Renata Pereira da Cruz

Cold tolerance at the germination stage of rice: methods of evaluation and characterization of genotypes

Rice cold tolerance at the germination stage is important in Rio Grande do Sul (RS) where temperatures below 15°C prevent or reduce germination and plant establishment in early sowings. The present study aimed at identifying an adequate method for cold tolerance evaluation of the rice germination stage and at verifying the variability among 24 rice genotypes of different origins. Cold tolerance was evaluated in experiment I, germination under two conditions: 13°C for 28 days and 28°C for seven days, and in experiment II, germination under 28°C for 72 hours, 13°C for 96 hours and again 28°C for 72 hours. In experiment I measured characteristics were germination index, percentage of seeds with coleoptile length superior to 5 mm and percentage of reduction in coleoptile length due to cold. In experiment II the measured characteristic was coleoptile regrowth after the cold period. Cold tolerance varied among genotypes studied in both experiments, but only the percentage of reduction in coleoptile length and coleoptile regrowth allowed a better distinction between the tolerant checks and the susceptible one. In general, genotypes belonging to the Japonica subspecies presented higher cold tolerance than Indica, but there was variability within subspecies. The most adequate method of evaluation of cold tolerance is through percentage of reduction in coleoptile length and coleoptile regrowth. Among Japonica genotypes, Quilla 64117 and Diamante presented the highest cold tolerance, and among Indica, cultivars BR-IRGA 410 and IRGA 416 were the most cold tolerant at the

Melhoramento genético para tolerância ao frio em arroz irrigado

Genetic breeding for cold tolerance is presented as a strategy to confront the problem of chilling temperature in Rio Grande do Sul during irrigated rice crop season. The occurence of chilling temperatures in other countries is compared to the situation in Southern Brazil. Plant developmental stages most affected by low temperatures, selection strategies available in controlled conditions and difficulties found for breeding this trait are discussed. Finally, promising selection strategies based on Physiology and Biotechnology studies are presented.

Inheritance of rice cold tolerance at the germination stage

Cold tolerance during germination is important for ensuring fast and uniform establishment of a rice crop early in the season. However, evaluation of this trait under field conditions is limited by environmental variation, which makes it difficult to identify genetically superior lines. Evaluation of cold tolerance under controlled temperature conditions may be performed by assessing percentage of reduction in coleoptile length and coleoptile growth. Our study determined the inheritance and heritability of cold tolerance at the germination stage in crosses between six rice genotypes. Diallel analysis showed that while both additive and non-additive gene action were involved, the non-additive action was relatively more important for percentage of reduction in coleoptile length and coleoptile growth. Our data shows that genotype Quilla 66304 would be the best parent in crosses aimed at increasing cold tolerance at the germination stage in rice due to its high general combining ability for both percentage of reduction in coleoptile length and coleoptile growth. Generation mean analysis was also performed for coleoptile growth in six cold-sensitive x cold-tolerant crosses and proved that non-additive effects were due to dominance and epistatic interactions. Though broad sense heritability values were high, the relative importance of the non-additive effects suggests that selection should be applied in advanced generations of the breeding program.

Alterations in fatty acid composition due to cold exposure at the vegetative stage in rice.

Rice is a tropical plant, so cold temperature may be detrimental to its development, depending on the genotype and environmental conditions. Degree of lipid unsaturation has been related to cold tolerance due to its effect on membrane stability. So, the aim of this study was to characterize the fatty acid composition and its alterations due to cold temperature in rice genotypes of diversified origin. Forty-four rice genotypes at the V4 stage were submitted to two temperature conditions: 10°C and 28°C for two days and after this they had their leaves collected for lipid extraction and quantification. Control plants were allowed to regrow until presenting four leaves fully expanded and then were subjected to 10°C for ten days for cold tolerance evaluation. Plant survival was measured seven days after recovery at 28°C and the genotypes were grouped in three cold tolerance classes: tolerant, intermediate and sensitive. These classes differed for total saturated and unsaturated fatty acids only under the cold temperature treatment. Further analysis of the more abundant fatty acids: linoleic, linolenic and palmitic, showed that the two last ones differed between tolerant and sensitive genotypes. Linolenic acid increased after cold exposure in cold tolerant genotypes while palmitic acid decreased, and an opposite behavior was found in the cold sensitive genotypes. These evidences indicate that these fatty acids are potential molecular markers useful for breeding programs as well as for future basic studies on cold tolerance in rice.

