Annals of Botany 91: 263-270, 2002
© 2002 Annals of Botany Company
Regulation of Submergence-induced Enhanced Shoot Elongation in Oryza sativa L.
,11 Department of Molecular Genetics, Ghent University (RUG-VIB), Ledeganckstraat 35, B-9000 Ghent, Belgium
* For correspondence. Fax 32-9-2645333, e-mail dostr{at}gengenp.rug.ac.be Present address: CropDesign NV, B-9052 Zwijnaarde, Belgium
Received: 28 October 2001; Returned for revision: 17 December 2001; Accepted: 1 February 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMOLECULAR ASPECTS OF HORMONE...MOLECULAR ASPECTS OF GROWTHCONCLUSIONS AND PERSPECTIVESLITERATURE CITED |
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Rice (Oryza sativa L.) is the only cereal that can be cultivated in the frequently flooded river deltas of South-East and South Asia. The survival strategies used by rice have been studied quite extensively and the role of several phytohormones in the elongation response has been established. Deep-water rice cultivars can diminish flooding stress by rapid elongation of their submerged tissues to keep up with the rising waters. Other rice cultivars may react by mechanisms of submergence tolerance. Aerenchyma and aerenchymatous adventitious roots are formed that facilitate oxygen diffusion to prevent anaerobic conditions in the submerged tissues. This paper discusses the molecular aspects of the mechanism that leads to shoot elongation (leaves of seedlings and internodes), the regulation of which involves metabolism of, and interactions between, ethylene, gibberellins and abscisic acid. Finally, the importance of new techniques in future research is assessed. Current molecular technology can reveal subtle differences in gene activity between tolerant and non-tolerant cultivars, and identify genes that are involved in the regulation of submergence avoidance and tolerance.
Key words: Review, rice, submergence, flooding, Oryza sativa, hypoxia, ethylene, gibberellin, abscisic acid.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMOLECULAR ASPECTS OF HORMONE...MOLECULAR ASPECTS OF GROWTHCONCLUSIONS AND PERSPECTIVESLITERATURE CITED |
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Flooding is a severe threat for survival of terrestrial plants, mainly because it severely impedes the gas diffusion rate. This leads, in submerged tissues, to a decline of the partial pressure of oxygen, whereas in general, the levels of both carbon dioxide and the gaseous phytohormone ethylene increase (Musgrave et al., 1972; Jackson, 1985; Armstrong et al., 1994). This paper discusses the mechanistic and molecular responses of the monocotyledonous species Oryza sativa L. to survive flooding. Rice ecosystems can be classified in four different groups: irrigated lowland, rain-fed lowland, upland and flood-prone. Each ecosystem can be occupied by different rice cultivars adapted to the local conditions. Deep-water rice and particularly floating rice, a subtype of deep-water rice, is cultivated in coastal river deltas in South and South-East Asia, areas that can be flooded during the monsoon season for more than 1 month. Deep-water rice responds to partial submergence with a stem elongation up to 25 cm/day to avoid complete submergence (Vergara et al., 1976). Besides enhanced shoot elongation to avoid inundation, rice possesses additional traits that contribute to its tolerance to submerged conditions and concomitant hypoxic conditions. Activation of
-amylases and the mobilization of starch to produce energy for maintenance of basic metabolic processes under hypoxic or anoxic conditions has been implicated as one of the major metabolic adaptations of rice plants under water (Setter et al., 1997; Dennis et al., 2000). In addition, a relatively low activity of lactate dehydrogenase, and therefore decreased cytoplasmic acidosis, might contribute to submergence tolerance (reviewed by Perata and Alpi, 1993). In addition, morphological adaptations occur. Rice grows well in flooded soils due to the ventilation efficiency that is acquired by formation of air spaces within the tissue to improve the exchange of gases between the submerged plant parts and the atmosphere. Moreover, submergence induces the formation of adventitious roots mediated by ethylene that also appeared to facilitate aerenchyma formation (Justin and Armstrong, 1991). The yield from deep-water rice cultivation is lower, and the culinary properties are inferior to those of irrigated lowland rice types. Therefore, the availability of lowland species that avoid submergence by a delimited shoot elongation in combination with submergence tolerance could be advantageous in agriculture. Excessive elongation of lowland rice would, however, be an undesirable trait, since tall plants might lodge when the water recedes. The understanding of the mechanisms that are used by plants to survive flooding might contribute to future biotechnological improvements of rice varieties. This paper describes some molecular aspects of leaf and stem elongation of rice. The biosynthesis and role of one of the key regulators of the cell elongation process, the gaseous phytohormone ethylene, and its interactions with gibberellins (GAs) and abscisic acid (ABA) are discussed.
