Xue Qiang Zhao

A phosphate starvation response regulator Ta-PHR1 is involved in phosphate signalling and increases grain yield in wheat.

Background and Aims Phosphorus deficiency is a major limiting factor for crop yield worldwide. Previous studies revealed that PHR1 and it homologues play a key role in regulating the phosphate starvation response in plants. However, the function of PHR homologues in common wheat (Triticum aestivum) is still not fully understood. The aim of the study was to characterize the function of PHR1 genes in regulating phosphate signalling and plant growth in wheat. Methods Wheat transgenic lines over-expressing a wheat PHR1 gene were generated and evaluated under phosphorus-deficient and -sufficient conditions in hydroponic culture, a soil pot trial and two field experiments. Key Results Three PHR1 homologous genes Ta-PHR1-A1, B1 and D1 were isolated from wheat, and the function of Ta-PHR1-A1 was analysed. The results showed that Ta-PHR1-A1 transcriptionally activated the expression of Ta-PHT1.2 in yeast cells. Over-expressing Ta-PHR1-A1 in wheat upregulated a subset of phosphate starvation response genes, stimulated lateral branching and improved phosphorus uptake when the plants were grown in soil and in nutrient solution. The data from two field trials demonstrated that over-expressing Ta-PHR1-A1 increased grain yield by increasing grain number per spike. Conclusions TaPHR1 is involved in phosphate signalling in wheat, and was valuable in molecular breeding of crops, with improved phosphorus use efficiency and yield performance.

Major quantitative trait loci for seminal root morphology of wheat seedlings

Vigorous early root growth at seedling stage has been shown to be important for efficient acquisition of nutrients in wheat (Triticum aestivum L.). Identifying quantitative trait loci (QTL) for early root growth can facilitate the selection of wheat varieties with efficient nutrient use. A recombinant inbred line population derived from two Chinese wheat varieties, Xiaoyan 54 and Jing 411, was grown hydroponically at seedling stage. The maximum root length (MRL), primary root length (PRL), lateral root length (LRL), total root length (TRL), and root tip number (RN) of seminal roots were measured using the WinRHIZO Root Analyser. Numerous QTL for the investigated root traits were detected with QTL numbers varying from two to six, depending on the traits. Among them, two loci had major effects on primary (MRL and PRL) and lateral (LRL and RN) root parameters, respectively. The QTL (namely qTaLRO-B1) between Xgwm210 and Xbarc1138.2 on chromosome 2B explained 68.0 and 59.0% of phenotypic variations in MRL and PRL, respectively; the major QTL between Xgwm570 and Xgwm169.2 on chromosome 6A explained 30.5 and 24.5% of phenotypic variations in LRL and RN, respectively. These two major loci showed linkage with previous reported QTL for yield component and nutrient uptake. Detailed analysis of qTaLRO-B1 indicated that the positive allele of qTaLRO-B1 showed dominance over the negative allele, which showed impairment in primary root elongation. The existence of major QTL for root trait and their linkage with agronomic traits and nutrient uptake will facilitate the design of root morphology for better yield performance and efficient nutrient use.

Auxin biosynthetic gene TAR2 is involved in low nitrogen-mediated reprogramming of root architecture in Arabidopsis

In plants, the plasticity of root architecture in response to nitrogen availability largely determines nitrogen acquisition efficiency. One poorly understood root growth response to low nitrogen availability is an observed increase in the number and length of lateral roots (LRs). Here, we show that low nitrogen-induced Arabidopsis LR growth depends on the function of the auxin biosynthesis gene TAR2 (tryptophan aminotransferase related 2). TAR2 was expressed in the pericycle and the vasculature of the mature root zone near the root tip, and was induced under low nitrogen conditions. In wild type plants, low nitrogen stimulated auxin accumulation in the non-emerged LR primordia with more than three cell layers and LR emergence. Conversely, these low nitrogen-mediated auxin accumulation and root growth responses were impaired in the tar2-c null mutant. Overexpression of TAR2 increased LR numbers under both high and low nitrogen conditions. Our results suggested that TAR2 is required for reprogramming root architecture in response to low nitrogen conditions. This finding suggests a new strategy for improving nitrogen use efficiency through the engineering of TAR2 expression in roots.

