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DEVELOPMENT AND HORMONE ACTION

High Humidity Induces Abscisic Acid 8'-Hydroxylase in Stomata and Vasculature to Regulate Local and Systemic Abscisic Acid Responses in Arabidopsis^1,[OA]

RIKEN Plant Science Center, Yokohama, Kanagawa 230–0045, Japan (M.O., Y.T., Y.K., M.S., E.N.); Department of Cell and Systems Biology, University of Toronto, Toronto, Ontario M5S 3B2, Canada (E.N.); The Centre for the Analysis of Genome Evolution and Function, University of Toronto, Toronto, Ontario M5S 3B2, Canada (E.N.); and Plant Biotechnology Institute, National Research Council of Canada, Saskatoon, Saskatchewan S7N 0W9, Canada (S.R.A.)

	ABSTRACT

TOP ABSTRACT RESULTS DISCUSSION MATERIALS AND METHODS LITERATURE CITED

 
Levels of endogenous abscisic acid (ABA) are changed dynamically in response to environmental conditions. The ABA 8'-hydroxylase is a key enzyme in ABA catabolism and is encoded by CYP707A genes. In this study, we examined physiological roles of Arabidopsis (Arabidopsis thaliana) CYP707As in the plant's response to changes in humidity. The cyp707a1 and cyp707a3 mutants displayed lower stomatal conductance under turgid conditions (relative humidity 60%) than the wild type. When wild-type plants were transferred to high-humidity conditions (relative humidity 90%), CYP707A1 and CYP707A3 transcript levels increased, followed by the reduction of ABA levels. The cyp707a3 mutant exhibited high ABA levels even after transferring to high-humidity conditions, whereas, under similar conditions, the cyp707a1 mutant exhibited low ABA levels comparable to the wild type. Analysis of spatial expression patterns by using transgenic plants harboring a promoter::β-glucuronidase gene indicated that high-humidity-induced expression of CYP707A1 and CYP707A3 occurred primarily in guard cells and vascular tissues, respectively. Furthermore, stomatal closure of the cyp707a1 mutant, but not cyp707a3 mutant, was ABA hypersensitive when epidermal peel was treated with exogenous ABA, suggesting that CYP707A1 is essential for ABA catabolism inside the guard cells. These results implicate that CYP707A3 reduces the amount of mobile ABA in vascular tissues in response to high humidity, whereas CYP707A1 inactivates local ABA pools inside the guard cells. Taken together, ABA catabolism in both vascular tissues and guard cells participates in the systemic ABA action that controls stomatal movement in response to high humidity.

The biosynthetic pathway of ABA in higher plants has been established by forward-genetic approaches (Seo and Koshiba, 2002; Zhu, 2002; Nambara and Marion-Poll, 2005). Biochemical and molecular genetic studies of ABA-deficient mutants from various plant species contributed to the identification and characterization of a number of enzymes of ABA biosynthesis. The 9-cis-epoxycarotenoid dioxygenase (NCED) cleaves 11, 12 double bonds of C₄₀ carotenoids and produces xanthoxin. This is a key regulatory step in de novo ABA biosynthesis (Qin and Zeevaart, 1999; Thompson et al., 2000; Iuchi et al., 2001). In the Arabidopsis (Arabidopsis thaliana) genome, there are five members (AtNCED2, 3, 5, 6, and 9) of the NCED family (Iuchi et al., 2001; Toh et al., 2008). Transcript levels of AtNCED3 rapidly increase in response to drought stress, and its protein levels are correlated with drought-induced ABA accumulation. These results indicate that AtNCED3 plays a crucial role in ABA accumulation during dehydration (Iuchi et al., 2001; Endo et al., 2008).

In the case of ABA catabolism, ABA is converted to inactive forms by oxidation or conjugation (Nambara and Marion-Poll, 2005). The major ABA catabolic pathway in higher plants involves oxidation of ABA by ABA 8'-hydroxylase to form 8'-hydroxy ABA. In the subsequent step, 8'-hydroxy ABA is spontaneously isomerized to phaseic acid (PA), which is further reduced to dihydrophaseic acid (DPA) by an unknown reductase (Gillard and Walton, 1976; Krochko et al., 1998; Cutler and Krochko, 1999). The genes encoding ABA 8'-hydroxylase were identified to be members of the subfamily of CYP707A (Kushiro et al., 2004; Saito et al., 2004). There are four members of CYP707A family in the Arabidopsis genome. The transcript levels of all four CYP707As are induced by dehydration and exogenous ABA treatment (Kushiro et al., 2004; Saito et al., 2004). CYP707A3 is induced strongly by rehydration after dehydration conditions. The cyp707a3 mutant contains higher ABA levels than the wild type during dehydration and rehydration, suggesting that CYP707A3 plays a predominant role in ABA catabolism in these conditions (Umezawa et al., 2006). On the other hand, ABA is also inactivated by conjugation with Glc (Xu et al., 2002; Chiwocha et al., 2005; Lim et al., 2005; Priest et al., 2006). ABA-Glc ester (ABA-GE) is one of the predominant ABA conjugates and may function as a storage form of releasable ABA (Dietz et al., 2000). In Arabidopsis, ABA-GE is hydrolyzed in response to dehydration by β-glucosidase (AtBG1), leading to an increase in the active ABA pool (Lee et al., 2006).