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MOLECULAR ASPECTS OF HORMONE BIOSYNTHESIS AND INTERACTIONS |
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Submergence-induced internode elongation and the maximum length that adult plants can reach varies amongst the different rice cultivars (Keith et al., 1986). The highest growth levels are displayed by floating rice varieties that are cultivated in areas with occasional flood-water levels of up to several metres (Sauter, 2000). Survival of floating rice depends on its ability to keep abreast of the rising waters because total submergence of plants for more than 1 week results in heavy crop losses (Vergara et al., 1976). Growth occurs at the youngest internode containing an intercalary meristem that is competent to respond to submergence signals. This meristem is located just above the second node and it releases cells into the elongation zone above (Kende et al., 1998). The growth response is mediated by at least three different hormones, ethylene, GA and ABA. Today, the proposed sequence of events upon submergence is that decreased gas diffusion and low oxygen concentration lead to increased ethylene concentration which causes a decrease in ABA level, an increase of GA1 concentration, and an increase of responsiveness to GA, with enhanced internodal elongation as a result (Raskin and Kende, 1984a; Kende et al., 1998). A comparable hormonal interaction regulates enhanced leaf growth upon complete submergence, which appeared to be common to both, lowland and deep-water rice seedlings (Van Der Straeten et al., 2001).
Ethylene biosynthesis and action in submerged leaves
Ethylene biosynthesis of seedlings is enhanced by hypoxic conditions (Satler and Kende, 1985) that arise under water due to a decrease in the efficiency of photosynthesis as a result of limited gas exchange and light intensity under water (Jackson, 1985). In addition, entrapment of gases cause the ethylene concentration to increase which can, in turn, induce its own biosynthesis due to positive feedback of the hormone (Yang and Hoffman, 1984; Jackson et al., 1987; Chae et al., 2000). Ethylene biosynthesis is regulated by tissue- and inducer-specific ACC synthase genes (ACS; Fig. 1). In 8-d-old seedlings growing under anaerobic conditions, OS-ACS1 was shown to be shoot specific and OS-ACS3 was root specific (Zarembinski and Theologis, 1993). Upon complete submergence of 9-d-old seedlings (Fig. 3A), the OS-ACS5 gene was expressed in the shoot. OS-ACS5 mRNA accumulates within 1 h after submergence to a level that is much higher than OS-ACS1 transcript levels (Van Der Straeten et al., 2001; Zhou et al., 2001). OS-ACS5 mRNA is localized in specific tissues and cells both during normal development and in response to complete submergence. The temporal and spatial regulation of OS-ACS5 expression was analysed by in situ hybridization and histochemical analysis of ß-glucuronidase (GUS) activity in transgenic rice carrying an OS-ACS5–GUS fusion (Z. Zhou and D. Van Der Straeten, unpubl. res.).