Responses of two rice cultivars differing in seedling-stage nitrogen use efficiency to growth under low-nitrogen conditions

Demand for low-input nitrogen sustainable rice is increasing to meet the need for environmentally friendly agriculture and thus development of rice with high nitrogen use efficiency (NUE) is a major objective. Hence, understanding how rice responds to growth under low-nitrogen conditions is essential to devise new ways of manipulating genes to improve rice NUE. In this study, using two rice varieties with different seedling-stage NUE obtained from previous field experiments, we investigated the physiological and molecular responses of young rice to low-nitrogen conditions. Our results suggest that glutamine synthetase (GS) and NADH-dependent glutamate synthase (NADH-GOGAT) play important roles in N assimilation of seedling rice roots under low-nitrogen conditions; the regulatory mechanisms of GS and NADH-GOGAT in seedling rice roots do not occur at the transcription level, and may be posttranscriptional; OsAMT1;1 play important roles in rice N acquisition by partially regulating N uptake under low-nitrogen conditions; and OsAMT1;1 and OsNRT2;1 also play important roles in rice N acquisition by partially regulating root growth and development under low-nitrogen conditions. The challenge for future studies is to characterize the functional roles of GS, NADH-GOGAT, OsAMT1;1, and OsNRT2;1 in young rice NUE using RNAi and mutant techniques.

A Wheat CCAAT Box-Binding Transcription Factor Increases the Grain Yield of Wheat with Less Fertilizer Input

The transcription factor TaNFYA-B1 is up-regulated by low-nitrogen and low-phosphorus treatment in wheat seedlings, and overexpressing this gene increases the grain yield of wheat under differing nitrogen and phosphorus supply levels. Increasing fertilizer consumption has led to low fertilizer use efficiency and environmental problems. Identifying nutrient-efficient genes will facilitate the breeding of crops with improved fertilizer use efficiency. This research performed a genome-wide sequence analysis of the A (NFYA), B (NFYB), and C (NFYC) subunits of Nuclear Factor Y (NF-Y) in wheat (Triticum aestivum) and further investigated their responses to nitrogen and phosphorus availability in wheat seedlings. Sequence mining together with gene cloning identified 18 NFYAs, 34 NFYBs, and 28 NFYCs. The expression of most NFYAs positively responded to low nitrogen and phosphorus availability. In contrast, microRNA169 negatively responded to low nitrogen and phosphorus availability and degraded NFYAs. Overexpressing TaNFYA-B1, a low-nitrogen- and low-phosphorus-inducible NFYA transcript factor on chromosome 6B, significantly increased both nitrogen and phosphorus uptake and grain yield under differing nitrogen and phosphorus supply levels in a field experiment. The increased nitrogen and phosphorus uptake may have resulted from the fact that that overexpressing TaNFYA-B1 stimulated root development and up-regulated the expression of both nitrate and phosphate transporters in roots. Our results suggest that TaNFYA-B1 plays essential roles in root development and in nitrogen and phosphorus usage in wheat. Furthermore, our results provide new knowledge and valuable gene resources that should be useful in efforts to breed crops targeting high yield with less fertilizer input.

Characterization of root response to phosphorus supply from morphology to gene analysis in field-grown wheat

The adaptations of root morphology, physiology, and biochemistry to phosphorus supply have been characterized intensively. However, characterizing these adaptations at molecular level is largely neglected under field conditions. Here, two consecutive field experiments were carried out to investigate the agronomic traits and root traits of wheat (Triticum aestivum L.) at six P-fertilizer rates. Root samples were collected at flowering to investigate root dry weight, root length density, arbusular-mycorrhizal colonization rate, acid phosphatase activity in rhizosphere soil, and expression levels of genes encoding phosphate transporter, phosphatase, ribonucleases, and expansin. These root traits exhibited inducible, inhibitory, or combined responses to P deficiency, and the change point for responses to P supply was at or near the optimal P supply for maximum grain yield. This research improves the understanding of mechanisms of plant adaptation to soil P in intensive agriculture and provides useful information for optimizing P management based on the interactions between soil P dynamics and root processes.

Phosphorus Enhances Al Resistance in Al-resistant Lespedeza bicolor but not in Al-sensitive L. cuneata Under Relatively High Al Stress

BACKGROUND AND AIMS Aluminium (Al) toxicity and phosphorus (P) deficiency often co-exist in acidic soils and limit crop production worldwide. Lespedeza bicolor is a leguminous forage species that grows very well in infertile, acidic soils. The objective of this study was to investigate the effects of Al and P interactions on growth of Lespedeza and the distributions of Al and P in two different Al-resistant species, and to explore whether P can ameliorate the toxic effect of Al in the two species. METHODS Two species, Lespedeza bicolor and L. cuneata, were grown for 30 d with alternate Al and P treatments in a hydroponics system. Harvested roots were examined using a root-system scanner, and the contents of Al, P and other nutrient elements in the plants were determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Haematoxylin staining was used to observe the distribution of Al in the roots of seedlings. After pre-culture with or without P application, organic acids in the exudates of roots exposed to Al were held in an anion-exchange resin, eluted with 2 m HCl and then analysed using high-performance liquid chromatography (HPLC). KEY RESULTS Lespedeza bicolor exhibited a stronger Al resistance than did L. cuneata; Al exclusion mechanisms may mainly be responsible for resistance. P application alleviated the toxic effect of Al on root growth in L. bicolor, while no obvious effects were observed in L. cuneata. Much less Al was accumulated in roots of L. bicolor than in L. cuneata after P application, and the P contents in both roots and shoots increased much more for L. bicolor than for L. cuneata. Lespedeza bicolor showed a higher P/Al ratio in roots and shoots than did L. cuneata. P application decreased the Al accumulation in root tips of L. bicolor but not in L. cuneata. The amount of Al-induced organic acid (citrate and malate) exudation from roots pre-cultured with P was much less than from roots without P application; no malate and citrate exudation was detected in L. cuneata. CONCLUSIONS P enhanced Al resistance in the Al-resistant L. bicolor species but not in the Al-sensitive L. cuneata under relatively high Al stress, although P in L. cuneata might also possess an alleviative potential. Enhancement of Al resistance by P in the resistant species might be associated with its more efficient P accumulation and translocation to shoots and greater Al exclusion from root tips after P application, but not with an increased exudation of organic acids from roots.