ABA and its catabolites are transported through the phloem and xylem flow (Zeevaart and Boyer, 1984; Sauter et al., 2002). There are many reports concerning ABA synthesis in root and its possible transport from root to shoot (Davies et al., 2005). Recent studies have highlighted the importance of drought- and osmotic stress-induced ABA synthesis in shoots (Holbrook et al., 2002; Christmann et al., 2005, 2007). Grafting experiments using a tomato (Lycopersicum esculentum) ABA-deficient mutant showed that shoot genotype is the primary determinant of stomatal closure during soil drying (Holbrook et al., 2002). Christmann et al. reported that the induction of a reporter gene driven by an Arabidopsis ABA-inducible ATHB6 promoter was observed in vascular tissues of shoots when the roots were subjected to osmotic stress (Christmann et al., 2005). Consistent with these findings, dehydration-induced AtNCED3 protein accumulation was observed in vascular parenchyma cells in shoots (Endo et al., 2008). These results indicated that the major tissues that supply dehydration-induced ABA pools are the vascular tissues in the shoot. Indeed, three de novo ABA biosynthesis enzymes, AtNCED3, short-chain dehydrogenase/reductase (ABA2), and abscisic aldehyde oxidase (AAO3) are localized in vascular tissues (Koiwai et al., 2004; Endo et al., 2008). Moreover, the expression of an ABA-inducible reporter gene is observed in vascular tissues after a short period of osmotic stress, followed by the reporter gene expression in guard cells (Christmann et al., 2005). These findings suggest that the vascular tissues provide a major ABA pool to regulate stomatal closure during dehydration. Nonetheless, the competence of the guard cells to regulate ABA metabolism in response to environmental signals remains unknown.

Stomatal aperture changes in response to humidity. This response is complex and is controlled by both ABA-dependent and ABA-independent mechanisms. Xie et al. (2006) reported that stomata of Arabidopsis ABA-deficient aba2 mutants are defective in the rapid response (within minutes) to low humidity (Xie et al., 2006). In contrast, Arabidopsis aba1 mutant and tomato flacca mutant reportedly change stomatal conductance in response to humidity (Bradford et al., 1983; Assmann et al., 2000). Humidity is also known to affect endogenous ABA levels. In spinach (Spinacia oleracea) and Tradescantia virginiana, ABA levels were lower in leaves grown in high-humidity conditions, compared with those grown in moderate humidity conditions (Zeevaart, 1974; Rezaei Nejad and van Meeteren, 2007, 2008). In contrast to recent progress in understanding the regulation of ABA biosynthesis, our knowledge of ABA catabolism in response to environmental cues is still limited. The aim of this study is to elucidate physiological roles of CYP707As in the humidity response in vegetative tissues by characterizing loss-of-function mutants in these genes. Our results indicate that CYP707A family members play differential roles in the systemic plant responses to changes in humidity.

	RESULTS

TOP ABSTRACT RESULTS DISCUSSION MATERIALS AND METHODS LITERATURE CITED

 

CYP707A1 and CYP707A3 Are Involved in the Regulation of Transpiration in Turgid Plants

In Arabidopsis, there are four members of CYP707A genes that encode ABA 8'-hydroxylases (Kushiro et al., 2004; Saito et al., 2004). A previous study showed that the CYP707A3 was induced predominantly among the CYP707A genes during dehydration and subsequent rehydration in plants. In addition, endogenous ABA levels in the cyp707a3 mutant were found to be higher than those in the wild type, and cyp707a3 mutants showed reduced transpiration compared to wild-type plants (Umezawa et al., 2006). This indicates that CYP707A3 is the major isoform for ABA 8'-hydroxylation in these conditions. However, it is still unclear whether other CYP707As contribute to the regulation of transpiration.

