Molecular Analyses of Transgenic Plants

Expression of a cucumber alanine aminotransferase2 gene improves nitrogen use efficiency in transgenic rice

Background

Rice can absorb less than 40% of applied nitrogen fertilizer, whereas the unabsorbed nitrogen fertilizer may cause environmental problems, such as algal blooms in freshwater and increased production of nitrous oxide, a greenhouse gas which is 300 times more potent than carbon dioxide. Development of nitrogen use efficient (NUE) rice is essential for more environmentally friendly rice production. Recently, NUE rice has been developed by root-specific expression of alanine aminotransferase (AlaAT) gene from barley, a monocot plant. Therefore, we tested the efficacy of AlaAT gene from cucumber in transgenic rice, aiming to provide evidence for the conservation of AlaAT gene function in monocot and dicot.

Results

AlaAT gene from cucumber (CsAlaAT2) has been successfully cloned and constructed on pCAMBIA1300 plant expression vectors under the control of tissue-specific promoter OsAnt1. Agrobacterium tumefaciens-mediated transformation of Indonesian rice cv. Fatmawati using this construct produced 14 transgenic events. Pre-screening of T1 seedlings grown in the agar medium containing low nitrogen concentration identified selected events that were superior in the root dry weight. Southern hybridization confirmed the integration of T-DNA in the selected event genomes, each of them carried 1, 2, or 3 T-DNA insertions. Efficacy assay of three lead events in the greenhouse showed that in general transgenic events had increased biomass, tiller number, nitrogen content, and grain yield compared to WT. One event, i.e., FAM13, showed an increase in yield as much as 27.9% and higher plant biomass as much as 27.4% compared to WT under the low nitrogen condition. The lead events also showed higher absorption NUE, agronomical NUE, and grain NUE as compared to WT under the low nitrogen condition.

Conclusions

The results of this study showed that root-specific expression of cucumber alanine aminotransferase2 gene improved nitrogen use efficiency in transgenic rice, which indicate the conservation of function of this gene in monocot and dicot.

Over-expression of OsPT2 under a rice root specific promoter Os03g01700

Identification of root-specific promoters is a good method to drive root-specific gene expression for nutrient uptake. Constitutive over-expression of OsPT2 may have negative effects on the growth of rice seedlings under high Pi condition. Thus, characterization and utilization of root-specific promoters are critical for genetic breeding. Here, a root-specific promoter (Os03g01700) with a number of specific regulatory elements has been confirmed. Interestingly, cis-regulatory element S449 is significantly enriched in the -1475∼-2013 bp and -1077∼-1475 bp regions of Os03g01700 promoter. The activities of several deletion derivatives of Os03g01700 promoter were analyzed using both transient expression and genetic transformation system. The results showed that the root-specific cis-acting elements might be present in the -2013 bp～-1475 bp and -1077 bp～-561 bp regions of Os03g01700 promoter. To determine the actual effect of root-specific expression of OsPT2, a construction consisting of Os03g01700 promoter and OsPT2 CDS was used to transform rice. Under Pi-sufficient condition, there were a series of symptoms of phosphorus toxicity in the shoots of OsPT2 over-expressing (Ov-OsPT2) seedlings. Under Pi-deficient condition, more soluble Pi was accumulated in the shoots of Ov-OsPT2 seedlings than that in the wild type. Our data provide a candidate root-specific promoter in the breeding of rice with high phosphorus uptake variety.

Reference genes for accurate gene expression analyses across different tissues, developmental stages and genotypes in rice for drought tolerance

BACKGROUND

Quantitative reverse transcription PCR (qRT-PCR) has been routinely used to quantify gene expression level. This technique determines the expression of a target gene by comparison to an internal control gene uniformly expressed among the samples analyzed. The reproducibility and reliability of the results depend heavily on the reference genes used. To achieve successful gene expression analyses for drought tolerance studies in rice, reference gene selection should be based on consistency in expression across variables. We aimed to provide reference genes that would be consistent across different tissues, developmental stages and genotypes of rice and hence improve the quality of data in qRT-PCR analysis.

FINDINGS

Ten candidate reference genes were screened from four ubiquitously expressed gene families by analyzing public microarray data sets that included profiles of multiple organs, developmental stages, and water availability status in rice. These genes were evaluated through qRT-PCR experiments with a rigorous statistical analysis to determine the best reference genes. A ubiquitin isogene showed the best gene expression stability as a single reference gene, while a 3-gene combination of another ubiquitin and two cyclophilin isogenes was the best reference gene combination. Comparison between the qRT-PCR and in-house microarray data on roots demonstrated reliability of the identified reference genes to monitor the differential expression of drought-related candidate genes.

CONCLUSIONS

Specific isogenes from among the regularly used gene families were identified for use in qRT-PCR-based analyses for gene expression in studies on drought tolerance in rice. These were stable across variables of treatment, genotype, tissue and growth stage. A single gene and/or a three gene set analysis is recommended, based on the resources available.

Biofortified indica rice attains iron and zinc nutrition dietary targets in the field

More than two billion people are micronutrient deficient. Polished grains of popular rice varieties have concentration of approximately 2 μg g(-1) iron (Fe) and 16 μg g(-1) zinc (Zn). The HarvestPlus breeding programs for biofortified rice target 13 μg g(-1) Fe and 28 μg g(-1) Zn to reach approximately 30% of the estimated average requirement (EAR). Reports on engineering Fe content in rice have shown an increase up to 18 μg g(-1) in glasshouse settings; in contrast, under field conditions, 4 μg g(-1) was the highest reported concentration. Here, we report on selected transgenic events, field evaluated in two countries, showing 15 μg g(-1) Fe and 45.7 μg g(-1) Zn in polished grain. Rigorous selection was applied to 1,689 IR64 transgenic events for insert cleanliness and, trait and agronomic performances. Event NASFer-274 containing rice nicotianamine synthase (OsNAS2) and soybean ferritin (SferH-1) genes showed a single locus insertion without a yield penalty or altered grain quality. Endosperm Fe and Zn enrichment was visualized by X-ray fluorescence imaging. The Caco-2 cell assay indicated that Fe is bioavailable. No harmful heavy metals were detected in the grain. The trait remained stable in different genotype backgrounds.