Deepwater Rice Cultivar Research Articles

Molecular characterization of submergence tolerance genes and locus in the deep-water rice cultivars

Most of the rice cultivars exhibit suspension of growth when submerged to overcome the reduced availability of oxygen. When the situation continues, majority of the cultivars unable to recover after the flood recedes. However, there are fortunately some rice genotypes that can withstand such submerged condition for up to two weeks by adapting two totally opposite mechanisms. One type of cultivars elongates enormously at a very short span of time and the leaves come above the water level. In the second type, they remain under water without any growth. Cultivars of both types tolerate the submergence but the first category easily lodges when flood water recede. In those lines, yields are reduced drastically. In this study, we focus on characterize the genetic variation at the Sub1 locus and to associate its relevance, if any, to submergence tolerance among the deep water landraces. As a first step, seeds of some rice cultivars collected from North-east Indian regions were initially selected for the characterization of genetic variation. The PCR based analysis involving several genes known to be associated with submergence tolerance did not reveal much difference. However, Southern hybridization revealed certain differences between submergence tolerant and susceptible cultivars. Although we did not notice major difference with regard to Sub1 genes when tried with EcoRI and BamHI, differences were noticed with adh1 and RAmy3C genes. Representative, Southern analysis showed the genetic variation among the deep-water cultivars as compared to Swarna and Sub1-Swarna. It is possible that deep-water rice cultivars may not differ in their genome at Sub1 locus but they respond through SNORKEL genes under submergence.

QTL mapping of sheath blight resistance in a deep-water rice cultivar

Sheath blight, caused by the pathogen Rhizoctonia solani Kuhn, is one of the most serious diseases of rice and leads to severe yield loss worldwide. A recombinant inbred line (RIL) population consisting of 121 lines was constructed from a cross between HH1B and RSB03, the latter of which is a deep-water rice variety. Five traits were used to evaluate sheath blight resistance, namely disease rating (DR), lesion length (LL), lesion height (LH), relative lesion length [RLL, the ratio of LL to plant height (PH)], and relative LH (RLH, the ratio of LH to PH). Using the RIL population and 123 molecular markers, we identified 28 quantitative trait loci (QTLs) for the five traits in two environments. These QTLs are located on nine chromosomes and most of them are environment specific. A major QTL for DR (qSBR1) on chromosome 1 was identified with contributions of 12.7% at Shanghai and 42.6% at Hainan, and it collocated with a QTL for PH. The allele at this locus from RSB03 enhances sheath blight resistance and increases PH. Another QTL for DR on chromosome 7 was adjacent to QTLs for heading date (HD) and four other disease traits. RSB03 also carries the resistant allele at this locus and shortens HD. The susceptible parent, HH1B, provides the resistance allele at the locus qSBR8, where QTLs for four other disease traits were identified. QTL mapping results showed that most QTLs for LL, LH, RLL, and RLH are collocated with QTLs for DR. Three QTLs for DR are independent from HD, PH, and four other disease traits, while four QTLs are closely related to HD and PH. Four QTLs for LL, LH, RLL, and RLH are independent from DR, HD, and PH, while there is only one region harboring QTLs for these four traits and HD. Correlation analysis and QTL mapping results indicated that LL, LH, RLL, and RLH might be important indices, like DR, for evaluating the level of resistance to rice sheath blight.

Phenotypic diversity of panicle and nutritional traits of some widely cultivated indigenous deep-water rice (Oryza sativa L.) cultivars of Dhemaji district, Assam (India)

Deep-water rice shows some morphological and nutritional peculiarities. To evaluate the phenotypic diversity in panicle and nutritional traits, ten widely cultivated indigenous deepwater rice cultivars (viz. Panikakuwa , Kakuwa , Miabao , Dalbao , Maguribao , Negheribao , Panidhan , Bhubao , Amana and Happybao ) collected from different paddy growing fields of Dhemaji district, Assam, have been taken into study. Kernel colour of the cultivars varies from white to dark-purple. Six cultivars have been identified as high protein cultivars (>10%). Amylose content of the selected cultivars ranges from 18.73% to 34.51%. Dalbao has shown the highest total carbohydrate (88.4%), starch (83.25%) and protein (13.37%) content along with highest length of the panicle (33.6 cm), whereas cultivar Bhubao shows highest amylose content (34.51). Dalbao has also been found to possess several characters of rice with ideal panicle type. Protein content shows negative correlation with amylose content (–0.5011). Dalbao , having superior nutritional quality and panicle traits may be considered for inclusion in deep-water rice breeding programmes.

Mapping of three QTLs that regulate internode elongation in deepwater rice

The internodes of deepwater rice can elongate in response to rises in water level. This unique character allows deepwater rice to survive severe flooding during the monsoon season in South and Southeast Asia. Our previous quantitative trait locus (QTL) analysis of a deepwater rice cultivar (Oryza sativa) detected QTLs on chromosomes 1, 3 and 12. In this study, we produced three nearly isogenic lines (NILs) possessing each of the three QTLs by backcross introduction of each chromosomal region into a non-deepwater rice cultivar. The NILs showed internode elongation under deepwater conditions, and we were able to demonstrate the existence of the QTLs and to evaluate the effect of each QTL. Using progenies of the NILs, we mapped all QTLs between molecular markers. Comparison of the location of the most effective QTL between the rice cultivar (O. sativa) and a wild rice species (O. rufipogon) indicated that the QTL on chromosome 12 is common and is the most important QTL for internode elongation in deepwater condition.