To examine the physiological role of other CYP707As in transpirational control, we first examined the rate of transpiration in cyp707a mutant plants in turgid conditions. Currently, allelic series of the cyp707a1, cyp707a2, and cyp707a3 mutants, except for the cyp707a4 mutant, are available from the Arabidopsis Biological Resource Center. A Leaf Porometer was used to measure stomatal conductance of these mutants and wild type. The stomatal conductance of cyp707a1 mutants was significantly reduced compared to that of wild type, and its value was similar to the cyp707a3 mutants (Fig. 1A ). In contrast, there was no difference in stomatal conductance between wild-type and cyp707a2 mutants. Next, we examined the cyp707a double mutants. Stomatal conductance of cyp707a1 cyp707a3 double mutant was reduced more strongly than that of each single mutant. On the other hand, transpiration of cyp707a1 cyp707a2 and cyp707a2 cyp707a3 double mutants was comparable with that of cyp707a1 and cyp707a3 mutants, respectively. These results suggest that CYP707A1 and CYP707A3, but not CYP707A2, are involved in the control of transpiration.

View larger version (38K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 1. Stomatal conductance and leaf surface temperatures of the cyp707a mutants. A, Stomatal conductance of wild-type and cyp707a plants. Stomatal conductance was measured with the intact 4-week-old plants of wild type, cyp707a mutants under 22°C, RH 60% condition. At least six leaves were measured in each experiment and stomatal conductance is shown by an average with SE from four experiments. B and C, Leaf surface temperatures of nonstress plants under well-watered conditions. Plants were grown on soil under well-watered conditions (22°C, RH 75%) for 4 weeks. B, Leaf surface temperature in cyp707a mutant. At least four plants were used in each experiment. Leaf surface temperature is shown by an average with SE from seven experiments. C, Typical thermal images of rosette leaves in cyp707a mutants. A lowercase letter in A and B indicates a significant difference judged by Student's t test; a, b, and c indicate the significant difference (P < 0.01) relative to wild type, cyp707a1-1 mutant, and cyp707a3-1 mutant, respectively. An asterisk indicates the significant difference when less stringent cutoff value was used (P < 0.05).

Regulation of ABA Biosynthesis and Catabolism in Response to High Humidity

We then examined whether high humidity affects endogenous ABA levels in Arabidopsis wild-type plants. High-humidity treatment was performed by transferring plants from normal humidity (relative humidity [RH] 60%) to high-humidity conditions (RH 90%) at 22°C. The ABA levels decreased by 40% at 30 min, and then by 80% at 1 h after initiation of the high-humidity treatment (Fig. 2A ). The levels of PA and DPA, catabolites of ABA 8'-hydroxylation pathway, increased, followed by the ABA reduction. The levels of these catabolites increased significantly within 10 min of humidity treatment and their levels remained at the high level until 30 min and decreased gradually thereafter (Fig. 2B). This suggests that high-humidity-induced ABA reduction occurs through ABA 8'-hydroxylation, not by alteration of ABA biosynthesis. Endogenous levels of other catabolites, such as ABA-GE and neoPA were much lower than PA and DPA levels. In addition, endogenous 7'-OH ABA could not be detected in this experiment (data not shown). These results suggest that, in response to high humidity, ABA is predominantly converted to PA and DPA. PA and DPA levels decline, indicating that DPA is further metabolized. Next, we examined the expression of AtNCED3 and CYP707A genes during the high-humidity treatment by quantitative reverse transcription (QRT)-PCR. The levels of AtNCED3 transcript were low and did not change significantly under the high-humidity treatment (Fig. 2C), suggesting that AtNCED3 expression is not responsive to high humidity. As for the CYP707A genes, the CYP707A3 transcript was the most abundant under normal humidity conditions. The transcript level of the CYP707A3 increased immediately after onset of the high-humidity treatment, reached a maximum within 10 min, and decreased thereafter. Although CYP707A1 transcript levels were relatively lower than those of CYP707A3, induction of CYP707A1 was also observed during the high-humidity treatment. The CYP707A1 transcript levels reached a maximum in 30 min, slower than the CYP707A3 induction, and decreased thereafter. On the other hand, CYP707A2 and CYP707A4 transcript levels were much lower than CYP707A1 and CYP707A3 transcript levels and were not changed significantly during high-humidity treatment. These results suggest that high humidity reduces the ABA levels by the activation of CYP707A1 and CYP707A3 rather than the down-regulation of the AtNCED3.