Haplotype analysis of the genes encoding glutamine synthetase plastic isoforms and their association with nitrogen-use- and yield-related traits in bread wheat.

Glutamine synthetase (GS) plays a key role in the growth, nitrogen (N) use and yield potential of cereal crops. Investigating the haplotype variation of GS genes and its association with agronomic traits may provide useful information for improving wheat N-use efficiency and yield. We isolated the promoter and coding region sequences of the plastic glutamine synthetase isoform (GS2) genes located on chromosomes 2A, 2B and 2D in bread wheat. By analyzing nucleotide sequence variations of the coding region, two, six and two haplotypes were distinguished for TaGS2-A1 (a and b), TaGS2-B1 (a-f) and TaGS2-D1 (a and b), respectively. By analyzing the frequency data of different haplotypes and their association with N use and agronomic traits, four major and favorable TaGS2 haplotypes (A1b, B1a, B1b, D1a) were revealed. These favorable haplotypes may confer better seedling growth, better agronomic performance, and improved N uptake during vegetative growth or grain N concentration. Our data suggest that certain TaGS2 haplotypes may be valuable in breeding wheat varieties with improved agronomic performance and N-use efficiency.

The Nitrate-Inducible NAC Transcription Factor TaNAC2-5A Controls Nitrate Response and Increases Wheat Yield

A nitrate-inducible transcription factor affects nitrate responses that enable wheat to yield more with less fertilizer. Nitrate is a major nitrogen resource for cereal crops; thus, understanding nitrate signaling in cereal crops is valuable for engineering crops with improved nitrogen use efficiency. Although several regulators have been identified in nitrate sensing and signaling in Arabidopsis (Arabidopsis thaliana), the equivalent information in cereals is missing. Here, we isolated a nitrate-inducible and cereal-specific NAM, ATAF, and CUC (NAC) transcription factor, TaNAC2-5A, from wheat (Triticum aestivum). A chromatin immunoprecipitation assay showed that TaNAC2-5A could directly bind to the promoter regions of the genes encoding nitrate transporter and glutamine synthetase. Overexpression of TaNAC2-5A in wheat enhanced root growth and nitrate influx rate and, hence, increased the root’s ability to acquire nitrogen. Furthermore, we found that TaNAC2-5A-overexpressing transgenic wheat lines had higher grain yield and higher nitrogen accumulation in aerial parts and allocated more nitrogen in grains in a field experiment. These results suggest that TaNAC2-5A is involved in nitrate signaling and show that it is an exciting gene resource for breeding crops with more efficient use of fertilizer.

Altered cell wall properties are responsible for ammonium‐reduced aluminium accumulation in rice roots

The phytotoxicity of aluminium (Al) ions can be alleviated by ammonium (NH4(+)) in rice and this effect has been attributed to the decreased Al accumulation in the roots. Here, the effects of different nitrogen forms on cell wall properties were compared in two rice cultivars differing in Al tolerance. An in vitro Al-binding assay revealed that neither NH4(+) nor NO3(-) altered the Al-binding capacity of cell walls, which were extracted from plants not previously exposed to N sources. However, cell walls extracted from NH4(+)-supplied roots displayed lower Al-binding capacity than those from NO3(-)-supplied roots when grown in non-buffered solutions. Fourier-transform infrared microspectroscopy analysis revealed that, compared with NO3(-)-supplied roots, NH4(+)-supplied roots possessed fewer Al-binding groups (-OH and COO-) and lower contents of pectin and hemicellulose. However, when grown in pH-buffered solutions, these differences in the cell wall properties were not observed. Further analysis showed that the Al-binding capacity and properties of cell walls were also altered by pHs alone. Taken together, our results indicate that the NH4(+)-reduced Al accumulation was attributed to the altered cell wall properties triggered by pH decrease due to NH4(+) uptake rather than direct competition for the cell wall binding sites between Al(3+) and NH4(+).