During drought, the plant hormone abscisic acid (ABA) triggers stomatal closure, thus reducing water loss. Using infrared thermography, we isolated two allelic Arabidopsis mutants ( ost1-1 and ost1-2 ) impaired in the ability to limit their transpiration upon drought. These recessive ost1 mutations disrupted ABA induction of stomatal closure as well as ABA inhibition of light-induced stomatal opening. By contrast, the ost1 mutations did not affect stomatal regulation by light or CO 2 , suggesting that OST1 is involved specifically in ABA signaling. The OST1 gene was isolated by positional cloning and was found to be expressed in stomatal guard cells and vascular tissue. In-gel assays indicated that OST1 is an ABAactivated protein kinase related to the Vicia faba ABA-activated protein kinase (AAPK). Reactive oxygen species (ROS) were shown recently to be an essential intermediate in guard cell ABA signaling. ABA-induced ROS production was disrupted in ost1 guard cells, whereas applied H 2 O 2 or calcium elicited the same degree of stomatal closure in ost1 as in the wild type. These results suggest that OST1 acts in the interval between ABA perception and ROS production. The relative positions of ost1 and the other ABA-insensitive mutations in the ABA signaling network ( abi1-1 , abi2-1 , and gca2 ) are discussed.

Light activates proton (H þ )-ATPases in guard cells, to drive hyperpolarization of the plasma membrane to initiate stomatal opening, allowing diffusion of ambient CO 2 to photosynthetic tissues. Light to darkness transition, high CO 2 levels and the stress hormone abscisic acid (ABA) promote stomatal closing. The overall H þ -ATPase activity is diminished by ABA treatments, but the significance of this phenomenon in relationship to stomatal closure is still debated. We report two dominant mutations in the OPEN STOMATA2 (OST2) locus of Arabidopsis that completely abolish stomatal response to ABA, but importantly, to a much lesser extent the responses to CO 2 and darkness. The OST2 gene encodes the major plasma membrane H þ -ATPase AHA1, and both mutations cause constitutive activity of this pump, leading to necrotic lesions. H þ -ATPases have been traditionally assumed to be general endpoints of all signaling pathways affecting membrane polarization and transport. Our results provide evidence that AHA1 is a distinct component of an ABAdirected signaling pathway, and that dynamic downregulation of this pump during drought is an essential step in membrane depolarization to initiate stomatal closure.

Tomato high pigment (hp) mutants are characterized by their exaggerated photoresponsiveness. Light-grown hp mutants display elevated levels of anthocyanins, are shorter and darker than wild-type plants, and have dark green immature fruits due to the overproduction of chlorophyll pigments. It has been proposed that HP genes encode negative regulators of phytochrome signal transduction. We have cloned the HP-2 gene and found that it encodes the tomato homolog of the nuclear protein DEETIOLATED1 (DET1) from Arabidopsis. Mutations in DET1 are known to result in con-stitutive deetiolation in darkness. In contrast to det1 mutants, tomato hp-2 mutants do not display any visible phenotypes in the dark but only very weak phenotypes, such as partial chloroplast development. Furthermore, whereas det1 mutations are epistatic to mutations in phytochrome genes, analysis of similar double mutants in tomato showed that manifestation of the phenotype of the hp-2 mutant is strictly dependent upon the presence of active phytochrome. Because only one DET1 gene is likely to be present in each of the two species, our data suggest that the phytochrome sig-naling pathways in which the corresponding proteins function are regulated differently in Arabidopsis and tomato.

Tomato high pigment ( hp ) mutants are characterized by their exaggerated photoresponsiveness. Light-grown hp mutants display elevated levels of anthocyanins, are shorter and darker than wild-type plants, and have dark green immature fruits due to the overproduction of chlorophyll pigments. It has been proposed that HP genes encode negative regulators of phytochrome signal transduction. We have cloned the HP-2 gene and found that it encodes the tomato homolog of the nuclear protein DEETIOLATED1 (DET1) from Arabidopsis. Mutations in DET1 are known to result in constitutive deetiolation in darkness. In contrast to det1 mutants, tomato hp-2 mutants do not display any visible phenotypes in the dark but only very weak phenotypes, such as partial chloroplast development. Furthermore, whereas det1 mutations are epistatic to mutations in phytochrome genes, analysis of similar double mutants in tomato showed that manifestation of the phenotype of the hp-2 mutant is strictly dependent upon the presence of active phytochrome. Because only one DET1 gene is likely to be present in each of the two species, our data suggest that the phytochrome signaling pathways in which the corresponding proteins function are regulated differently in Arabidopsis and tomato. INTRODUCTIONLight is a critical environmental signal controlling many aspects of plant development. For example, dark-grown plants display a typical etiolated morphology with elongated hypocotyls, closed apical hooks, and unexpanded cotyledons, whereas plants grown in the light have short hypocotyls, opened apical hooks, and expanded photosynthetically active cotyledons. Light is perceived by a series of photoreceptors that can detect light within a wide spectral range. The phytochromes are the best characterized of these photoreceptors and are able to intercept light primarily within the red and far-red regions of the spectrum (Furuya and Schäfer, 1996). They exist as multigene families, for example, PHYA to PHYE in Arabidopsis, and each phytochrome is likely to have a specific photoperceptory function during plant development (Quail et al., 1995). In addition, plants contain blue/UV-A-absorbing cryptochromes and UV-Babsorbing photoreceptors.Several models for light signal transduction in plants have been proposed. One has been deduced largely by microinjection experiments with tomato and involves G proteins, calcium, and cGMP (Bowler et al., 1994b;Mustilli and Bowler, 1997). Others are based on the genetic analysis of Arabidopsis mutants, such as deetiolated ( det ) and constitutively photomorphogenic ( cop ), which display characteristics of light-grown plants when grown in complete darkness, for example, reduced hypocotyl length, cotyledon opening and expansion, chloroplast development, and expression of light-induced genes (Chory et al., 1989(Chory et al., , 1996Deng et al., 1991;Chamovitz and Deng, 1996).Although several COP and DET genes have been identified, it is not clear how the activities of their gene products are regulated by phytochrome or by the calcium-an...

Drought stress is the major limitation to crop productivity. However, crops are genetically complex with many loci contributing quantitatively to a given physiological trait. Nonetheless, significant in-roads into the molecular mechanisms of drought-adaptive responses have been made from the use of Arabidopsis thaliana. In this special review, we will discuss results gleaned from reverse and forward genetic studies that revealed the involvement of both ABA-dependent and ABA-independent components. In particular, mutant analyses have highlighted the surprising prevalence of RNA metabolism in many key steps. We will also discuss our recent use of infrared thermography to visualize stomatal closure in response to dehydration as a means to identify novel regula-tory genes. This has allowed us to recover mutations belonging to at least eight complementation groups. Analysis of six of these loci revealed that all of their corresponding mutations affect either abscisic acid (ABA) biosynthesis or perception. Hence, in contrast to molecular studies on gene networks which pointed to the clear existence of multiple ABA-independent pathways in the control of dehydration tolerance, our results reinforce ABA-based signalling pathways as the predominant factor in primary or rapid responses. Finally, we will provide some details learned from the molecular analysis of OPEN STOMATA1 (OST1), a gene that encodes an ABA-activated kinase issued from this targeted genetic approach.