View larger version (25K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 2. ABA biosynthesis and catabolism in response to high humidity. Changes of ABA (A) and its catabolite (B) levels after transferring to high-humidity treatment. C, Changes of AtNCED3 and CYP707A transcript levels in response to high-humidity treatment. Four-old-week plants were transferred from RH 60% to 90%. Rosette leaves were collected every 10 min after the onset of the treatment. Experiments were performed three times and an average is shown with SE. FW, Fresh weight.

To examine stomatal opening after transferring to high-humidity conditions, we measured stomatal apertures. Stomatal aperture in wild-type plants increased by approximately 25% under the high-humidity treatment (from RH 60% to 90% at 22°C) for 1 h (Fig. 3 ). We then compared stomatal apertures of cyp707a mutants to test if this stomatal opening could be attributed to the activation of CYP707A1 and CYP707A3. Stomata of cyp707a1 and cyp707a3 mutants, but not cyp707a2, were more closed compared to that of wild type under normal conditions (Fig. 3). After 1 h of high-humidity treatment, the degree of stomatal opening in cyp707a2 was similar to that in wild type, whereas stomatal opening of cyp707a1 and cyp707a3 was repressed compared to that of wild type. Next, we examined stomatal aperture in the cyp707a double mutants. The cyp707a1 cyp707a3 double mutant exhibited more drastic stomatal closure than other mutants under normal conditions. In addition, stomatal opening of cyp707a1 cyp707a3 double mutant was strongly repressed under high-humidity conditions. On the other hand, CYP707A2 mutation did not affect stomatal response to humidity in cyp707a1 cyp707a2 and cyp707a2 cyp707a3 double mutants relative to their parental single mutants, cyp707a1 and cyp707a3, respectively. This result indicates that both CYP707A1 and CYP707A3 are important for stomatal opening under high-humidity treatment.

View larger version (30K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 3. Stomatal aperture of the cyp707a mutants in response to high humidity. Stomatal aperture in cyp707a single and double mutants before and 1 h after transferring to high-humidity conditions. Four-week-old plants were transferred from RH 60% to 90% at 22°C. Images of stomatal aperture from intact leaves were obtained using SUMP method. Images were observed by light microscopy. Experiments were performed at least four times using rosette leaves from independent plants. An average (n 60) is shown with SE. A lowercase letter indicates a significant difference judged by Student's t test. a, b, and c indicate the significant difference (P < 0.01) relative to wild type, cyp707a1-1 mutant, and cyp707a3-1 mutant, respectively. An asterisk indicates the significant difference when less stringent cutoff value was used (P < 0.05).

View larger version (26K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 4. Phenotypic analysis of the cyp707a mutants in high-humidity conditions. A, ABA levels of the cyp707a mutants under high-humidity conditions for 1 h. B, Expression of ABA-responsive genes in cyp707a single and double mutants under high-humidity conditions for 1 h. ABF3 and ATHB6 expression in cyp707a mutants are shown in right and left graph, respectively. Experiments were performed at least four times and an average is shown with SE. A lowercase letter indicates a significant difference judged by Student's t test. a, b, and c indicate the significant difference (P < 0.01) relative to wild type, cyp707a1-1 mutant, and cyp707a3-1 mutant, respectively. An asterisk indicates the significant difference when less stringent cutoff value was used (P < 0.05).

To investigate the site of CYP707As expression in response to high humidity, the promoter of CYP707A1 or CYP707A3 was fused to a GUS gene and transgenic Arabidopsis plants were generated. Under normal humidity conditions, both pCYP707A1::GUS and pCYP707A3::GUS lines showed only faint GUS staining at the edge of the veins (Fig. 5, A and E ). The pCYP707A1::GUS lines also showed weak GUS staining in guard cells at the leaf edges. Coincident with the transcript levels, high-humidity induction of the reporter gene was observed (Fig. 5, B and F). After 2 h of high-humidity treatment, GUS staining for pCYP707A1::GUS was observed in guard cells of rosette leaves (Fig. 5, C and D). In addition, weak but reproducible GUS staining of pCYP707A1::GUS was also in vascular tissues of rosettes leaves (Fig. 5C). On the other hand, strong GUS staining for pCYP707A3::GUS was observed in vascular tissues of high-humidity-treated plants (Fig. 5G). It is worth mentioning that GUS staining in guard cell was observed in pCYP707A1::GUS, but not in pCYP707A3::GUS (Fig. 5, D and H). These results indicate that the major sites of CYP707A1 and CYP707A3 expression in response to high humidity are the guard cells and vascular tissues, respectively.

