First Report of White Leaf Streak of Rice Caused by Mycovellosiella oryzae in Texas.

Rice (Oryza sativa L.) plants in experimental plots in Beaumont, TX developed symptoms on leaf blades similar to white leaf streak caused by Mycovellosiela oryzae (Deighton & Shaw) Deighton (synonym Ramularia oryzae Deighton & Shaw) (1,3,4) during the late summer of 2009. Symptoms were observed on several rice cultivars and breeding lines including Cocodrie, Cypress, and Lemont. Lesions usually appeared on lower leaves and were approximately 2 to 7 mm long and linear with whitish or grayish centers surrounded by a narrow brown-or-dark brown margin. Symptoms were similar to narrow brown leaf spot caused by Cercospora janseana (Racib.) O. Const. (synonym C. oryzae Miyake) (3,4), but the centers of lesions were slightly wider (up to 2 mm). Symptoms were visible on the upper and lower leaf surfaces. Some lesions on heavily infected leaves were long (10 to 15 × 0.5 to 2 mm) whitish streaks parallel to the midrib. Leaves with typical symptoms were collected and incubated in a petri dish lined with moistened filter paper for 3 to 4 days at room temperature under a 12-h fluorescent photoperiod. Conidiophores were produced on external mycelium growing out through stomata on the lesion surface. Conidiophores were hyaline, straight, and 7 to 22 µm long and 2 to 3 µm wide with conidial scars. Conidia were washed from the lesions, diluted in sterilized distilled water, and placed on acidified potato dextrose agar. After 6 to 7 days of incubation at room temperature, slow-growing colonies were transferred onto potato dextrose agar (PDA). Three isolates were obtained from single-spore cultures. The colonies of these isolates grew similarly and very slowly on PDA and their radial growth averaged 0.8 mm/day at room temperature. The colonies were dense, grayish, and did not produce any pigments. Conidia were formed singly or in chains and measured 12 to 30 µm long. They were hyaline, straight, cylindrical, typically with no or one septum, a few with two to three septa, and had a hilum and tapered ends. Pathogenicity of these three isolates was assessed in greenhouse tests by spraying a conidial suspension (105 conidia/ml) onto 12 plants of each of the cvs. Cocodrie, Cypress and Lemont at the late tillering stage. Inoculum was obtained by harvesting conidia from the colonies grown on PDA for 3 weeks at room temperature under a 12-h fluorescent photoperiod. Plants sprayed with sterilized distilled water served as the controls. All plants were maintained in a humid chamber for 2 days and then grown in a greenhouse at 20 to 31°C. After 15 days, early lesions appeared on inoculated leaf blades; after 4 weeks, typical symptoms similar to those observed in the field developed. Control plants did not exhibit any symptoms. M. oryzae was reisolated from symptomatic plants, confirming that the disease was caused by this pathogen. To our knowledge, this is the first report of white leaf streak on rice in Texas and the second report after Louisiana (2). This disease has been reported in Papua New Guinea, the Solomon Islands, North Borneo, Sierra Leone, and Nigeria (3,4). References: (1) F. C. Deighton and D. Shaw. Trans. Br. Mycol. Soc. 43:515, 1960. (2) A. K. M. Shahjahan et al. Plant Dis. 82:1282, 1998. (3) B. C. Sutton and A. K. M. Shahjahan. Nova Hedwigia 25:197, 1981. (4) R. K. Webster and P. S. Gunnell. Compendium of Rice Diseases. The American Phytopathological Society, St. Paul, MN, 1992.

Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. I. Biomass and grain yield.

To understand the genetic basis of inbreeding depression and heterosis in rice, main-effect and epistatic QTL associated with inbreeding depression and heterosis for grain yield and biomass in five related rice mapping populations were investigated using a complete RFLP linkage map of 182 markers, replicated phenotyping experiments, and the mixed model approach. The mapping populations included 254 F(10) recombinant inbred lines derived from a cross between Lemont (japonica) and Teqing (indica) and two BC and two testcross hybrid populations derived from crosses between the RILs and their parents plus two testers (Zhong 413 and IR64). For both BY and GY, there was significant inbreeding depression detected in the RI population and a high level of heterosis in each of the BC and testcross hybrid populations. The mean performance of the BC or testcross hybrids was largely determined by their heterosis measurements. The hybrid breakdown (part of inbreeding depression) values of individual RILs were negatively associated with the heterosis measurements of their BC or testcross hybrids, indicating the partial genetic overlap of genes causing hybrid breakdown and heterosis in rice. A large number of epistatic QTL pairs and a few main-effect QTL were identified, which were responsible for >65% of the phenotypic variation of BY and GY in each of the populations with the former explaining a much greater portion of the variation. Two conclusions concerning the loci associated with inbreeding depression and heterosis in rice were reached from our results. First, most QTL associated with inbreeding depression and heterosis in rice appeared to be involved in epistasis. Second, most ( approximately 90%) QTL contributing to heterosis appeared to be overdominant. These observations tend to implicate epistasis and overdominance, rather than dominance, as the major genetic basis of heterosis in rice. The implications of our results in rice evolution and improvement are discussed.

Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. II. Grain yield components.

