在异倍性水平多胚苗水稻的特征特性研究

通过无融合生殖方式固定水稻杂种优势是水稻育种中极具诱惑力的研究内容之一。自从1979年以来，我国水稻无融合生殖充种研究主要集中在筛选和鉴定水稻多胚苗材料。根据前人的研究结果，具有无融合生殖特性的植物大多数为多倍体。本试验以多胚苗水稻APIV及其衍生系为研究材料，在二倍体[APIV_((2))]、同源三倍体[APIV_((3))]和同源四倍体[APIV_((4))]水平，对其多个世代中的特征特性，其中包括形态学特征，遗传学和胚胎学特性进行了研究，旨在探讨在多倍性水平筛选水稻无融合生殖种质的可能性，获得如下主要研究结果： 1.在二倍体APIV_((2))的多个世代中，只发现单胚苗、双胚苗和三胚苗植株，多胚苗频率比较低(4.67～5.14%)，多胚苗性状的表达在一定程度上受到环境因素的影响，并且，多胚苗植株的成活率比较低(11.70～17.39%)，大部分多胚苗植株在三叶期之前死亡，这很可能与其胚乳的营养供应有关。根据APIV_((2))的多胚苗性状在多个自交世代和杂交世代中的表达特点，多胚苗性状不是显性性状，也不是隐性性状，而是一种比较特殊的数量性状。 2.胚胎学的研究结果表明，在APIV_((2))的2857个子房的胚囊中没有观察到与不定胚生殖有关的特异生殖现象;APIV_((2))在发生受精之前的胚囊构型包括正常蓼型胚囊（76.5%）、退化型胚囊（16.0%）和变异型胚囊（7.5%），在变异型胚囊中包括双卵卵器胚囊（86.7%）和三卵卵器胚囊（13.3%）;APIV_((2))的双受精与前人在正常二倍体水稻中所观察到的结果大致相符;在不同季节的颖花和同一季节同一稻穗的不同颖花内多卵和多胚苗频率存在着明显差异。 3.在同源四倍体水稻的诱导中对种芽进行预处理，促使其胚芽鞘明显伸长后再进行诱导处理是诱导成功的关键技术。在同源四倍体APIV_((4))的同一稻穗中强势颖花的多卵频率要明显地高于弱势颖花的多卵频率。在APIV_((4))去雄后的颖花中意外地观察到了单胚和双胚现象。同源四倍体水稻APIV_((4))的有性生殖能力明显变弱，在花药内正常花粉粒少而败育花粉粒多;在受精前正常胚囊数少而退化胚囊数比较多（36.0%）;花粉管进入胚囊的时间比较晚;双受精频率低而单受精频率高。 4.异倍性水稻间具有一定的可交配性，但其结实率比较低（0.20～-1.64%），通过常规杂交方法所获得的同源三倍体成活植株的频率更低（0.07%）。在湖南湘潭的秋季条件下同源三倍体水稻植株雄性完全败育，但有部分稻穗能结实（0.59%～7.71%)，由此可获得饱满种子和不饱满种子。 5.通过子房培养可以获得异倍性水稻间杂种植株。在APIV_((4))/APIV_((2))杂交组合的子房培养中出现了一株双胚苗;同源三倍体成活植株的获得率仍然比较低（0.78%），但比通过常规杂交法首先获得种子，进而获得同源三倍体成活植株的效率要倍出11.14倍。根据试验结果，提出了6个问题进行讨论。认为在杂交后代中有可能筛选到多胚苗发生频率更高的单株;多胚苗性状是比较复杂的数量性状;同源四倍体水稻的有性生殖能力明显变弱;通过合理配组和复重授粉可以提高异倍性水稻间的杂并结实率;通过子房培养可以明显提高获得同源三倍体成活植株的效率;在多倍体水稻中筛选无融合生殖种质在比二倍体水稻中筛选可能更容易成功。

Genetic Analysis of Tolerance to Rice Tungro Bacilliform Virus in Rice (Oryza sativa L.) Through Agroinoculation

Balimau Putih [an Indonesian cultivar tolerant to rice tungro bacilliform virus (RTBV)] was crossed with IR64 (RTBV, susceptible variety) to produce the three filial generations F1, F2 and F3. Agroinoculation was used to introduce RTBV into the test plants. RTBV tolerance was based on the RTBV level in plants by analysis of coat protein using enzyme-linked immunosorbent assay. The level of RTBV in cv. Balimau Putih was significantly lower than that of IR64 and the susceptible control, Taichung Native 1. Mean RTBV levels of the F1, F2 and F3 populations were comparable with one another and with the average of the parents. Results indicate that there was no dominance and an additive gene action may control the expression of tolerance to RTBV. Tolerance based on the level of RTBV coat protein was highly heritable (0.67) as estimated using the mean values of F3 lines, suggesting that selection for tolerance to RTBV can be performed in the early selfing generations using the technique employed in this study. The RTBV level had a negative correlation with plant height, but positive relationship with disease index value

Changes in Level of Virus Accumulation and Incidence of Infection are Critical in the Characterization of Rice Tungro Bacilliform Virus (RTBV) Resistance in Rice