View larger version (72K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 5. Spatial expression patterns of CYP707A1 and CYP707A3 in response to high humidity. Expression of promoter CYP707A1::GUS before (A) and after transferring to high humidity conditions (B–D). Expression of promoter CYP707A3::GUS before (E) and after transferring to high-humidity conditions (F–H). Approximately 3-week-old plants were grown on soil in pots under RH 60% at 22°C condition. High-humidity treatment (from RH 60% to 90% at 22°C) was performed for 2 h. Bars = 1 mm for A, B, E, and F; 50 µm for C and G; 10 µm for D and H.

CYP707A1 and CYP707A3 are involved in stomatal opening in response to high humidity (Fig. 3), but only CYP707A1 was expressed in guard cells (Fig. 5, D and H). To investigate the function of CYP707A in guard cells, we examined ABA-induced stomatal closure using epidermal peels. In this experiment, we used natural type of ABA [(+)-S-ABA] rather than racemic ABA, because (–)-R-ABA was found not to be a substrate of CYP707As (Kushiro et al., 2004; Saito et al., 2004), and its weak biological activity in guard cells but strong activity in other ABA-related processes (for review, see Zaharia et al., 2005). Among the cyp707a mutants, ABA-induced stomatal closure of the cyp707a1 mutant was hypersensitive to (+)-S-ABA (Fig. 6, A and B ). The stomata of the cyp707a2 and cyp707a3 mutants responded to exogenous (+)-S-ABA similarly to the wild type. These results indicate that CYP707A1 is involved in ABA catabolism inside guard cells, whereas CYP707A3 is likely to inactivate ABA outside the guard cells.

View larger version (58K): [in this window] [in a new window] [as a PowerPoint slide]   Figure 6. The effect of exogenous ABA treatment on the stomatal apertures of cyp707a mutants in epidermal peels. A, ABA sensitivity of epidermal peels in cyp707a mutants. B, Typical images of ABA-induced stomatal closure on 1 µM (+)-S-ABA. Epidermal peels were incubated with opening buffer under light condition for 2 h to open stomata. The opened epidermal peels were incubated with (+)-S-ABA solution for 2 h. Experiments were performed using at least four independent plants' rosettes leaves. An average (n 60) is shown with SE. An asterisk indicates a significant difference relative to wild type by Student's t test (**P < 0.05 and *P < 0.01).

	DISCUSSION

TOP ABSTRACT RESULTS DISCUSSION MATERIALS AND METHODS LITERATURE CITED

Humidity can change rapidly (within minutes) in the natural environment. To date, there are a number of reports about the regulation of ABA metabolism in response to low humidity, specifically dehydration (Qin and Zeevaart, 1999; Iuchi et al., 2001; Tan et al., 2003; Lee et al., 2006; Endo et al., 2008). On the other hand, information of physiological roles of ABA metabolism and its regulatory mechanism in response to high humidity is limited. Previous reports described that endogenous ABA levels in leaves grown under high humidity was lower than under moderate humidity (Zeevaart, 1974; Rezaei Nejad and van Meeteren, 2007, 2008). However, these previous reports studied the effect of the long term under high humidity. In addition, long-term treatment under high humidity causes changes of stomatal properties such as stomatal development and response to ABA (Spence et al., 1986; Xia, 1994; Rezaei Nejad and van Meeteren, 2007, 2008). In contrast, our present study revealed that the endogenous ABA levels decreased after 20 min of high-humidity treatments (Fig. 2A). Concomitant with the reduction of ABA levels, stomata were opened in response to high-humidity treatment (Fig. 3). Expression and phenotypic analyses indicate that ABA levels were reduced primarily by activation of ABA catabolism (Fig. 2B). Rapid activation of ABA 8'-hydroxylation pathway in response to high humidity is important for the reduction of ABA levels. Our analysis revealed that CYP707A3 plays a major role, whereas CYP707A1 has a relatively minor role, in regulating ABA levels in shoots in response to high humidity (Figs. 2C and 4). It is interesting to note that plants' response to dehydration is triggered by the rapid induction of the AtNCED3. Dehydration induction of the CYP707A genes is slow, and is involved in the fine tuning of ABA levels (Kushiro et al., 2004; Umezawa et al., 2006). This study indicates that the expression of the CYP707A3 and CYP707A1, but not AtNCED3, responded rapidly to high humidity. The defects in the cyp707a3 and cyp707a1 mutants support the notion that ABA 8'-hydroxylation is the principal pathway for ABA metabolism to respond to high humidity. The metabolic balance between ABA biosynthesis and catabolism is differentially regulated under different environmental cues to control the hormone levels properly.

