid (IAA) across the cambial meristem and the differentiating derivatives in Scots pine (Pinus sylvestris L.) trees that had different rates of cambial growth. This approach allowed us to investigate the relationship between growth rate and the concentration of endogenous IAA in the dividing cells. We also tested the hypothesis that IAA is a positional signal in xylem development (C. Uggla, T. Moritz, G. Sandberg, B. Sundberg [1996] Proc Natl Acad Sci USA 93: 9282–9286). This idea postulates that the width of the radial concentration gradient of IAA regulates the radial number of dividing cells in the cambial meristem, which is an important component for determining cambial growth rate. The relationship between IAA concentration in the dividing cells and growth rate was poor, although the highest IAA concentration was observed in the fastest-growing cambia. The radial width of the IAA concentration gradient showed a strong correlation with cambial growth rate. The results indicate that IAA gives positional information in plants.

Tomato Flower Abnormalities Induced by Low Temperatures Are Associated with Changes of Expression of MADS-Box Genes1

Flower and fruit development in tomato (Lycopersicon esculentum Mill.) were severely affected when plants were grown at low temperatures, displaying homeotic and meristic transformations and alterations in the fusion pattern of the organs. Most of these homeotic transformations modified the identity of stamens and carpels, giving rise to intermediate organs. Complete homeotic transformations were rarely found and always affected organs of the reproductive whorls. Meristic transformations were also commonly observed in the reproductive whorls, which developed with an excessive number of organs. Scanning electron microscopy revealed that meristic transformations take place very early in the development of the flower and are related to a significant increase in the floral meristem size. However, homeotic transformations should occur later during the development of the organ primordia. Steady-state levels of transcripts corresponding to tomato MADS-box genes TM4, TM5, TM6, and TAG1 were greatly increased by low temperatures and could be related to these flower abnormalities. Moreover, in situ hybridization analyses showed that low temperatures also altered the stage-specific expression of TM4.

Analysis of Cell Division and Elongation Underlying the Developmental Acceleration of Root Growth in Arabidopsis thaliana1

To investigate the relation between cell division and expansion in the regulation of organ growth rate, we used Arabidopsis thaliana primary roots grown vertically at 20°C with an elongation rate that increased steadily during the first 14 d after germination. We measured spatial profiles of longitudinal velocity and cell length and calculated parameters of cell expansion and division, including rates of local cell production (cells mm−1 h−1) and cell division (cells cell−1 h−1). Data were obtained for the root cortex and also for the two types of epidermal cell, trichoblasts and atrichoblasts. Accelerating root elongation was caused by an increasingly longer growth zone, while maximal strain rates remained unchanged. The enlargement of the growth zone and, hence, the accelerating root elongation rate, were accompanied by a nearly proportionally increased cell production. This increased production was caused by increasingly numerous dividing cells, whereas their rates of division remained approximately constant. Additionally, the spatial profile of cell division rate was essentially constant. The meristem was longer than generally assumed, extending well into the region where cells elongated rapidly. In the two epidermal cell types, meristem length and cell division rate were both very similar to that of cortical cells, and differences in cell length between the two epidermal cell types originated at the apex of the meristem. These results highlight the importance of controlling the number of dividing cells, both to generate tissues with different cell lengths and to regulate the rate of organ enlargement.

Auxin as a positional signal in pattern formation in plants.

By using a novel, extremely sensitive and specific gas chromatography-mass spectrometry technique we demonstrate in Pinus sylvestris (L.) trees the existence of a steep radial concentration gradient of the endogenous auxin, indole-3-acetic acid, over the lateral meristem responsible for the bulk of plant secondary growth, the vascular cambium. This is the first evidence that plant morphogens, such as indole-3-acetic acid, occur in concentration gradients over developing tissues. This finding gives evidence for a regulatory system in plants based on positional signaling, similar to animal systems.

A rice homeobox gene, OSH1, is expressed before organ differentiation in a specific region during early embryogenesis.

Homeobox genes encode a large family of homeodomain proteins that play a key role in the pattern formation of animal embryos. By analogy, homeobox genes in plants are thought to mediate important processes in their embryogenesis, but there is very little evidence to support this notion. Here we described the temporal and spatial expression patterns of a rice homeobox gene, OSH1, during rice embryogenesis. In situ hybridization analysis revealed that in the wild-type embryo, OSH1 was first expressed at the globular stage, much earlier than organogenesis started, in a ventral region where shoot apical meristem and epiblast would later develop. This localized expression of OSH1 indicates that the cellular differentiation has already occurred at this stage. At later stages after organogenesis had initiated, OSH1 expression was observed in shoot apical meristem [except in the L1 (tunica) layer], epiblast, radicle, and their intervening tissues in descending strength of expression level with embryonic maturation. We also performed in situ hybridization analysis with a rice organless embryo mutant, orl1, that develops no embryonic organs. In the orl1 embryo, the expression pattern of OSH1 was the same as that in the wild-type embryo in spite of the lack of embryonic organs. This shows that OSH1 is not directly associated with organ differentiation, but may be related to a regulatory process before or independent of the organ determination. The results described here strongly suggest that, like animal homeobox genes, OSH1 plays an important role in regionalization of cell identity during early embryogenesis.

A mechanism for inducing plant development: the genesis of a specific inhibitor.

Parasitic strategies are widely distributed in the plant kingdom and frequently involve coupling parasite organogenesis with cues from the host. In Striga asiatica, for example, the cues that initiate the development of the host attachment organ, the haustorium, originate in the host and trigger the transition from vegetative to parasitic mode in the root meristem. This system therefore offers a unique opportunity to study the signals and mechanisms that control plant cell morphogenesis. Here we establish that the biological activity of structural analogs of the natural inducer displays a marked dependence on redox potential and suggest the existence of a semiquinone intermediate. Building on chemistry that exploits the energetics of such an intermediate, cyclopropyl-p-benzoquinone (CPBQ) is shown to be a specific inhibitor of haustorial development. These data are consistent with a model where haustorial development is initiated by the completion of a redox circuit.