We have investigated the stomatal and phototropic responses to blue light of a number of single and double mutants at various loci that encode proteins involved in blue-light responses in Arabidop-sis. The stomatal responses of light-grown mutant plants (cry1, cry2, nph1, nph3, nph4, cry1cry2, and nph1cry1) did not differ significantly from those of their wild-type counterparts. Second positive phototropic responses of etiolated mutant seedlings, cry1, cry2, cry1cry2, and npq1-2, were also similar to those of their wild-type counterparts. Although npq1 and single and double cry1cry2 mutants showed somewhat reduced amplitude for first positive phototropism, threshold, peak, and saturation fluence values for first positive phototropic responses of etiolated seedlings did not differ from those of wild-type seedlings. Similar to the cry1cry2 double mutants and to npq1-2, a phyAphyB mutant showed reduced curvature but no change in the position or shape of the fluenceresponse curve. By contrast, the phototropism mutant nph1-5 failed to show phototropic curvature under any of the irradiation conditions used in the present study. We conclude that the chromoproteins cry1, cry2, nph1, and the blue-light photoreceptor for the stomatal response are genetically separable. Moreover, these photoreceptors appear to activate separate signal transduction pathways. LITERATURE CITED AhmadM, Cashmore AR (1993) HY4 gene of A. thaliana encodes a protein with characteristics of a blue-light photoreceptor. Nature 366: 162-166 Ahmad M, Jarillo JA, Smirnova O, Cashmore AR (1998) Cryptochrome blue-light photoreceptors implicated in phototropism. Nature 392: 720-723 Briggs WR, Huala E (1999) Blue-light photoreceptors in higher plants. Annu Rev Cell Dev Biol (in press) Briggs WR, Liscum E (1996) Blue light-activated signal transduction in higher plants. In P Aducci, ed, Signal Transduction in Plants. Birkhauser Verlag, Basel, Switzerland, pp 107-135 Briggs WR, Liscum E (1997) The role of mutants in the search for the photoreceptor for phototropism in higher plants. Plant Cell Environ 20: 768-771 Bruggemann E, Handwerger K, Essex C, Storz G (1996) Analysis of fast neutron-generated mutants at the Arabidopsis thaliana HY4 locus. Plant J 10: 755-760 Chamovitz DA, Deng X-W (1996) Light signaling in plants. Crit Rev Plant Sci 15: 455-478 Chory J (1992) A genetic model for light-regulated seedling development in Arabidopsis. Development 115: 167-172 Christie JM, Reymond P, Powell GK, Bernasconi P, Raibekas A, Liscum E, Briggs WR (1998) Arabidopsis NPH1: a flavoprotein with the properties of a photoreceptor for phototropism. Science

Stomatal responses to light oi Arabidopsis thaiiana wildtype plants and mutant plants deficient in starch (phosphoglucomutase deficient) were compared in gas exchange experiments. Stomatal density, size and ultrastructure were identical for the two phenotypes, but no starch was observed in guard cells of the mutant plants whatever the time of day. The overall extent of changes in stomatal conductance during 14 h light-10 h dark cycles was similar for the two phenotypes. However, the slow endogenous stomatai opening occurring in darkness in the wild type was not observed in the mutant plants. Stomata in the mutant plants responded much more slowly to blue light (70 ;Umol m~^ s~*) though the response to red light (250 /imol m~^ s~') was similar to that of wild-type plants. In paradermal sections, stomatal responses to red light (300 ^mol m""^ s~^) were weak for wild-type plants as well as for mutant plants. Stomatal opening was greater under low blue light (75 /imol m"^ s~*) than under red light for the two genotypes. However, in mutant plants, a high chloride concentration (50 mol m~^) was necessary to achieve the same stomatal aperture as observed for the wild-type plants. These results suggest that starch metabolism, via the synthesis of a counter-ion to potassium (probably malate), is required for full stomatal response to blue light but is not involved in the stomatal response to red light.

Photosynthetic carbon assimilation, carbon partitioning and foliar carbon budgets were measured in the leaves of transformed tomato plants expressing a maize sucrose-phosphate synthase (SPS) gene in addition to the native enzyme, and in untransformed controls. The maize SPS gene was expressed under control of either the promoter of the small subunit of ribulose 1,5-bisphosphate carboxylase (rbcS promoter; lines 2, 9 and 18) or the 35S promoter from cauliflower mosaic virus (CaMV promoter; line 13). The rate of sucrose synthesis was increased relative to that of starch and sucrose/starch ratios were higher throughout the photoperiod in the leaves of all plants expressing high SPS activity. The leaf carbon budget over the day/night cycle in air at low irradiance (180μmol photon m−2 s−1) was similar in all plants. Net photosynthesis measured in air and at elevated CO2 (800–1500 μl l−1) on whole plants grown in air at 400μmol m−2 s−1 irradiance was significantly increased in the high SPS expressors compared to the untransformed controls and was highest where SPS activity was greatest. At high CO2 the stimulation of photosynthesis was more pronounced. We conclude that SPS activity is a major point of control of photosynthesis particularly under saturating light and CO2.

Stomatal responses to ABA and CO2 were investigated in Arabidopsis thaliana (L.) Heynh. wild-type and ABA insensitive mutants (abi1-1, abi2-1, abi1-1abi2-1) at the whole plant and at the isolated epidermis levels. In wild-type plants, feeding roots with ABA (1–50 µM) triggered a rapid drop in leaf conductance which levelled off during the following photoperiods, and strongly inhibited the increase in conductance induced by light. The rapid response was strongly inhibited in abi1-1, abi2-1 and abi1-1abi2-1 double mutants, but a residual long-term decrease in leaf conductance was still observed. In wild-type plants, exogenous ABA strongly enhanced the response to CO2 removal. Conversely, in the absence of CO2 the effect of ABA was drastically reduced in epidermal strip experiments. These results reveal a strong interaction between sensing of ABA and CO2 in stomata of A. thaliana. Despite an initially wide stomatal aperture in abi-1, abi-2 and double mutant plants, their stomatal responses to light and CO2 removal were half those of wild-type plants. Moreover these responses were totally independent of the presence of ABA, suggesting that ABI1 and ABI2 are either directly involved in the interaction between the two signalling pathways or, alternatively located upstream of this point of interaction.

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