ABA Catabolism Inside Guard Cells

Guard cells are one of the well-known target cells for the ABA action. Although a number of genes have been identified as ABA-signaling components in guard cells, ABA metabolism inside guard cells has not been characterized (Schroeder et al., 2001; Nilson and Assmann, 2007; Wasilewska et al., 2008). It has been reported that the expression of AAO3, last step enzyme for ABA biosynthesis, is detected in guard cells of dehydrated Arabidopsis plants (Koiwai et al., 2004). Moreover, a transient expression of AtNCED3 or AAO3 fused to a GFP in Vicia faba guard cells led to stomatal closure, suggesting that guard cell itself is competent to synthesize ABA (Melhorn et al., 2008). Endo et al. (2008) reported that drought-induced accumulation of mRNA and protein of AtNCED3 is observed in the vascular tissues, but not in guard cells. It remains unclear whether the guard cell itself contributes to the change in ABA levels in response to environmental signals. In this study, our results showed that CYP707A1 was induced in guard cells in response to high humidity (Fig. 5), and the cyp707a1 mutant, but not cyp707a3 mutant, showed the ABA-hypersensitive stomatal closure when applied exogenous ABA was applied to epidermal peels (Fig. 6). These results indicate that CYP707A1 has an essential role for ABA catabolism in the guard cell. It is also interesting to note that ABA responsiveness in stomata is reportedly shown to be regulated by environmental conditions as light intensity and quality, atmospheric concentration of CO₂, and by phytohormones other than ABA (for review, see Zeiger, 1983). Some of known ABA-signaling components possibly regulate ABA catabolism in guard cells rather than signaling pathways. Elucidation of cross talk between CYP707A1 and these factors will be the next challenge for fully understanding stomatal movement.

ABA Metabolism in Vegetative Tissues and Its Involvement in Systemic ABA Responses

ABA is a mobile signal and activates various adaptive responses in stressed conditions (Davies et al., 2005; Schachtman and Goodger, 2008). Recent work reported that osmotic- and dehydration-inducible gene expression occurs in a systemic manner (Christmann et al., 2005; Endo et al., 2008). Recent advances in the identification of key regulatory genes enable linkage of the metabolic and signaling pathways of ABA action. Nevertheless, it remains unclear at the molecular levels how ABA is transported and how it acts in particular tissues/cell types. Identification of the cell type that synthesizes and inactivates ABA provides an insight into the nature of ABA transport and its dynamic mode of action in a plant system.

Recent studies indicated that major ABA pools are supplied mainly by vascular tissues in dehydrated plants. De novo ABA biosynthesis enzymes (AtNCED3, ABA2, and AAO3) are abundantly localized in vascular tissues of shoots during dehydration (Endo et al., 2008). In addition, the early expression of ATHB6, an ABA-inducible gene, is observed in vascular tissues of shoots (Christmann et al., 2005). Interestingly, the expression of ATHB6 is also observed in guard cell after prolonged osmotic stress (Christmann et al., 2005). It is likely that vascular tissues of shoots are the main site of ABA biosynthesis, and ABA is transported to other cells, including guard cells. This study showed that the CYP707A3, the most abundant member of the CYP707A genes, was predominantly expressed in vascular tissues in response to high humidity (Fig. 5, F and G). The prominent effect of the CYP707A3 in transpiration is likely due to inactivation of the major ABA pool in the vascular tissues. We propose that the vascular tissue is the main site for regulating the levels of major ABA pool for mobile ABA. It is also worth mentioning that the pCYP707A1::GUS lines showed an intense staining in guard cells (Fig. 5D), but GUS staining was also observed reproducibly in vascular tissues (Fig. 5C). Therefore, it is likely that the CYP707A1 also play a minor role in regulating bulk ABA levels in vascular tissues.

Our present study suggests that ABA is transferred from vascular tissues to guard cell for controlling stomatal movement. The ABA movement from xylem strands to the guard cells takes place likely through the apoplast, because of the absence of plasmodesmata between guard cells and neighboring cells (Plaeviz and Hepler, 1985). In the future, as microtracer techniques or chemical probe methods become available, it would be important to determine the actual movement of ABA, because ABA levels are determined by not only the balance between synthesis and catabolism, but also by mobilization (Seo and Koshiba, 2002; Davies et al., 2005; Schachtman and Goodger, 2008).

In conclusion, we have characterized the physiological role of ABA 8'-hydroxylase by expression analysis of CYP707As and phenotypic analysis of cyp707a single and double mutants in response to high humidity. Our result indicates that CYP707A3 plays a role in regulating the levels of major ABA pool provided by the vascular tissues, whereas CYP707A1 is essential for ABA catabolism inside guard cells. ABA catabolism in both vascular tissues and guard cells is indispensable for controlling stomatal movement.

