Oxygen deficit alleviates phosphate overaccumulation toxicity in OsPHR2 overexpression plants

Sourcing DNA parts for synthetic biology applications in plants

Transgenic approaches are now standard in plant biology research aiming to characterize gene function or improve crops. Recent advances in DNA synthesis and assembly make constructing transgenes a routine task. What remains nontrivial is the selection of the DNA parts and optimization of the transgene design. Early career researchers and seasoned molecular biologists alike often face difficult decisions on what promoter or terminator to use, what tag to include, and where to place it. This review aims to inform about the current approaches being employed to identify and characterize DNA parts with the desired functionalities and give general advice on basic construct design. Furthermore, we hope to share the excitement about new experimental and computational tools being developed in this field.

Molecular mechanisms and genetic improvement of low-phosphorus tolerance in rice

Phosphorus (P) is a macronutrient required for plant growth and reproduction. Orthophosphate (Pi), the preferred P form for plant uptake, is easily fixed in the soil, making it unavailable to plants. Limited phosphate rock resources, low phosphate fertilizer use efficiency and high demands for green agriculture production make it important to clarify the molecular mechanisms underlying plant responses to P deficiency and to improve plant phosphate efficiency in crops. Over the past 20 years, tremendous progress has been made in understanding the regulatory mechanisms of the plant P starvation response. Here, we systematically review current research on the mechanisms of Pi acquisition, transport and distribution from the rhizosphere to the shoot; Pi redistribution and reuse during reproductive growth; and the molecular mechanisms of arbuscular mycorrhizal symbiosis in rice (Oryza sativa L.) under Pi deficiency. Furthermore, we discuss several strategies for boosting P utilization efficiency and yield in rice.

Root epidermis-specific expression of a phosphate transporter TaPT2 enhances the growth of transgenic Arabidopsis under Pi-replete and Pi-depleted conditions

Although attempts to improve the phosphate (Pi) uptake and use efficiency by constitutively overexpressing phosphate transporters have resulted in enhanced Pi or total phosphorous contents, growth promotion by Pi acquisition was observed in only a few cases. This study examined the effect of the tissue-specific overexpression of phosphate transporter on Pi acquisition and plant growth. We cloned cDNA for a wheat phosphate transporter, TaPT2, using PCR and confirmed its Pi transport activity in Arabidopsis suspension cells. The overexpression of TaPT2 by the Arabidopsis Shaker family inward rectifying potassium channel 1 (AKT1) promoter, dominantly expressed in root epidermal cells, resulted in increased root and shoot growth of transgenic Arabidopsis under Pi-replete and Pi-depleted conditions. However, their Pi and total P contents did not increase. The overexpression of TaPT2 by the constitutive promoter, actin8 (ACT8), increased shoot total P contents in transgenic plants, but did not promote their growth. These results suggested that enhanced Pi uptake in root epidermal cells is suitable as a driving force for Pi transport from roots to shoots, improving subsequent Pi use in shoots. Thus, the root epidermal cell-specific expression of TaPT2 may be a simple and promising strategy for enhancing plant Pi uptake and efficiency.

Transgenic expression of rice OsPHR2 increases phosphorus uptake and yield in wheat

Oryza sativa PHOSPHATE RESPONSE2 (OsPHR2) can promote the uptake and use of phosphorus (P) in rice. We introduced OsPHR2 into the winter wheat (Triticum aestivum L.) variety "Zhengmai0856." OsPHR2 was integrated into the wheat genome with two copy numbers and could be correctly transcribed and expressed. OsPHR2 was mainly expressed in the leaves at the seedling stage. From the jointing to filling stage, OsPHR2 was mainly expressed in the roots, followed by the leaves, with a low expression level in detected the tassels and stems. The transgenic lines exhibited higher P accumulation at each growth stage and increased P uptake intensity in comparison to the wild type under low P and high P conditions. Analysis of the root characteristics showed that the transgenic expression of OsPHR2 increased the maximum root length, total root length, root-to-shoot ratio, and root volume under the conditions of P deficiency or low P. A field experiment showed that the transgenic lines had a higher grain yield than the wild type under low P and high P conditions. The yield of the transgenic lines increased by 6.29% and 3.73% on average compared with the wild type under low P and high P conditions, respectively. Thus, the transgenic expression of OsPHR2 could increase P uptake and yield in wheat, but the effect was more prominent under low P conditions.

