Daisuke Fujita

Plant Drought Stress: Effects, Mechanisms and Management

Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in crop yield probably exceeds losses from all other causes, since both the severity and duration of the stress are critical. Here, we have reviewed the effects of drought stress on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in plants. This article also describes the mechanism of drought resistance in plants on a morphological, physiological and molecular basis. Various management strategies have been proposed to cope with drought stress. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant water relations and reduces water-use efficiency. Plants display a variety of physiological and biochemical responses at cellular and whole-organism levels towards prevailing drought stress, thus making it a complex phenomenon. CO2 assimilation by leaves is reduced mainly by stomatal closure, membrane damage and disturbed activity of various enzymes, especially those of CO2 fixation and adenosine triphosphate synthesis. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactive oxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increased diffusive resistance, enhanced water uptake with prolific and deep root systems and its efficient use, and smaller and succulent leaves to reduce the transpirational loss. Among the nutrients, potassium ions help in osmotic adjustment; silicon increases root endodermal silicification and improves the cell water balance. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, and polyols, are crucial to sustain cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberrellins, cytokinin and abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce the adverse effects of water deficit. At molecular levels several drought-responsive genes and transcription factors have been identified, such as the dehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant drought tolerance can be managed by adopting strategies such as mass screening and breeding, marker-assisted selection and exogenous application of hormones and osmoprotectants to seed or growing plants, as well as engineering for drought resistance.

NAL1 allele from a rice landrace greatly increases yield in modern indica cultivars

Significance This work reports discovery of a unique gene important for rice agriculture. A significant yield enhancement in rice modern cultivar was achieved by identification of a gene, SPIKELET NUMBER (SPIKE) in Indonesian rice landrace. The SPIKE increased grain yield of an indica cultivar IR64, which is widely grown in the tropics, over four seasons at the field level and improved plant architecture without changing grain quality or growth period, which are important for regional adaptability. These results indicate finding of SPIKE will be extremely valuable for contributing to increase grain production of indica rice cultivars. Increasing crop production is essential for securing the future food supply in developing countries in Asia and Africa as economies and populations grow. However, although the Green Revolution led to increased grain production in the 1960s, no major advances have been made in increasing yield potential in rice since then. In this study, we identified a gene, SPIKELET NUMBER (SPIKE), from a tropical japonica rice landrace that enhances the grain productivity of indica cultivars through pleiotropic effects on plant architecture. Map-based cloning revealed that SPIKE was identical to NARROW LEAF1 (NAL1), which has been reported to control vein pattern in leaf. Phenotypic analyses of a near-isogenic line of a popular indica cultivar, IR64, and overexpressor lines revealed increases in spikelet number, leaf size, root system, and the number of vascular bundles, indicating the enhancement of source size and translocation capacity as well as sink size. The near-isogenic line achieved 13–36% yield increase without any negative effect on grain appearance. Expression analysis revealed that the gene was expressed in all cell types: panicles, leaves, roots, and culms supporting the pleiotropic effects on plant architecture. Furthermore, SPIKE increased grain yield by 18% in the recently released indica cultivar IRRI146, and increased spikelet number in the genetic background of other popular indica cultivars. The use of SPIKE in rice breeding could contribute to food security in indica-growing regions such as South and Southeast Asia.

Rice Resistance to Planthoppers and Leafhoppers

For over 50 years, host-plant resistance has been regarded as an efficient method to reduce yield losses to rice caused by delphacid and cicadelid hoppers. Already a number of resistant rice varieties have been developed and deployed throughout Asia. To date, over 70 hopper resistance genes have been identified in rice; however, less than 10 genes have been deliberately introduced to commercial rice varieties. Currently, due to recent brown planthopper (Nilaparvata lugens [Stål]) and whitebacked planthopper (Sogatella furcifera [Horvath]) outbreaks occurring at an unprecedented scale, researchers are working toward a second generation of resistant varieties using newly identified gene loci and applying new molecular breeding methods. This paper reviews advances in the identification of resistance genes and QTLs against hoppers in rice. It collates all published information on resistance loci and QTLs against the major rice planthoppers and leafhoppers and presents information on gene locations, genetic markers, differential varieties, and wild rice species as sources of resistance. The review indicates that, whereas progress in the identification of genes has been rapid, considerable tidying of the information is required, especially regarding gene nomenclature and resistance spectra. Furthermore, sound information on gene functioning is almost completely lacking. However, hopper responses to resistance mechanisms are likely to be similar because a single phenotyping technique has been applied by most national and international breeding programs during germplasm screening. The review classifies genes occurring at two chromosome regions associated with several identified resistance loci and highlights these (Chr4S: BphR-R and Chr12L: BphR-R) as general stress response regions. The review calls for a greater diversity of phenotyping methods to enhance the durability of resistant varieties developed using marker-aided selection and emphasizes a need to anticipate the development of virulent hopper populations in response to the field deployment of genes.