	MATERIALS AND METHODS

TOP ABSTRACT RESULTS DISCUSSION MATERIALS AND METHODS LITERATURE CITED

 

Plant Materials and Growth Conditions

The wild-type and all mutant plants were Columbia accession of Arabidopsis (Arabidopsis thaliana). The cyp707a single and double mutants used in this study were described previously (Kushiro et al., 2004; Okamoto et al., 2006). In all experiments except for Figure 1A, cyp707a single mutants used were cyp707a1-1, cyp707a2-1, and cyp707a3-1, whereas cyp707a1-1 cyp707a2-1, cyp707a1-1 cyp707a3-1, and cyp707a2-1 cyp707a3-1 double mutants were used in this study. Plants were grown for 2 weeks on 1% agar plates containing one-half Murashige and Skoog and 0.5% Suc, transferred to soil in 200-mL pots, and grown under 22°C, RH 60% condition. Soil-grown plants were watered every 3 d from the bottom. For high-humidity treatment, 4-week-old plants were transferred from RH 60% to 90% under 22°C by using two humidity-controlled growth chambers (model LPH-350S, NK system, Nihonika Co.).

Measurement of Stomatal Conductance

Stomatal conductance was measured for intact 4-week-old plants of wild type, cyp707a single, and double mutants using Leaf Porometer (Decagon Devices, Inc.) under 22°C, RH 60% condition into growth chamber (model MCP-201CP system, SANYO). Experiments were repeated four times and at least six leaves were measured for each experiment.

Thermal Imaging

Thermal images of nonstress plants were obtained using thermal video system TVS-8500 (Nippon Avionics Co., Ltd.). Temperature range of thermal image was restricted to between 22.0°C and 17.3°C. Plants used in this experiment were grown on soil pot under well-watered condition (22°C, RH 75%) for 4 weeks. Growth chamber used model LHP-350S (NK system, Nihon-ika Co.). Images and leaf surface temperature of plants were analyzed by PE Professional software (Nippon Avionics Co., Ltd.). Experiments were repeated seven times and at least four plants were used for each experiment.

QRT-PCR

Four-week-old plants were grown on soil in pots at 22°C, RH 60%. High-humidity treatment was performed at 22°C, RH 90% condition. Total RNA was isolated using TRIZOL Reagent (Invitrogen) according to manufacturer's protocol. cDNA synthesized from 2 µg of total RNA using QuantiTec reverse transcription kit (QIAGEN) as described previously (Okamoto et al., 2006). QRT-PCR using Taq-Man probe was performed as described previously (Kushiro et al., 2004). The sequences of primers and Taq-Man probe for AtNCED3 and CYP707A1-A4 were used as described in Kushiro et al. (2004). The following primer sets were used: for ABF3, forward primer (5'-TGGTGGGAGTTTGCAGAGAC-3') and reverse primer (5'-TCCCTAACCACACCAGCCCT-3'); for ATHB6, forward primer (5'-GGGCTTGTCGGAGAAGAAGAGAA-3') and reverse primer (5'-CTTCCACCGAGCACGACGGT-3'). For normalization of data, 18S rRNA was used as an internal standard.

Determination of ABA and Its Catabolites

For determination of ABA and its catabolites, approximately 300 mg of plants materials (fresh weight) was used. Deuterium-labeled d₆-ABA purchased from ICON SERVICES and deuterium-labeled d₃-PA, d₃-DPA, d₅-ABA-GE, d₃-neoPA, and d₄-7'OH-ABA (Priest et al., 2006) were used as internal standards. After adding 1 ng of internal standards, extraction and purification of ABA and its catabolites were performed as described previously (Saika et al., 2007). The hormone and metabolites were quantified by liquid chromatography (LC)- (ACQUITY UPLC system, Waters) tandem mass spectrometry (MS/MS; Q-Tof premier, Micromass) system. The LC conditions were same as described previously (Saika et al., 2007). The retention times of the compounds were 9.1 min (DPA and d₃-DPA), 15.5 min (PA and d₃-PA), 16.5 min (ABA-GE and d₅-ABA-GE), 19.2 min (7'OH-ABA and d₄-7'OH-ABA), 20.7 min (neoPA and d₃-neoPA), 23.3 min (d₆-ABA), and 23.6 min (ABA). MS/MS conditions were as follows: Capillary (kV) = 2.6, source temperature (°C) = 80, desolvation temperature (°C) = 400, cone gas flow (l/h) = 0, desolvation gas flow (l/h) = 500, collision energy (eV) = 8.0 (ABA and d₆-ABA), 12.0 (PA and d₃-PA, 7'OH-ABA and d₄-7'OH-ABA, neoPA and d₃-neoPA, ABA-GE and d₅-ABA-GE), 18.0 (DPA, d₃-DPA). MS/MS transitions (m/z) were: 263/153 (ABA), 269/159 (d₆-ABA), 279/139 (PA), 282/142 (d₃-PA), 281/171 (DPA), 284/174 (d₃-DPA), 425/263 (ABA-GE), 430/268 (d₅-ABA-GE), 217/279 (7'OH-ABA), 221/283 (d₄-7'OH-ABA), 279/205 (neoPA), and 282/208 (d₃-neoPA). Data analysis was performed using the spectrometer software (MassLynx v. 4.1, Micromass).

