Plants grown under dense canopies perceive through the phytochrome system a reduction in the ratio of red to far-red light as a warning of competition, and this triggers a series of morphological changes to avoid shade. Several phytochrome signaling intermediates acting as positive regulators of accelerated elongation growth and induction of flowering in shade avoidance have been identified. Here we report that a negative regulatory mechanism ensures that in the presence of far-red-rich light an exaggerated plant response does not occur. Strikingly, this unpredicted negative regulatory mechanism is centrally involved in the attenuation of virtually all plant responses to canopy shade.Supplemental material is available at http://www.genesdev.org.

Summary. In order to improve the ultrastructural preservation of the female gametophyte of Petunia x hybrida and Brassica napus we tested several cryofixation techniques and compared the results with those of conventional chemical fixation methods. Ovules fixed with glutaraldehyde and osmium tetroxide in the presence or absence of potassium ferrocyanide showed poor cell morphological and ultrastructural preservation. In ovules cryo-fixed by plunging into liquid propane, the cell morphology was well preserved. However, at the ultrastructural level structure-distorting ice crystals were detected in all tissues. Due to the large size of the ovules, cryofixation by plunging in liquid propane is not adequate for ultrastructural studies. In contrast, P. x hybrida and B. napus ovtfles cryo-fixed by high pressure freezing showed improved cell morphological as well as ultrastructural preservation of the embryo sac and the surrounding integumentary tissues. The contrast of the cellular membranes after fi'eeze substitution with 2% osmium tetroxide and 0.1% uranyI acetate in dry acetone was high. At the ultrastructural level, the most prominent improvements were: straight plasma membranes which were appressed to the ceil walls; turgid appearing organeltes with smooth surface contours; minimal extraction of cytoplasmic and extracelluIar substances. In contrast to the chemically fixed ovules, in high pressure frozen ovules numerous microtubules and multivesicular bodies could be distinguished.

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