Transgenic tobacco plants that express an oat phytochrome gene (phyA) under control of the cauliflower mosaic virus (CaMV) 35S promoter and display altered photophysiology were used to test the role of light as a vehicle of information in dominance relationships between neighboring plants. Compared with the isogenic wild type, phyAoverexpressing plants showed dramatically reduced morphological responsivity to changes in the red/far red ratio of the incident light and to the proximity of neighboring plants in spacing experiments. In transgenic canopies an increase in stand density caused the small plants of the population to be rapidly suppressed by their neighbors. In wild-type canopies, plants responded to increased density with large morphological changes, and there appeared to be an inverse relationship between the magnitude of this morphological response and the ranking of the individual plant in the population size hierarchy. In these wild-type populations, size inequality increased only moderately with density within the time frame of the experiments. Our results suggest that, in crowded stands, the ability of individual plants to acquire information about their light environment via phytochrome plays a central role in driving architectural changes that, at the population level, delay the development of size differences between neighbors.Higher plants respond to the proximity of other plants with plastic morphological and physiological changes. Some of these responses are simple changes in growth rate caused by variation in the supply of environmental resources (e.g., light, nutrients) imposed by neighboring individuals (1). Others are triggered by systems that appear to have evolved specifically to acquire information about the nearness of other plants (2). One such system is driven by phytochrome, a family of photochromic plant photoreceptors (3) that are sensitive to the red (R)-to-far red (FR) ratio (R/FR) of the incident light (4,5 (23,24). The lack of information on the role of light signaling in the genesis of population structure may have far reaching consequences, as the idea of engineering photomorphogenically "blind" genotypes to "improve" crop plants is currently being actively debated (25).Abbreviations: CV, coefficient of variation; FR, far-red radiation; LAI, leaf area index; PPFD, photosynthetic photon flux density; R, red radiation; wt, wild type.tTo whom reprint requests should be addressed at the * address. 10094The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

The N-terminus of phytochrome A is important for the structural integrity and biological activity of the photoreceptor. Mutational analysis of the N-terminus by two different strategies created two distinct photoreceptors, one inactive and the other hyperactive when expressed in transgenic tobacco, suggesting that this region has multiple functional domains. To identify critical residues within this N-terminal region, a series of smaller deletions of oat phytochrome A were created, designated NB (delta49-62), NC (delta6-47), ND (delta7-21), NE (delta2-5), and NF (delta6-12), and compared with a previously characterized deletion mutant NA (delta7-69) and full-length oat phytochrome A. Using photochemical properties as a measure of chromoprotein structure, it was found that the region between residues 13 and 62 was important for the spectral integrity of the photoreceptor. These deletion mutants were also biologically inactive when expressed in both mature tobacco plants and seedlings grown under continuous far-red or red light. In contrast, deletion of the serine-rich region between residues 6 and 12 did not alter the photochemical properties but did produce a hyperactive photoreceptor, indicating this region may be involved in down-regulating phytochrome A activity. The data show that the N-terminus of phytochrome A contains two functional domains, one necessary for conformational stability and biological activity (residues 13-62), and the other involved in attenuating phytochrome responses (residues 6-12).

Ectopic overexpression of an oat PHYA cDNA in tobacco under the cauliflower mosaic virus 35S promoter results in plants with reduced morphological responses to far-red radiation (FR). We have tested the hypothesis that it is possible to molecularly 'mask' stems and leaves to FR-induced elongation and senescence responses by targeting the overexpression ol'PHYA with appropriate promoters. Oat PHYA was expressed in tobacco {Nicotiana tahacum L. cv Xanthi) under the 35S and two Arabidopsis promoters: UBQl and CAB. The internodes of wild type, UBQ.PHYA, and CAB.PHYA plants, which exhibited little or no oat PHYA overexpression, responded to locali/.ed FR treatments with a marked increase in elongation. In contrast, 35S:PHYA plants, which overexpressed PHYA to high levels in all parts of the shoot, did not respond to FR treatments directed to their stems. Leaf senescence responses to FR were remarkably localized, and sensitivity to FR was also inversely correlated with the local PHYA expression level. Thus, chlorophyll content, specific leaf weight, and nitrate reductase activity in leaf spots treated with FR were highly reduced in wild type and UBQ:PHYA plants, but not in the CAB.PHYA and 35S:PHYA counterparts. Our results suggest that it may be feasible to obtain transgenie crop plants in which certain organs or tissues are made 'blind' to phytochrome-perceived signals of canopy density, but whose general photoniorpbogenic competence is not greatly disturbed.

To investigate the biological functions of phytochromes in monocots, we generated, by electric discharge particle bombardment, transgenic rice (Oryza sativa cv Culfmont) that constitutively expresses the oat phytochrome A apoprotein. l h e introduced 124-kD polypeptide bound chromophore and assembled into a red-and far-red-light-photoreversible chromoprotein with absorbance spectra índistinguishable from those of phytochrome purified from etiolated oats. Transgenic lines expressed up to 3 and 4 times more spectrophotometrically detectable phytochrome than wild-type plants in etiolated and green seedlings, respectively. Upon photoconversion to the far-red-absorbing form of phytochrome, oat phytochrome A was degraded in etiolated seedlings with kinetics similar to those of endogenous rice phytochromes (half-life approximately 20 min). Although plants overexpressing phytochrome A were phenotypically indistinguishable from wild-type plants when grown under high-fluence white light, they were more sensitive as etiolated seedlings to light pulses that established very low phytochrome equilibria. lhis indicates that the introduced oat phytochrome A was biologically active. Thus, rice ectopically expressing PHY genes may offer a useful model to help understand the physiological functions of the various phytochrome isoforms in monocotyledonous plants.