Os06g0205100

Please input one-sentence summary here.

Annotated Information

Abiotic stress; Flavonoid pathway; Rice; Secondary metabolism; Transcription factors Molecular basis of regulation of abiotic stress responses and the flavonoid biosynthesis in rice was investigated. The role of the regulatory gene OsC1-Myb, encoding a MYB class of transcription activators in the stress-induced expression of the structural genes, OsDfr and OsAns, was analyzed. Northern analysis of shoot tissues of rice, Nagina 22, (Oryza sativa L. sub sp. indica) seedlings under dehydration stress or high salt or abscisic acid (ABA) showed a significant enhancement of transcript level and/or transcript stability of OsDfr and OsAns. Enhanced levels of the OsC1-myb transcript were also detected. The expression pattern of these three genes indicates that the stress responsive accumulation of OsDfr and OsAns transcripts is mediated by the transcription factor, OsC1-MYB. The 5′ upstream region of the OsDfr and OsAns genes carry several regulatory domains, which share homology with some of the known stress responsive genes in plants. In addition, several putative myb and myc responsive domains were identified in the promoter region of the genes, OsDfr and OsAns. The recombinant OsC1-MYB protein binds in vitro to the myb responsive elements (MREs) in the OsDfr and OsAns promoters, suggesting that it is a potential transcription activator of stress-induced expression of structural genes of the flavonoid p

Expression

Plants inherit different adaptive mechanisms to acclimate to abiotic stress situations through a cascade of events that begin with stress perception and end with expression of a wide spectrum of stress responsive genes and gene products, and eventually stress tolerant phenotype. Recent studies in model plants suggest that different biochemical pathways appear to share common elements that are associated with adaptation to a range of stress factors [1] and [2]. However, only in selected plant species such pathways of stress response have been rigorously analyzed. Further, association between the enhanced expression of genes belonging to these pathways and the stress tolerance phenotype, for instance, drought tolerance, is not yet established in any major crop plant. Molecular dissection of the regulatory elements governing expression, temporally and spatially, of a host of the genes in model crop plants such as rice will help in elucidation of the connection between different genes and pathways, and the stress tolerance phenotype.

Earlier studies in many higher plants revealed a multitude of biochemical pathways associated with stress responses, though the exact relationships between many of them are not obvious from the data. Two such well-studied pathways are the abscisic acid (ABA)-mediated abiotic stress response pathway and the ubiquitous flavonoid biosynthetic pathway. The ABA-mediated stress response pathway has been thoroughly investigated in the model plant Arabidopsis [3] but very little is known about it in other agronomically important crop species, including rice. One common element between the ABA mediated pathway and the flavonoid pathway is the role of the MYB and MYC class of transcription activators on target gene expression [3]. In Arabidopsis, the Atmyb2 and rd22BP1 genes regulate rd22 gene expression in the ABA-dependent abiotic stress response pathway [3]. In maize, product of the C1-myb gene regulates expression of the flavonoid pathway genes in cooperation with a MYC family protein encoded by the R gene [4] and [5]. However, not much is known about the drought and salt stress responsive expression of the genes belonging to the flavonoid pathway, though this has been one of the most thoroughly investigated pathways in higher plants that is reported to be stress responsive [6]. The flavonoid biosynthetic pathway genes have been well-characterized and assigned with functions in two major cereal crop plants such as maize and rice [7], [8] and [9]. The rice flavonoid pathway is reported to respond to biotic and abiotic stress situations. Flavonoid accumulation in response to UV-B [10], cold [11], and drought [12] were reported earlier. All these reports are based mainly on phenotypic or chemical analysis and almost nothing is known about the flavonoid gene expression under stress situations.

