Rice Genetics IV - IRRI books - International Rice Research Institute

However, to detect genetic variability in complex traits, a reproducible screening techniqueis much needed. For example, stringent and uniform submergence screeningallows for recovery of lines with quantitative enhancement in submergence tolerance.On the other hand, considerable variability exists in grain yields among the mutantlines under drought stress and control environments, illustrating the difficulties inidentifying quantitative response using parameters that could be affected by multiplefactors. It appears that measurement of multiple characters with adequate replicationswould be essential in detecting mutations that affect quantitative traits.Another factor to consider in conducting a productive mutant screening is thelikelihood of detecting a gain or loss in functions in the parental genotype. Using theparental phenotype as a guide, we need to design screening methods with sufficientsensitivity to detect deviations from the parental value. For example, IR64 has anintermediate level of tolerance of zinc deficiency. Whether we can detect variation inoversensitivity or enhanced tolerance may depend on the level of available zinc in thetest soil to differentiate the phenotypic extremes.Given the abundance of useful traits in IR64, opportunities exist to search forconditional mutants under laboratory and field conditions. Possible candidates arevariants under photosynthesis-limiting conditions, tolerance of soil toxicity, and quantitativeresistance to brown planthopper. Recently, Wurtzel et al (2001) described asimple approach to isolate rice mutants exhibiting a block in carotenoid biosynthesis.Although this class of mutants may not be useful in developing pro-vitamin A rice,the approach of isolating mutants blocking micronutrient biosynthesis and availabilitymay prove useful for identifying genes to improve nutrients in rice grain.Reverse genetics systemsOur original goal in producing a deletion stock is to create a sizable gap in a genesuch that it can be physically detected by PCR or hybridization methods. To examinewhether deletions can be detected in our mutant population, we selected diseasesusceptiblemutants and examined a set of putative disease-resistance-gene sequences(nucleotide-binding site and leucine-rich repeat, NBS-LRR, sequences) for detectingdeletions. In one experiment, DNA from several blast-susceptible mutants was probedwith three NBS-LRR sequences—R4, R12, and R5—previously mapped on chromosome11 (Leister et al 1998). A small deletion was detected in a line (GR131) with R4but not with the other probes (Fig. 4, M. Ramos-Pamplona and H. Leung, unpublished).Cosegregation of this deletion and loss of resistance is in progress to determinewhether R4 is responsible for the change from resistance to susceptibility. Thisexperiment indicates that deletion can be detected if a collection of candidate genesand preselected mutants is available. However, the challenge is to be able to applyDNA sequences to screen DNA from a pool of random mutants and isolate the mutantswith specific genetic lesions.At least two screening methods have been developed as reverse genetics tools innontransgenic mutants. Liu et al (1999) described the use of PCR screening of a DNApool from a deletion stock of Caenorhabditis elegans. This approach uses gene-spe-Deletion mutants for functional genomics: . . . 247