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Whole-mount in situ hybridization (WISH) revealed that in air-grown rice seedlings OS-ACS5 was expressed at a low level in the shoot apex, meristems, leaf and adventitious root primordia, and in vascular tissues of non-elongated stems and leaf sheaths. In response to complete submergence, the expression in vascular bundles of young stems and leaf sheaths was strongly induced. The results of histochemical GUS assays (Fig. 2) were consistent with those found by WISH. Our findings suggest that OS-ACS5 plays a role in vegetative growth of rice under normal conditions and is also recruited for enhanced growth upon complete submergence. With respect to cell type specificity, OS-ACS5 expression was mainly associated with longitudinal vascular tissues. Parenchymatous cells surrounding the differentiating vascular bundle showed GUS activity in young leaf tissues, which disappeared in the differentiated mestome sheath cells that envelop the mature vascular bundles of leaf sheath (Fig. 2B). Expression in the thin-walled xylem parenchyma cells that form the envelope of the protoxylem lacunae might indicate that ethylene plays a role in the formation of the lacunae by inducing the destruction of the protoxylem vessel during internode elongation.
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Programmed cell death also takes place in the cortex of adventitious roots to form aerenchyma. In maize this process is ethylene and hypoxia induced (Drew et al., 2000) and, in some rice varieties, ethylene also has a stimulatory effect on aerenchyma formation (Justin and Armstrong, 1991; Kawai et al., 1998). GUS-expression was also observed in xylem parenchyma cells between the xylem and phloem (Fig. 2B). These cells are connected with plasmodesmata and with phloem parenchyma cells that may imply that the xylem parenchyma cells absorb solute from the transpiration stream and transfer it to neighbouring cells (Chonan, 1993).
Under water in the absence of transpiration, long-distance mass flow of water in aquatic plants is controlled by root pressure (Pedersen, 1993; Pedersen et al., 1997). Based on what is known in several submerged aquatic species, it can be assumed that water flux, controlled by root pressure, also occurs in fully submerged rice plants. The spatial correlation of OS-ACS5 expression with conducting tissues may therefore be directly related to ACC transport. Although there is ample evidence for the importance of ACC transport in root–shoot signalling of flooded tomato plants (English et al., 1995), it remains unclear if this process is involved in the adaptive response of rice to submergence. A careful analysis of ACC flux rates in conducting tissues could provide a clue whether changes in ACC transport occur and influence downstream processes. Because OS-ACS5 expression was subjected to a positive feedback control by ACC/ethylene (Van Der Straeten et al., 2001), ACC transport might further enhance the signal over the vascular system. OS-ACS5 might therefore play a prominent role in ethylene production under water, especially during the first few hours after submergence. Together with an enhanced ACC synthase gene activity and increased ACC levels, also ACC oxidase activity in vitro and ethylene emanation from de-submerged seedlings increased (Van Der Straeten et al., 2001). Applying exogenous ACC (20 µM) to submerged rice seedlings seemed to increase their life span, an assumption that was based upon a significant slower decrease of the chlorophyll content of the leaves under water in a lowland cultivar, IR36 (Van Der Straeten et al., 2001). The oxygen concentration in the flooding water increased during the light period in the experimental conditions used, indicating that oxidation of ACC to ethylene is possible. However, the induction of some senescence-associated genes was comparable in ACC-treated and untreated plants, and did not reflect the differences in chlorophyll content of the tissues (W. Vriezen and D. Van Der Straeten, unpubl. res.).
Ethylene biosynthesis and its role in internode elongation
Métraux and Kende (1983) showed that ethylene treatment of a deep-water rice variety stimulated growth of the internodes of plants older than 28 days. In contrast, the growth of a non-deep-water variety was not induced by this treatment. During submergence, endogenous ethylene concentration appeared to rise to 1 µl l–1, a concentration sufficiently high to induce growth, also when exogenously applied to air-grown plants. This increase was attributed to a combination of accumulation due to entrapment and an enhanced capacity to produce ethylene by internode sections (measured by accumulation after de-submergence). The role of ethylene was emphasized by the inhibition of ethylene biosynthesis by AVG (aminoethoxyvinylglycine) that prevented growth induction of partially submerged deep-water rice (Métraux and Kende, 1983). Upon submergence, in vivo ACC synthase activity (ACC accumulation in a nitrogen atmosphere) increased within 2 h and reached a maximum concentration after 4 h. An eight-fold and five-fold increase was found compared with the basal level (grown in air) in the intercalary meristem and the elongation zone above it, respectively (Fig. 3B) (Cohen and Kende, 1987). Also low oxygen concentration <13 % (v/v) appeared to induce ACC synthesis, especially in the node, in the intercalary meristem and in the elongation zone.