Mapping and pyramiding of two major genes for resistance to the brown planthopper (Nilaparvata lugens [Stål]) in the rice cultivar ADR52

The brown planthopper (BPH), Nilaparvata lugens (Stål), is one of the most serious and destructive pests of rice, and can be found throughout the rice-growing areas of Asia. To date, more than 24 major BPH-resistance genes have been reported in several Oryza sativa ssp. indica cultivars and wild relatives. Here, we report the genetic basis of the high level of BPH resistance derived from an Indian rice cultivar, ADR52, which was previously identified as resistant to the whitebacked planthopper (Sogatella furcifera [Horváth]). An F2 population derived from a cross between ADR52 and a susceptible cultivar, Taichung 65 (T65), was used for quantitative trait locus (QTL) analysis. Antibiosis testing showed that multiple loci controlled the high level of BPH resistance in this F2 population. Further linkage analysis using backcross populations resulted in the identification of BPH-resistance (antibiosis) gene loci from ADR52. BPH25 co-segregated with marker S00310 on the distal end of the short arm of chromosome 6, and BPH26 co-segregated with marker RM5479 on the long arm of chromosome 12. To characterize the virulence of the most recently migrated BPH strain in Japan, preliminary near-isogenic lines (pre-NILs) and a preliminary pyramided line (pre-PYL) carrying BPH25 and BPH26 were evaluated. Although both pre-NILs were susceptible to the virulent BPH strain, the pre-PYL exhibited a high level of resistance. The pyramiding of resistance genes is therefore likely to be effective for increasing the durability of resistance against the new virulent BPH strain in Japan.

Molecular mapping of a novel gene, Grh5, conferring resistance to green rice leafhopper (Nephotettix cincticeps Uhler) in rice, Oryza sativa L.

The green rice leafhopper (GRH), Nephotettix cincticeps Uhler, is one of the most serious insect pests affecting cultivated rice (Oryza sativa L.) in temperate regions of East Asia. An accession of the wild rice species, Oryza rufipogon Griff. (W1962), was found to be highly resistant to GRH by an antibiosis test. To understand the genetic basis of the GRH resistance, a BC1F1 population derived from a cross between a susceptible Japonica variety, Taichung 65 (T65), and a highly resistant accession W1962 was analyzed by quantitative trait loci (QTL) mapping. A single major QTL for GRH resistance was detected on rice chromosome 8. A nearly isogenic population containing segments of the targeted QTL region derived from W1962 was then developed through advanced backcrossing with marker-assisted selection. Further molecular mapping using a BC4F2 population revealed that a new resistance gene, designated as Green rice leafhopper resistance 5 (Grh5), was located on the distal region of the long arm of chromosome 8 and tightly linked to the simple sequence repeat markers RM3754 and RM3761. A nearly isogenic line (NIL) carrying Grh5 was subsequently developed in the progeny of the mapping population. The resistance level of Grh5-NIL was compared with those of developed NILs for GRH resistance and was found to have the highest resistance. The DNA markers found to be closely linked to Grh5 would be useful for marker-assisted selection for the improvement of resistance to GRH in rice.

qEMF3, a novel QTL for the early-morning flowering trait from wild rice, Oryza officinalis, to mitigate heat stress damage at flowering in rice, O. sativa

Highlight qEMF3, a novel QTL for the early-morning flowering trait to mitigate heat-induced spikelet sterility at flowering in rice, was identified using a wild rice, Oryza officinalis, as a genetic resource.

