ABA says NO to UV-B: a universal response?

Abscisic acid (ABA) signaling pathways have been widely characterized in plants, whereas the function of ABA in animals is less well understood. However, recent advances show ABA production by a wide range of lower animals and higher mammals. This enables a new evaluation of ABA signaling pathways in different organisms in response to common environmental stress, such as ultraviolet (UV)-B. In this opinion article, we propose that the induction of common signaling components, such as ABA, nitric oxide (NO) and Ca(2+), in plant and animal cells in response to high doses of UV-B, suggests that the evolution of a general mechanism activated by UV-B is conserved in divergent multicellular organisms challenged by a changing common environment.

Interaction of light and hormone signals in germinating seeds.

Seed germination is regulated by several environmental factors, such as moisture, oxygen, temperature, light, and nutrients. Light is a critical regulator of seed germination in small-seeded plants, including Arabidopsis and lettuce. Phytochromes, a class of photoreceptors, play a major role in perceiving light to induce seed germination. Classical physiological studies have long suggested the involvement of gibberellin (GA) and abscisic acid (ABA) in the phytochrome-mediated germination response. Recent studies have demonstrated that phytochromes modulate endogenous levels of GA and ABA, as well as GA responsiveness. Several key components that link the perception of light and the modulation of hormone levels and responsiveness have been identified. Complex regulatory loops between light, GA and ABA signaling pathways have been uncovered.

Interactions between circadian and hormonal signalling in plants.

Growth and development of plants is controlled by external and internal signals. Key internal signals are those generated by hormones and the circadian clock. We highlight interactions between the circadian clock and hormonal signalling networks in regulating the physiology and growth of plants. Microarray analysis has shown that a significant proportion of transcripts involved in hormonal metabolism, catabolism, perception and signalling are also regulated by the circadian clock. In particular, there are interactions between the clock and abscisic acid, auxin, cytokinin and ethylene signalling. We discuss the role of circadian modulation ('gating') of hormonal signals in preventing temporally inappropriate responses. A consideration of the daily changes in physiology provides evidence that circadian gating of hormonal signalling couples the rhythmic regulation of carbon and water utilisation to rhythmic patterns of growth.

Control of gravitropic orientation. II. Dual receptor model for gravitropism.

Gravitropism of vascular plants has been assumed to require a single gravity receptor mechanism. However, based on the evidence in Part I of this study, we propose that maize roots require two. The first mechanism is without a directional effect and, by itself, cannot give rise to tropism. Its role is quantitative facilitation of the second mechanism, which is directional like the gravitational force itself and provides the impetus for tropic curvature. How closely coupled the two mechanisms may be is, as yet, unclear. The evidence for dual receptors supports a general model for roots. When readiness for gravifacilitation, or gravifacilitation itself, is constitutive, orthogravitropic curvature can go to completion. If not constitutively enabled, gravifacilitation can be weak in the absence of light and water deficit or strong in the presence of light and water deficit. In either case, it can decay and permit roots to assume reproducible non-vertical orientations (plagiogravitropic or plagiotropic orientations) without using non-vertical setpoints. In this way roots are deployed in a large volume of soil. Gravitropic behaviours in shoots are more diverse than in roots, utilising oblique and horizontal as well as vertical setpoints. As a guide to future experiments, we assess how constitutive v. non-constitutive modes of gravifacilitation might contribute to behaviours based on each kind of setpoint.

Low irradiances affect abscisic acid, indole-3-acidic acid, and cytokinin levels of wheat (Triticum aestivum L.) tissues.

Wheat (Triticum aestivum L.) plants were grown under four irradiance levels: 1,400, 400, 200, and 100 micromol m-2 s-1. Leaves and roots were sampled before, during, and after the boot stage, and levels of abscisic acid (ABA), indole-3-acetic acid (IAA), zeatin, zeatin riboside, dihydrozeatin, dihydrozeatin riboside, isopentenyl adenine, and isopentenyl adenosine were quantified using noncompetitive indirect ELISA systems. Levels of IAA in leaves and roots of plants exposed to 100 micromol m-2 s-1 of irradiance were 0.7 and 2.9 micromol kg-1 dry mass (DM), respectively. These levels were 0.2 and 1.0 micromol kg-1 DM, respectively, when plants were exposed to 1,400 micromol m-2 s-1. Levels of ABA in leaves and roots of plants exposed to 100 micromol m-2 s-1 were 0.65 and 0.55 micromol kg-1 DM, respectively. They were 0.24 micromol kg-1 DM (both leaves and roots) when plants were exposed to 1,400 micromol m-2 s-1. Levels of isopentenyl adenosine in leaves (24.3 nmol kg-1 DM) and roots (29.9 nmol kg-1 DM) were not affected by differences in the irradiance regime. Similar values were obtained in a second experiment. Other cytokinins could not be detected (<10 nmol kg 1 DM) in either experiment with the sample sizes used (150-600 mg DM for roots and shoots, respectively).