Analysis by enzyme-linked immunosorbent assay showed that Rice tungro bacilliform virus (RTBV) accumulated in a cyclic pattern from early to late stages of infection in tungro-susceptible variety, Taichung Native 1 (TN1), and resistant variety, Balimau Putih, singly infected with RTBV or co-infected with RTBV+Rice tungro spherical virus (RTSV). These changes in virus accumulation resulted in differences in RTBV levels and incidence of infection. The virus levels were expressed relative to those of the susceptible variety and the incidence of infection was assessed at different weeks after inoculation. At a particular time point, RTBV levels in TN1 or Balimau Putih singly infected with RTBV were not significantly different from the virus level in plants co-infected with RTBV+RTSV. The relative RTBV levels in Balimau Putih either singly infected with RTBV or co-infected with RTBV+RTSV were significantly lower than those in TN1. The incidence of RTBV infection varied at different times in Balimau Putih but not in TN1, and to determine the actual infection, the number of plants that became infected at least once anytime during the 4wk observation period was considered. Considering the changes in RTBV accumulation, new parameters for analyzing RTBV resistance were established. Based on these parameters, Balimau Putih was characterized having resistance to virus accumulation although the actual incidence of infection was >75%.

The Biology, Epidemiology, and Management of Rice Tungro Disease in Asia

Many farmers in South and Southeast Asia describe rice tungro disease as a cancer disease because of the severe damage it causes and the difficulty of controlling it (121). As the most important of the 14 rice viral diseases, tungro was first recognized as a leafhopper-transmitted virus disease in 1963 (88). However, tungro, which means “degenerated growth” in a Filipino dialect, has a much longer history. It is almost certain that tungro was responsible for a disease outbreak that occurred in 1859 in Indonesia, which was referred to at the time as mentek (83). In the past, a variety of names has been given to tungro, including accep na pula in the Philippines, penyakit merah in Malaysia, and yelloworange leaf in Thailand (83).

Rice Tungro Bacilliform

Many well-known specialists have contributed to this book which presents for the first time an in-depth look at the viruses, their satellites and the retrotransposons infecting (or occuring in) one plant family: the Poaceae (Gramineae). After molecular and biological descriptions of the viruses to species level, virus diseases are presented by crop: barley, maize, rice, rye, sorghum, sugarcane, triticales, wheats, forage, ornamental and lawn. A detailed index of the viruses and taxonomic lists will help readers in the search for information.

Differentiation of Rice Tungro Bacilliform Virus Strains by Restriction Analysis and DNA Hybridization

Rice tungro bacilliform virus (RTBV) is one of the two viruses that cause tungro disease. Four RTBV strains maintained in the greenhouse for 4 years, G1, G2, Ic, and L, were differentiated by restriction fragment length polymorphism (RFLP) analysis of the native viral DNA. Although strains G1 and Ic had identical restriction patterns when cleaved with Pst1, BamHI, EcoRI, and EcoRV, they can be differentiated from strains G2 and L by EcoRI and EcoRV digestion. These same endonucleases also differentiate strain G2 from strain L. When total DNA extracts from infected plants were used instead of viral DNA, and digested with EcoRV, identical restriction patterns for each strain (G2 and L) were obtained from roots, leaves, and leaf sheaths of infected plants. The restriction patterns were consistent from plant to plant, in different varieties, and at different times after inoculation. This technique can be used to differentiate RTBV strains and determine the variability of a large number of field samples.

Mechanism of Resistance to Rice Tungro Spherical Virus (RTSV)

RTSV is one of two viruses that cause tungro disease. RTSV is independently transmitted, whereas the other virus, rice tungro bacilliform virus (RTBV), is dependent on RTSV for its transmission by the green leafhopper (GLH), Nephotettix virescens. The occurrence and spread of tungro disease therefore depend on the presence of RTSV in the field. Resistance to RTSV infection would slow down the spread of the disease.

Differentiation of Rice Tungro Spherical Virus Variants by RTPCR and RFLP

Differentiation of rice tungro spherical virus variants by RTPCR and RFLP tungro bacilliform virus (RTBV), the other causal agent, which causes the symptoms. RTSV is a single-stranded RNA virus of 12,180 nucleotides (Hull 1996).

Sequence Changes in Six Variants of Rice Tungro Bacilliform Virus and Their Phylogenetic Relationships

The DNA of three biological variants, G1, Ic and G2, which originated from the same greenhouse isolate of rice tungro bacilliform virus (RTBV) at the International Rice Research Institute (IRRI), was cloned and sequenced. Comparison of the sequences revealed small differences in genome sizes. The variants were between 95 and 99% identical at the nucleotide and amino acid levels. Alignment of the three genome sequences with those of three published RTBV sequences (Phi-1, Phi-2 and Phi-3) revealed numerous nucleotide substitutions and some insertions and deletions. The published RTBV sequences originated from the same greenhouse isolate at IRRI 20, 11 and 9 years ago. All open reading frames (ORFs) and known functional domains were conserved across the six variants. The cysteine-rich region of ORF3 showed the greatest variation. When the six DNA sequences from IRRI were compared with that of an isolate from Malaysia (Serdang), similar changes were observed in the cysteine-rich region in addition to other nucleotide substitutions and deletions across the genome. The aligned nucleotide sequences of the IRRI variants and Serdang were used to analyse phylogenetic relationships by the bootstrapped parsimony, distance and maximum-likelihood methods. The isolates clustered in three groups: Serdang alone; Ic and G1; and Phi-1, Phi-2, Phi-3 and G2. The distribution of phylogenetically informative residues in the IRRI sequences shared with the Serdang sequence and the differing tree topologies for segments of the genome suggested that recombination, as well as substitutions and insertions or deletions, has played a role in the evolution of RTBV variants. The significance and implications of these evolutionary forces are discussed in comparison with badnaviruses and caulimoviruses.