In the present study, we have investigated the response of the flavonoid biosynthetic pathway genes to dehydration, high salt and ABA treatments, and the role of the transcription factor, the OsC1-MYB, in regulating the stress responsive gene expression in rice. We have chosen rice as a model plant for valid reasons. Drought and salinity tolerance in rice are two of the most important agronomic traits, and unraveling the genetic and molecular basis of them is of paramount importance in genetic improvement of this crop for saline and water limited environments. Most importantly, with the availability of the complete genome sequence, saturated genetic maps and good EST coverage, rice serves as a great model crop plant to study stress response mechanisms at the genetic and molecular level. Further, the flavonoid biosynthetic pathway has been one of the most thoroughly investigated pathways in plants and the products have been used extensively as visible markers in genetics and breeding experiments. This pathway is amenable for investigation in rice because of several advantages: availability of cDNA clones for all major genes of the pathway, a wealth of literature on gene expression and regulation, an array of specific mutants, clearly visible phenotypes and well-established extraction and purification protocols. Molecular dissection of the regulatory elements and the mechanisms by which they regulate the expression, temporally and spatially, of a host of genes belonging to diverse but functionally connected pathways will help in elucidation of the genetic basis of abiotic stress response in rice.

Evolution

Rice lines were originally obtained from the Tamil Nadu Agricultural University, Coimbatore, India and the Directorate of Rice Research, Hyderabad, India and maintained in either green house or fields. These were repeatedly selfed before using in stress experiments. The lines belonging to all three classes [10], i.e., cyanic (Purple Puttu and R27), moderately cyanic (G962), and acyanic (Nagina 22, Prasanna and Hamsa) were screened for the accumulation of anthocyanins under stress treatment. The rice line Nagina 22 was proved suitable for detailed studies because it accumulates pigments in seed pericarp indicating that the line is genetically competent to produce anthocyanins [13]. Moreover, this line is known to be drought tolerant [14] and does not accumulate any visible pigmentation in vegetative tissues, thus providing a low basal level against which changes during various stress treatments could be scored. Rice seeds were imbibed in water, surface sterilized with 5% (v/v) sodium hypochlorite for 5 min, thoroughly washed with sterile water and germinated in water upon filter papers in dark. Two-day-old germinated seeds were transferred to growth chambers and were supplemented with 1/2 strength MS medium [15] without agar. The average temperature during seedling culture was 28±1 °C and a photoperiod of 18 h light and 6 h dark was maintained. Seedling trays were examined twice daily to maintain constant moisture content. Ten-day-old seedlings were subjected to either dehydration stress (using 20% (w/v) polyethylene glycol (PEG 8000)) or high salt stress (using 150 mM sodium chloride) or 100 μM ABA (mixed isomers, Sigma, USA) solution prepared in 1/2 strength MS medium. Control plants received only 1/2 strength MS medium. The treatments were given 3 h after beginning of light period. Samples were collected at different intervals up to 48 h after treatment.

Labs working on this gene

Molecular basis of regulation of abiotic stress responses and the flavonoid biosynthesis in rice was investigated. The role of the regulatory gene OsC1-Myb, encoding a MYB class of transcription activators in the stress-induced expression of the structural genes, OsDfr and OsAns, was analyzed. Northern analysis of shoot tissues of rice, Nagina 22, (Oryza sativa L. sub sp. indica) seedlings under dehydration stress or high salt or abscisic acid (ABA) showed a significant enhancement of transcript level and/or transcript stability of OsDfr and OsAns. Enhanced levels of the OsC1-myb transcript were also detected. The expression pattern of these three genes indicates that the stress responsive accumulation of OsDfr and OsAns transcripts is mediated by the transcription factor, OsC1-MYB. The 5′ upstream region of the OsDfr and OsAns genes carry several regulatory domains, which share homology with some of the known stress responsive genes in plants. In addition, several putative myb and myc responsive domains were identified in the promoter region of the genes, OsDfr and OsAns. The recombinant OsC1-MYB protein binds in vitro to the myb responsive elements (MREs) in the OsDfr and OsAns promoters, suggesting that it is a potential transcription activator of stress-induced expression of structural genes of the flavonoid pathway.

References

Nagabhushana Ithal;Arjula R. Reddy