A Major QTL Confers Rapid Internode Elongation in Response to Water Rise in Deepwater Rice

To avoid drowning under flooded conditions, deepwater rice responds to rising water level by rapid internode elongation. Quantitative trait locus (QTL) analysis, using a deepwater rice cultivar (Oryza sativa) and a wild rice species (O. rufipogon) with deepwater characteristics, revealed the presence of major QTLs (qTIL12, qNEI12 and qLEI12) in a common chromosomal region that regulates internode elongation. Genetic analysis revealed that a QTL inherited in a dominant manner, was located on the long arm of chromosome 12. A nearly isogenic line (NIL), produced by backcross introduction of a chromosome fragment carrying this major QTL into non-deepwater rice, exhibited a dramatic internode elongation in response to water rise. This indicates that the difference between deepwater rice and non-deepwater rice is associated with the presence of the QTL, and that this sequence is sufficient to confer deepwater characteristics. Although deepwater rice and non-deepwater rice share a common machinery for internode elongation; non-deepwater rice cannot activate the machinery in response to flooding, unlike deepwater rice which harbors this major QTL.

Genotypic variability in callus induction and plant regeneration through somatic embryogenesis of five deepwater rice (Oryza sativa L.) cultivars of Bangladesh

Experiments were carried out to induce embryogenic callus and plant regeneration from five different deepwater rice cultivars. Using mature embryos as explant, all the cultivars demonstrated high callus induction and plant regeneration frequencies. When coleoptile and root segments used as explants, they had low frequencies of embryogenesis. Depending on different genotypes, the best plant regeneration was obtained on LS (Linsmaier and Skoog, 1965) based medium supplemented with 2 mg l -1 BAP + 1.5 mg l -1 2,4-D. Large variabilities in callus growth and plant regeneration potential were revealed among the cultivars tested. Cultivar HA-8 formed a high frequency (78%) of callus than that of other cultivars. In contrast cv. HA-1 produced the highest percentage (72%) of plant regeneration. The callus growth potential was not correlated with the plant regeneration potential. Coleoptiles and root segments produced calli, which did not develop any shoot bud in regeneration media. Moreover, the calli turned blackish, watery and translucent after 25-28 days of culture. It is clear that mature seed scutellum (MSS) is the best explant for callus induction and plant regeneration.

Regulation of submergence-induced enhanced shoot elongation in Oryza sativa L.

Rice (Oryza sativa L.) is the only cereal that can be cultivated in the frequently flooded river deltas of South-East and South Asia. The survival strategies used by rice have been studied quite extensively and the role of several phytohormones in the elongation response has been established. Deep-water rice cultivars can diminish flooding stress by rapid elongation of their submerged tissues to keep up with the rising waters. Other rice cultivars may react by mechanisms of submergence tolerance. Aerenchyma and aerenchymatous adventitious roots are formed that facilitate oxygen diffusion to prevent anaerobic conditions in the submerged tissues. This paper discusses the molecular aspects of the mechanism that leads to shoot elongation (leaves of seedlings and internodes), the regulation of which involves metabolism of, and interactions between, ethylene, gibberellins and abscisic acid. Finally, the importance of new techniques in future research is assessed. Current molecular technology can reveal subtle differences in gene activity between tolerant and non-tolerant cultivars, and identify genes that are involved in the regulation of submergence avoidance and tolerance.

Regulation of Internodal Elongation of Rice Seedlings by Plant Growth Regulators.

We investigated the role of each plant hormone in the internodal elongation of rice seedlings using japonica rice cultivar 'Sasanishiki', and deepwater rice cultivar 'Leb Mue Nahng 111, and 'Habiganj Aman VIII'. Rice seedlings were grown in the light conditions at 30°C and applied with various plant growth regulators : abscisic acid (ABA), benzyl-adenine (BA), brassinolide (BR), ethylene (ET), gibberellin-A3 (GA3), indole-3-acetic acid (IAA) and triazinone (TA). (1) Among the seven growth regulators, only GA3 induced internode elongation when applied alone. In japonica rice, the elogation was not induced when applied with low level of GA3 (10-5M∼10-6M). The elongation, however, was induced by GA3 (10-5M∼10-6M) in the presence of polyethylene glycol (PEG) or TA (gibberellin synergist). In the deepwater rice, low level of GA3 (10-5M∼10-6M) induced the elongation. (2) GA3 or BR stimulated the coleptile growth. GA3, BR or IAA promoted the mesocotyl growth. GA3 promoted the leaf growth. GA3 and ET stimulated the internode elongation but the BR or BA inhibited the internode growth. ABA produced both an inhibition and a promotion of the internode growth. This may be due to the stage dependent response of the internode to ABA. These results may suggest that the regulation mechanism of the internode growth may include both organ-specific steps and the common process to other organs.

Screening Deepwater Rice Cultivars for Drought Tolerance Under Field Conditions

Screening Deepwater Rice Cultivars for Drought Tolerance Under Field Conditions