Cloning of the ethylene biosynthesis genes encoding ACC synthase (Zarembinski and Theologis, 1993, 1997; Van Der Straeten et al., 1997, 2001) and ACC oxidase (Mekhedov and Kende, 1996; Chae et al., 2000) made it possible to study the regulation of ethylene biosynthesis on a molecular level. In elongating deep-water rice internodes of 8- to 12-week-old plants (Fig. 3B), OS-ACS1 mRNA concentration is relatively low in the intercalary meristem compared with its concentration in the rest of the internode. Within 12 h of partial submergence the expression level of OS-ACS1 increased five-fold in the intercalary meristem and elongation zone after which it remained at a level comparable to that in the rest of the internode (Zarembinski and Theologis, 1997). This response was also observed at an oxygen concentration of 2·5 % or less. The ACC concentration increased, however, before an increase was observed of the OS-ACS1 messenger concentration, suggesting that other ACC synthase genes might play a role in submergence-induced ACC production. Alternatively, ACC oxidase enzyme activity might be restricted under the experimental conditions. PCR with OS-ACS2, 3, 4 and 5 specific primers and cDNA synthesized of RNA from each zone of internodes showed, however, that there were no significant messenger levels of the other genes present after 24 h of partial submergence (Zarembinski and Theologis, 1997). The gene specific primers were based upon DNA sequences from lowland rice (cv. IR36), whereas in the mentioned experiments DNA from deep-water rice (cv. Habiganj Aman II) was used as a template. This might explain why Van Der Straeten et al. (2001) did find expression of the OS-ACS5 gene in stem sections using the 3'-terminal part of the OS-ACS5 coding sequence for hybridization of RNA gel blots. OS-ACS5 proved to be expressed one order of magnitude higher than OS-ACS1 in submerged stem sections of adult plants as was found in 9-d-old seedling leaves. In addition, OS-ACS5 gene activity was induced within 1 h of submergence, suggesting that it might play a fundamental role in the early internodal ACC production as described above (Cohen and Kende, 1987; Van Der Straeten et al., 2001). The transcript levels of OS-ACO1, the ACC oxidase gene expressed in internodes, reached a maximum after 15 h of submergence in the intercalary meristem, elongation zone and differentiation zone of the internodes (Mekhedov and Kende, 1996). In vitro ACC oxidase enzyme activity also increased upon submergence in these internode zones. Although the highest mRNA levels were found in the intercalary meristem, a relatively weak induction of the enzyme activity was measured in this tissue compared with the rest of the internode (Mekhedov and Kende, 1996). This suggests that in the intercalary meristem translational regulation or enzymatic inhibition influence the in vitro ACC oxidase enzyme activity.