Quantitative Trait Loci Mapping for Leaf Length and Leaf Width in Rice cv. IR64 Derived Lines

The present study was conducted to identify quantitative trait loci (QTLs) for leaf size traits in IR64 introgression lines (INLs). For this purpose, selected F(2) populations derived from crosses between recurrent parent IR64 and its derived INLs, unique for leaf length and leaf width, were used to confirm QTLs. A total of eight QTLs, mapped on three chromosomes, were identified for the four leaf size traits in six F(2) populations. A QTL for leaf length, qLLnpt-1, in HKL69 was identified around simple sequence repeat (SSR) marker RM3709 on chromosome 1. Two QTLs for flag leaf length, qFLLnpt-2 and qFLLnpt-4, in HFG39 were indentified on chromosomes 2 and 4, respectively. For flag leaf width, a QTL, qFLWnpt-4, in HFG39 was identified around RM17483 on chromosome 4. While another QTL for flag leaf width, qFLWnpt-1, in HFG27 was identified around RM3252 on chromosome 1. A QTL for leaf width, qLWnpt-2, in HKL75 was identified around RM7451 on chromosome 2. For leaf width, two QTLs, qLWnpt-4a, qLWnpt-4b, in HKL48 and HKL99 were identified around RM7208 and RM6909, respectively on chromosome 4. Results from this study suggest the possibilities to use marker-assisted selection and pyramiding these QTLs to improve rice water productivity.

Characterization of near-isogenic lines carrying QTL for high spikelet number with the genetic background of an indica rice variety IR64 (Oryza sativa L.)

Total spikelet number per panicle (TSN) is one of the most important traits associated with rice yield potential. This trait was assessed in a set of 334 chromosomal segment introgression lines (ILs: BC3-derived lines), developed from new plant type (NPT) varieties as donor parents and having the genetic background of an indica-type rice variety IR64. Among the 334 ILs, five lines which had different donor parents and showed significantly higher TSN than IR64 were used for genetic analysis. Quantitative trait locus (QTL) analysis was conducted using F2 populations derived from crosses between IR64 and these ILs. As a result, a QTL for high TSN (one from each NPT donor variety) was detected on common region of the long arm of chromosome 4. The effect of the QTL was confirmed by an increase in TSN of five near-isogenic lines (NILs) developed in the present study. The variation in TSN was found among these NILs, attributing to the panicle architecture in the numbers of primary, secondary and tertiary branches. The NILs for TSN and the SSR markers linked to the TSN QTLs are expected to be useful materials for research and breeding to enhance the yield potential of rice varieties.

Responses and adaptation by Nephotettix virescens to monogenic and pyramided rice lines with Grh-resistance genes

The green leafhopper, Nephotettix virescens (Distant) (Hemiptera: Cicadellidae), occasionally damages rice in Asia either directly, by feeding on the host phloem, or indirectly by transmitting tungro virus. We assessed the nature of resistance against the leafhopper in monogenic and pyramided near‐isogenic rice lines containing the resistance genes Grh2 and Grh4. Only the pyramided line was resistant to leafhopper damage. Leafhopper nymphs and adults had high mortality and low weight gain when feeding on the pyramided line and adults laid few eggs. In contrast, although there was some minor resistance in 45‐day‐old plants that possessed either Grh2 or Grh4 genes, the monogenic lines were generally as susceptible to the leafhopper as the recurrent parent line Taichung65 (T65). Resistance in the pyramided line was stable as the plant aged and under high nitrogen, and affected each of five Philippine leafhopper populations equally. Furthermore, in a selection study, leafhoppers failed to adapt fully to the pyramided resistant line: nymph and adult survival did improve during the first five generations of selection and attained similar levels as on T65, but egg‐laying failed to improve over 10 generations. Our preliminary results suggested that resistance was associated with physiological costs to the plants in some experiments. The results of this study demonstrate the success of pyramiding resistance genes through marker‐assisted breeding, to achieve a strong and potentially durable resistance. We discuss the utility of gene pyramiding and the development of near‐isogenic lines for leafhopper management.

Fine mapping of a quantitative trait locus for spikelet number per panicle in a new plant type rice and evaluation of a near-isogenic line for grain productivity

qTSN12.1 and qTSN12.2 genes for total spikelet number per panicle were detected, qTSN12.2 on rice chromosome 12. Grain yield of a near-isogenic line carrying fine-mapped qTSN12.2 was increased by 18–36%.