The genetic basis underlying inbreeding depression and heterosis for three grain yield components of rice was investigated in five interrelated mapping populations using a complete RFLP linkage map, replicated phenotyping, and the mixed model approach. The populations included 254 F(10) recombinant inbred lines (RILs) derived from a cross between Lemont (japonica) and Teqing (indica), two backcross (BC) and two testcross populations derived from crosses between the RILs and the parents plus two testers (Zhong413 and IR64). For the yield components, the RILs showed significant inbreeding depression and hybrid breakdown, and the BC and testcross populations showed high levels of heterosis. The average performance of the BC or testcross hybrids was largely determined by heterosis. The inbreeding depression values of individual RILs were negatively associated with the heterosis measurements of the BC or testcross hybrids. We identified many epistatic QTL pairs and a few main-effect QTL responsible for >65% of the phenotypic variation of the yield components in each of the populations. Most epistasis occurred between complementary loci, suggesting that grain yield components were associated more with multilocus genotypes than with specific alleles at individual loci. Overdominance was also an important property of most loci associated with heterosis, particularly for panicles per plant and grains per panicle. Two independent groups of genes appeared to affect grain weight: one showing primarily nonadditive gene action explained 62.1% of the heterotic variation of the trait, and the other exhibiting only additive gene action accounted for 28.1% of the total trait variation of the F(1) mean values. We found no evidence suggesting that pseudo-overdominance from the repulsive linkage of completely or partially dominant QTL for yield components resulted in the overdominant QTL for grain yield. Pronounced overdominance resulting from epistasis expressed by multilocus genotypes appeared to explain the long-standing dilemma of how inbreeding depression could arise from overdominant genes.

A "defeated" rice resistance gene acts as a QTL against a virulent strain of Xanthomonas oryzae pv. oryzae.

The genetic components responsible for qualitative and quantitative resistance of rice plants to three strains (CR4, CXO8, and CR6) of Xanthomonas oryzae pv. oryzae (Xoo) were investigated using a set of 315 recombinant inbred lines (RILs) from the cross Lemont (japonica) x Teqing (indica) and a complete linkage map with 182 well distributed RFLP markers. We mapped a major gene (Xa4) and ten quantitative trait loci (QTLs) which were largely responsible for segregation of the resistance phenotype in the RILs. The Teqing allele at the Xa4 locus, Xa4T, acted as a dominant resistance gene against CR4 and CXO8. The breakdown of Xa4T-associated resistance mediated by the mutant allele at the avrXa4 locus in the virulent strain CR6 results from significant changes in both gene action (lose of dominance) and the magnitude of gene effect (approximately 50% reduction). Nevertheless, Xa4T still acted as a recessive QTL with a significant residual effect against CR6. The mutant alleles at the avrXa4 locus in CXO8 and CR6 that lead to a reduction in effect, or "breakdown", of Xa4T were apparently accompanied by corresponding penalties for their fitness. The quantitative component of resistance to Xoo in the RILs was largely due to a number of resistance QTLs. Most resistance QTLs mapped to genomic locations where major resistance genes and/or QTLs for resistance to Xoo, blast and sheath blight were identified in the same cross. Most QTLs showed consistent levels of resistance against all three Xoo strains. Our results suggest that a high level of durable resistance to Xoo may be achieved by the cumulative effects of multiple QTLs, including the residual effects of "defeated" major resistance genes.

RFLP mapping and race specificity of bacterial blight resistance genes (QTLs) in rice.

By using a set of 315 recombinant inbred lines (RILs) from the cross Lemont (japonica) x Teqing (indica) and a complete linkage map with 186 well distributed RFLP markers and 3 morphological markers, a major gene (Xa4) and 10 QTLs and 9 pairs of epistasis loci conferring horizontal resistance to three strains of Xanthomonas oryxa pv oryza (Xoo) were mapped. The Teqing allele at Xa4 on chromosome 11 acts as a dominant resistant gene against pathogen race CR4 and CX08, but as an additive QTL with a significantly (47%) reduced effect against the virulent strain, CR6. The major gene Xa4 exhibited stronger degree of race specificity. Most QTLs showed consistent levels of resistance against all three Xoo strains. The results suggest that a high level durable resistance to Xoo may be achieved by cumulative effects of multiple QTL.

Genetic and physical analysis of the rice bacterial blight disease resistance locus, Xa21.

Nearly isogenic lines (NILs) of rice (Oryza sativa) differing at a locus conferring resistance to the pathogen Xanthomonas oryzae pv. oryzae were surveyed with 123 DNA markers and 985 random primers using restriction fragment length plymorphism (RFLP) and random amplified polymorphic DNA (RAPD) analysis. One chromosome 11 marker (RG103) detected polymorphism between the NILs that cosegregated with Xa21. All other chromosome 11 DNA markers tested were monomorphic between the NILs, localizing the Xa21 introgressed region to an 8.3 cM interval on chromosome 11. Furthermore, we identified two polymerase chain reaction (PCR) products (RAPD2148 and RAPD818) that detected polymorphisms between the NILs. Genomic sequences hybridizing with RAPD818, RAPD248 and RG103 were duplicated specifically in the Xa21 NIL. All three markers cosegregated with the resistance locus, Xa21, in a F2 population of 386 progeny. Based on the frequency with which we recovered polymorphic Xa21-linked markers, we estimated the physical size of the introgressed region to be approximately 800 kb. This estimation was supported by physical mapping (using pulsed field gel electrophoresis) of the sequences hybridizing with the three Xa21-linked DNA markers. The results showed that the three Xa21-linked markers are physically close to each other, with one copy of the RAPD818 sequences located within 60 kb of RAPD248 and the other copy within 270 kb of RG103. None of the enzymes tested generated a DNA fragment that hybridized with all three of the markers indicating that the introgressed region containing the resistance locus Xa21 is probably larger than 270 kb.