The role of ABA and GA
During submergence, ABA levels decrease in the intercalary meristem and in the cell elongation zone of deep-water rice, a process that also could be induced by ethylene treatment (Azuma et al., 1995; Hoffmann-Benning and Kende, 1992). Conversely, applying ABA to submerged deep-water rice stem sections strongly inhibited cell elongation. This effect of ABA could be reversed by GA, indicating that the balance of ABA and GA, regulated by ethylene, modulates submergence-induced growth (Hoffmann Benning and Kende, 1992). A comparable ABA–GA interaction was found in 9-d-old seedlings concerning submergence-induced leaf elongation. Treatment with ABA inhibited leaf growth of 9-d-old submerged deep-water rice seedlings (Van Der Straeten et al., 2001). In addition, submergence of 9-d-old seedlings caused a strong decrease in shoot ABA concentration within 4 h and, at the same time, an increase of the bioactive gibberellin GA20. Under hypoxic conditions, exogenously applied GA up-regulates and ABA down-regulates OS-ACS5 activity in both lowland and deep-water rice seedlings (Van Der Straeten et al., 2001). This might function as a feedback regulatory system of the ethylene–ABA–GA cascade in vivo. In addition, ethylene causes a decrease in ABA levels (Hoffmann Benning and Kende, 1992) suggesting an environmental-dependent balance in the concentration of the three hormones. However, it remains to be proven that the basal endogenous ABA concentration is high enough to inhibit cell elongation or OS-ACS5 gene transcription. Inhibition of GA biosynthesis repressed internode and leaf elongation under water, which showed that GA is required for growth of adult plants (Raskin and Kende, 1984b). The time-lag between GA treatment and growth induction is shorter than the time-lag between ethylene treatment and growth induction. Thus, ethylene exerts its effect through GA, probably by inducing GA biosynthesis or a shift in sensitivity to GA (Raskin and Kende, 1984a, b; Sauter and Kende, 1992).
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TOPABSTRACTINTRODUCTIONMOLECULAR ASPECTS OF HORMONE...MOLECULAR ASPECTS OF GROWTHCONCLUSIONS AND PERSPECTIVESLITERATURE CITED |
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Cell division
Internode growth was shown to be caused by elongation of the meristematic cells during the first 2 h of GA treatment. Cells that moved from the meristematic region through the elongation zone were finally two and a half times longer than fully elongated cells in internodes before the start of the GA treatment (Sauter and Kende, 1992). In addition, Sauter and Kende (1992) showed that the percentage of nuclei of meristematic cells in the S phase and G2 phase increased after GA treatment indicating an enhanced cell division rate. This process was probably initiated by GA-induced transcription of cyclin genes (cyc2Os1 and 2) and cdc2Os-2, which encodes a p34cdc2-like histone H1 kinase (Sauter et al., 1995). The serine/threonine p34cdc2 histone H1 kinase plays a central role in the regulation of cell division in eukaryotes (Nurse, 1990).
Cell elongation
Besides enhanced cell division, cell elongation gives rise to the physical increase of internode length. Cell expansion in one direction is a highly coordinated process that involves regulation of microtubule orientation, creep of cell wall polymers and biosynthesis, transport and incorporation of new cell wall components (Cosgrove, 1997).
Cortical microtubules are generally oriented transversely to the direction of cell expansion, providing a cytoplasmic template determining the axis of cell growth (Green, 1980). The membrane-associated cytoskeleton is thought to channel the movement of intramembranous cellulose-synthesizing complexes in the plasma membrane, thereby causing cellulose to be deposited parallel to the microtubules. The direction of cell expansion can be influenced through changing the alignment of the cortical microtubules by many factors, amongst which are the hormones auxin, ABA, GA and ethylene (Lloyd and Seagull, 1985; Shibaoka, 1991). In newly formed epidermal cells in the intercalary meristem of rice internodes, the orientation of cellulose microfibrils is transversal, perpendicular to the direction of growth. The microfibril orientation changes to oblique when the cells reach the end of the elongation phase (Sauter et al., 1993). Assuming that an oblique to longitudinal orientation of cellulose microfibrils inhibits elongation (Green, 1980), it is possible that because of their faster growth, GA-treated cells can reach a greater length before the cellulose microfibrils are in the oblique position. The oblique orientation of microtubules and cellulose microfibrils from cells that were already displaced from the intercalary meristem was not changed by subsequent GA treatment, indicating that GA cannot induce microtubule reorientation (Sauter et al., 1993). It is also possible that GA stabilizes cortical microtubules in the transverse orientation perhaps due to association of the microtubules with the plasma membrane and to the association of the plasma membrane with the cell wall (Shibaoka, 1993).