Rice cold tolerance at the reproductive stage in a controlled environment

A tolerância do arroz (Oryza sativa L.) ao frio no periodo reprodutivo e importante para garantir alto rendimento em ambientes com temperatura baixa. No entanto, a selecao em condicoes de campo nao permite identificar fontes adequadas de tolerância e limita a selecao de linhas segregantes devido a temperatura variavel. Este trabalho teve por objetivo definir uma metodologia capaz de distinguir genotipos de arroz quanto a sua tolerância ao frio no periodo reprodutivo quando avaliados sob temperatura controlada. O efeito do frio foi investigado em seis genotipos de arroz submetidos a 17°C por periodos variaveis de tempo (tres, cinco, sete e dez dias) em dois estadios do periodo reprodutivo (microsporogenese e antese). A tolerância ao frio foi avaliada por meio da porcentagem de reducao na exercao da panicula e na fertilidade de espiguetas. O resultados indicaram que avaliar a tolerância ao frio por meio da reducao na exercao da panicula nao permitiu separar genotipos tolerantes ao frio de genotipos sensiveis e, quando avaliada por meio da reducao na fertilidade de espiguetas, no minimo sete dias foram necessarios para diferenciar os genotipos quanto a tolerância ao frio. Eles foram mais sensiveis ao frio na antese que na microsporogenese e, como estes estadios foram altamente correlacionados, a selecao sob frio poderia ser feita somente neste estadio, que e de mais facil determinacao. Logo, a tolerância do arroz ao frio no periodo reprodutivo pode ser avaliada por meio da reducao na fertilidade de espiguetas devido a temperatura baixa (17°C) aplicada por sete dias no estadio de antese.

Cold tolerance in rice germinating seeds revealed by deep RNAseq analysis of contrasting indica genotypes

Rice productivity is largely affected by low temperature, which can be harmful throughout plant development, from germination to grain filling. Germination of indica rice cultivars under cold is slow and not uniform, resulting in irregular emergence and small plant population. To identify and characterize novel genes involved in cold tolerance during the germination stage, two indica rice genotypes (sister lines previously identified as cold-tolerant and cold-sensitive) were used in parallel transcriptomic analysis (RNAseq) under cold treatment (seeds germinating at 13 °C for 7 days). We detected 1,361 differentially expressed transcripts. Differences in gene expression found by RNAseq were confirmed for 11 selected genes using RT-qPCR. Biological processes enhanced in the cold-tolerant seedlings include: cell division and expansion (confirmed by anatomical sections of germinating seeds), cell wall integrity and extensibility, water uptake and membrane transport capacity, sucrose synthesis, generation of simple sugars, unsaturation of membrane fatty acids, wax biosynthesis, antioxidant capacity (confirmed by histochemical staining of H2O2), and hormone and Ca(2+)-signaling. The cold-sensitive seedlings respond to low temperature stress increasing synthesis of HSPs and dehydrins, along with enhanced ubiquitin/proteasome protein degradation pathway and polyamine biosynthesis. Our findings can be useful in future biotechnological approaches aiming to cold tolerance in indica rice.