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.

Positive interactions of major-effect QTLs with genetic background that enhances rice yield under drought

To improve the grain yield of the lowland-adapted popular rice variety Samba Mahsuri under reproductive-stage drought (RS) and to understand the interactions between drought QTLs, two mapping populations were developed using marker-assisted selection (MAS) and marker-assisted recurrent selection (MARS). The mean grain yield of pyramided lines (PLs) with qDTY _2.2  + qDTY _4.1 in MAS is significantly higher under RS and irrigated control than lines with single QTLs. Among MARS PLs, lines with four qDTYs (qDTY _1.1  + qDTY _2.1  + qDTY _3.1  + qDTY _11.1 ) and two QTLs (qDTY _1.1  + qDTY _11.1 ) yielded higher than PLs with other qDTY combinations. The selected PLs showed a yield advantage of 0.3-2.0 t ha^-1 under RS. An allelic profile of MAS PLs having same qDTY combination but with different yields under drought was studied. Hierarchical clustering grouped together the selected lines with high yield under drought. Epistasis test showed the interaction of qDTY _4.1 and qDTY _9.1 loci with qDTY _7.1 significantly increased yield under drought and all the lines with higher yield under drought possessed the conserved region of qDTY _7.1 on chromosome 7. The positive interactions among QTLs, effectiveness of QTLs in different backgrounds, introgression of DTY QTLs together with resistance to biotic stresses shall help enhance grain yield under RS.

Molecular interaction between PHO2 and GIGANTEA reveals a new crosstalk between flowering time and phosphate homeostasis in Oryza sativa

Plants are often confronted to nutrient limiting conditions, such as inorganic phosphate (Pi) deficiency, resulting in a reduction in growth and yield. PHO2, encoding a ubiquitin-conjugating E2 enzyme, is a central component of the Pi-starvation response signalling pathway. A yeast-two-hybrid screen using Oryza sativa (rice) PHO2 as bait, revealed an interaction between OsPHO2 and OsGIGANTEA, a key regulator of flowering time, which was confirmed using bimolecular fluorescence complementation (BiFC). Characterization of rice Osgi and Ospho2 mutants revealed that they displayed several similar phenotypic features supporting a physiological role for this interaction. Reduced growth, leaf tip necrosis, delayed flowering and over-accumulation of Pi in leaves compared to wild type were shared features of Osgi and Ospho2 plants. Pi analysis of individual leaves demonstrated that Osgi, similar to Ospho2 mutants, were impaired in Pi remobilization from old to young leaves, albeit to a lesser extent. Transcriptome analyses revealed more than 55% of the genes differentially expressed in Osgi plants overlapped with the set of differentially expressed genes in Ospho2 plants. The interaction between OsPHO2 and OsGI links high-level regulators of Pi homeostasis and development in rice.

Mutation of OsGIGANTEA Leads to Enhanced Tolerance to Polyethylene Glycol-Generated Osmotic Stress in Rice

Water deficit is one of the most important environmental stresses limiting plant growth and crop yield. While the identification of many key factors involved in the plant water deficit response has greatly increased our knowledge about the regulation system, the mechanisms underlying dehydration tolerance in plants are still not well understood. In our current study, we investigated the roles of the key flowering time regulator, OsGIGANTEA (OsGI), in the osmotic stress tolerance in rice. Results showed that mutation of OsGI conferred tolerance to osmotic stress generated by polyethylene glycol (PEG), increased proline and sucrose contents, and accelerated stomata movement. In addition, qRT-PCR and microarray analysis revealed that the transcript abundance of some osmotic stress response genes, such as OsDREB1E, OsAP37, OsAP59, OsLIP9, OsLEA3, OsRAB16A, and OsSalT, was significantly higher in osgi than in WT plants, suggesting that OsGI might be a negative regulator in the osmotic stress response in rice.