Brassinosteroids (BRs) play a role in the formation of the intercalary meristem and in the longitudinal growth of internode cells (Yamamuro et al., 2000). A rice dwarf mutant, d61, mutated in OsBRI1, encoding a putative BR receptor kinase, showed a random orientation of microtubules in cells of the first elongating internode, whereas in wild-type plants the microtubules in cells of elongating internodes were arranged perpendicular to the direction of elongation (Yamamuro et al., 2000). The d1 mutant, which is of the same type as the d61 mutant (dm-type or specific retardation of the second internode) is mutated in a gene encoding a G -protein (Ueguchi et al., 2000). In this mutant the GA-signalling pathway is partially impaired, in particular concerning the induction of -amylase and the induction of GA-inducible genes in the aleurone and the internode elongation. The phenotype of d1 is completely suppressed in a d1/slr homozygous double mutant (Ueguchi et al., 2000). The slr mutant is caused by the loss of function of a rice GAI (GA-Insensitive) or RGA homologue, which are negative regulators of GA signaling (Peng et al., 1997; Silverstone et al., 1998), indicating that D1 functions above SLR in the same GA signalling pathway. GAI and RGA are proteins that belong to the GRAS family of plant transcriptional regulators (Peng et al., 1997). A mutated arabidopsis allele of GAI, gai, conferred dominant partial insensitivity to GA (Koornneef et al., 1985). Recently, a GA-insensitive transgenic rice line was created by introducing the mutated form of the arabidopsis GAI gene into Basmati 370 rice, a tall species with weak culms (Peng et al., 1999). The level of dwarfism was shown to be related with the strength of the expression of the gai transgene (Fu et al., 2001). Conclusively, it can be hypothesized that the BR and GA signalling pathways are interacting as they both regulate cell elongation in the second internode.
BRs were shown to induce rice xyloglucan endotransglycosylase genes (OsXET1 and 3) in a comparable pattern as GA (Uozu et al., 2000). XET mediates cross-linking of the cell wall cellulose microfibrils by internally cleaving xyloglucan polymers and ligating the newly formed reducing ends to other xyloglucan chains (Nishitani and Tominaga, 1992; Vissenberg et al., 2000). OsXET2 and 4 mRNA levels were at a constant level in elongating internodes, whereas OsXET1 and 3 expression was induced by BR and GA in the elongating and divisional zones in the internodes (Uozu et al., 2000). The exact role of XETs is not fully understood, but they are thought to have different functions in processes like wall loosening (Xu et al., 1995), softening (Redgwell and Fry, 1993) and lysis of cell walls (Saab and Sachs, 1996). In addition, XET activity might be necessary for the integration of new xyloglucan chains during elongation and cell wall formation.
Expansin genes are differentially expressed in the major vegetative parts of rice plants, and their messengers are most abundant in actively growing organs. They can be grouped into two families, the -expansins and the ß-expansins (Cosgrove, 2000). -Expansins are the main catalytic proteins responsible for acid-induced cell wall relaxation that are activated by low pH. They appeared to loosen the cell wall by disrupting noncovalent adhesion of wall matrix polysaccharides to one another or to cellulose microfibrils (McQueen-Mason et al., 1992; McQueen-Mason and Cosgrove, 1994; Shcherban et al., 1995). The rice genome contains more than 26 -expansin genes (Lee et al., 2001) and four -expansin genes, Os-EXP1 to Os-EXP4, are expressed in a tissue-specific manner and their expression pattern is correlated with growth (Cho and Kende, 1997a). Messenger levels of OS-EXP4 increased upon GA treatment and submergence before an increase in internodal growth could be observed. Low pH dependent extensibility of native cell walls of internode segments correlated positively to the OS-EXP4 messenger level. Moreover, -expansin mRNA and protein levels were high in the intercalary meristem and elongation zone (Cho and Kende, 1997a). Acid-induced growth, mediated by -expansins was shown to be restricted to cell walls derived from growing cells indicating that, besides the presence of -expansins, also the susceptibility of the wall to these proteins plays a crucial role (Cho and Kende, 1997b). Mature cell walls may be rigidified by different mechanisms, as for instance de-esterification of pectins or cross-linking of phenolic groups (reviewed by Cosgrove, 1997). The role of ß-expansins in vegetative tissues is largely unknown; however, wall-loosening activity has been observed in monocots (Cosgrove et al., 1997). Recently it was shown that five ß-expansin genes are expressed in the elongating region of rice internodes and that their expression was induced by treatment with GA3 (Lee and Kende, 2001; Lee et al., 2001).