Measurement of Stomatal Aperture

For analysis of intact plants, images of stomatal apertures were obtained by Suzuki's Universal Micro-Printing (SUMP) method using SUMP liquid and SUMP plate B (SUMP Laboratory) as described previously (Tanaka et al., 2005). The copied SUMP images were observed by light microscopy (model BX60, Olympus). For analysis of epidermal peels, abaxial epidermis was peeled from rosettes leaves of 4-week-old plants 6 h after the beginning of morning in 16/8-h light/dark cycles. Epidermal peels were floated on opening buffer (5 mM KCl, 50 µM CaCl₂, and 10 mM MES, pH 6.15) and incubated under light condition for 2 h to open the stomata. Subsequently, the epidermal peels with preopened stomata were transferred to the same buffer containing (+)-S-ABA (Toray Co., Ltd.) and incubated for 2 h to close the stomata. Stomatal image was obtained by light microscopy and the height and width of stomatal aperture were measured with ImageJ 1.37v software (National Institutes of Health).

Generation of Transgenic Plants

For reporter gene analysis, a promoter fragment (for CYP707A1, 2,244 bp; for CYP707A3, 2,776 bp) of the translational start of each CYP707A was amplified by PCR and cloned into pENTR/D-TOPO vector (Invitrogen). The following primer sets were used: for CYP707A1, 5'-CACCTATTTTTCTCCGCTCCGCCAAGTCAATTGA-3' and 5'-TTTTGTTTTTGTTCTTTGATCAGAAAAAAACCCACTTCG-3' for CYP707A3, 5'-CACCCACATAAGCGGGTTCCCTTACT-3' and 5'-TATCTTCGTCTTTTAAACAATGGGAATTCGAAG-3'. The adaptor sites of pENTR/D-TOPO are shown in italic. After nucleotide sequences were checked, the cloned cDNAs were cloned into the binary vector, pGWB3, with a recombination cassette for the expression of GUS-fused protein (Nakagawa et al., 2007). The resulting plasmids were electroporated into Agrobacterium strain GV3101, which was used to transform wild-type Arabidopsis accession Columbia plants by floral-dipping method (Desfeux et al., 2000). Transgenic plants were obtained by selection with kanamycin and hygromycin. GUS staining was performed as described previously (Tan et al., 2003). Approximately 20 T₂-independent transgenic lines were analyzed. Detail analysis was performed using at least between five and 10 T₃ independent transgenic lines.

	ACKNOWLEDGMENTS

 
We thank Drs. Irina Zaharia and Ken Nelson (National Research Council of Canada, Plant Biotechnology Institute) for supplying the deuterated ABA catabolites, Dr. Tsuyoshi Nakagawa (Shimane University) for providing binary vector, pGWB3, Dr. Yusuke Jikumaru (RIKEN Plant Science Centre) for performing the LC-MS/MS experiments, Dr. Tetsuo Kushiro (University of Tokyo) for initial work on characterization of CYP707As in Arabidopsis, Dr. Akira Endo (University of Toronto) for valuable discussion, and Arabidopsis Biological Resource Center, SALK Institute, and the University of Wisconsin for providing the T-DNA-tagged lines.

Received October 3, 2008; accepted November 23, 2008; published November 26, 2008.

	FOOTNOTES

 
¹ This work was supported by the Special Postdoctoral Researcher's Program from RIKEN (to M.O.), Incentive Research Grant from RIKEN (to M.O.), and NSERC Discovery Grant (to E.N.).

The author responsible for the distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Eiji Nambara (eiji.nambara{at}utoronto.ca).

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^* Corresponding author; e-mail eiji.nambara{at}utoronto.ca.

	LITERATURE CITED

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