Inheritance of rice cold tolerance at the seedling stage

A tolerância ao frio nos estadios iniciais de desenvolvimento do arroz (Oryza sativa L.) e uma caracteristica altamente desejavel para incorporar em cultivares do Rio Grande do Sul (RS), porem a selecao para esta caracteristica deve ser feita sob condicoes de temperatura controlada, o que limita o numero de linhas que pode ser avaliada. O conhecimento da heranca desta caracteristica e importante para definir estrategias de melhoramento genetico, assim o objetivo do presente trabalho e estudar a base genetica da tolerância ao frio no estadio vegetativo do arroz. Seis genotipos com reacoes contrastantes de tolerância ao frio foram cruzados num esquema dialelico sem os reciprocos. Os genitores e as geracoes F1 e F2 foram cultivados em casa-de-vegetacao ate o estadio V4, quando foram submetidos a 10°C por dez dias e avaliados quanto a sobrevivencia de plantas apos sete dias de recuperacao sob temperatura normal. Os resultados obtidos pela analise dialelica da geracao F1 indicaram significância tanto dos efeitos aditivos quanto nao-aditivos, mas a capacidade geral de combinacao foi mais importante. A avaliacao da geracao F2 revelou uma heranca oligogenica com um ou dois alelos dominantes responsaveis pela tolerância ao frio nos genitores tolerantes e dois genes complementares com alelos recessivos segregando nos cruzamentos envolvendo genotipos sensiveis e intermediarios.

Piramidização de genes de resistência às ferrugens em cereais

The rusts are the most damaging diseases of cereal crops, being capable of causing big yield tosses. Cereal breeding for rust resistance has made use of resistance conditioned by one or few major genes. However, the short time which pathogens break this type of host resistance has raised the question ofusing alternative strategies to increase resistance durability. The objective of this review is to discuss aspects of the process of pyramiding genes as a way to increase rust resistance durability. The pyramiding process and durable resistance are discussed in relation to the effectivity of the genes used and the practical limitations of this strategy. The use of DNA markers is pointed as a promising tool for combining several resistance genes in one genotype. Examples of durable resistance are presented and their relation with pyramiding is discussed.

Herança da exerção da panícula em arroz

Incomplete panicle exsertion is one of the symptoms of cold injury at the reproductive stage of the rice plant (Oryza sativa L.), which damages grain yield and raises disease incidence. For this reason, panicle exsertion is a better indicator of cold tolerance under field conditions than spikelet sterility, which may also be affected by other climatic factors. This work studies the variability of degree of panicle exsertion in rice, under the Southern Brazilian environmental conditions and determines the inheritance and heritability of this trait. Four rice genotypes of different cool temperature reactions at the reproductive stage were crossed and field evaluated, with the F 2 generation, in relation to the degree of panicle exsertion and spikelet fertility. There was variability among the genotypes for panicle exsertion. The F 2 generation of the crosses presented continuous distribution and transgressive segregation towards incomplete exsertion, indicating that genes controlling this trait may be complementary distributed between the parents. Heritability was moderate, so selection for complete panicle exsertion should be applied in advanced generations.

Severidade da ferrugem-da-folha e seus efeitos sobre caracteres da panícula de aveia

Crown rust (Puccinia coronata f.sp. avenae) was quantified in four F3 oat (Avena sativa L.) populations with the objective of studying disease severity and its effects on six panicle characters. Populations used were UFRGS 881920 x UFRGS 7, UFRGS 881920 x UFRGS 14, UFRGS 15 x UFRGS 7 and UFRGS 15 x UFRGS 14, with genotypes UFRGS 881920 and UFRGS 15 being resistant and the others susceptible. Crown rust infection was quantified at flowering stage and near grain maturity 10 plants at random were harvested per crown rust score in each population, and the following determinations were made: number of panicles, mean panicle weight, number of grains per plant and per panicle, grain weight and 1,000 grains weight. Variability for crown rust severity was analised through frequency distribution of crown rust scores in each population. Evaluation of crown rust effects on panicle characters was done through analysis of variance, polinomial regression analysis and phenotypic correlations. Results obtained demonstrate great variation for crown rust severity in the populations studied, and the most affected characters by disease occurence were mean panicle weight and 1,000 grains weight.