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Rice internode elongation has been under study for many years. Much knowledge has been gained about hormone biosynthesis pathways and their role during submergence of rice. Ethylene, in combination with hypoxia, induces growth due to an increase in GA concentration and sensitivity, and a concomitant decrease in ABA level (Kende et al., 1998; Van Der Straeten et al., 2001). The degree of the induction of growth, cell division and expansion, is one of the differences between rice cultivars that elongate fast during submergence and those with a less-pronounced response. The physiological processes that determine whether enhanced growth will occur are likely specific interactions between hormones and especially the increase of gibberellin sensitivity and concentration at high ethylene concentration. Flooded sensitive terrestrial species show the opposite response to ethylene, as was observed in Rumex acetosa (Rijnders et al., 1997). The molecular basis of the ethylene–GA interaction seems therefore crucial together with the mechanisms that regulate hormone sensitivity. However, these mechanisms are not fully understood. In the case of ethylene, responsiveness to this hormone is modulated in several species and appeared to be tissue specific and developmentally regulated (Porat et al., 1995; Voesenek et al., 1997; Lashbrook et al., 1998; Sato-Nara et al., 1999). Although much is known about the signalling pathway of ethylene and many signalling components have been isolated (reviewed by Stepanova and Ecker, 2000), the mechanism behind enhanced ethylene sensitivity remains to be elucidated.
Leaf and internodal elongation are processes that occur in all rice species during development of the foliage. Tolerance to temporary complete submergence and maintenance of growth under water are traits indispensable for survival. The metabolic adaptations that form the basis of these traits, however, are not specific for flooding tolerant cultivars. It is the strength of the responses that seem to make the difference. Therefore, it is not very likely that one rice cultivar possesses a specific set of genes. It is more likely that differences in properties of signalling proteins, like transcription factors, kinases and phosphatases, are responsible for the specific gene regulation or interactions between hormone pathways. The challenge for the future is to identify these flooding responsive genes. New molecular biological approaches to quantitatively display the activity of virtually all genes from a species promises the identification of genes involved in tolerance and avoidance of submergence. The rice genome has been sequenced by Monsanto Corporation (St Louis, MO, USA; http://www.monsanto.com) and is being sequenced by an international consortium organized in the International Rice Genome sequencing Project (IRGSP; http://www.rgp.dna. affrc.go.jp/rgp/rgpintro.html) and might soon be released by a Chinese consortium. It is likely that it is only a matter of time before micro-arrays will be available with all rice genes. The availability of these molecular data will definitely establish rice as the monocotyledonous model system as is Arabidopsis thaliana for the dicotyledonons. Furthermore, reproducible PCR-based cDNA AFLP (Vos et al., 1995) can be used to display and quantify all mRNAs that are present in a certain tissue at any moment. Knockout and over-expression experiments with the identified genes have to be performed to analyse the gene function in detail and also the relationship with the physiological response of the transformed rice compared with wild type. Finally, it will be possible, to unravel the mechanism that initiates the response, which is of crucial importance, as it determines the fundamental difference between being flooding tolerant or flooding sensitive.
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ACKNOWLEDGEMENT |
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This work was sponsored by the European Union, contract number: RTN1-1999-00086
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