Evidence for different QTL underlying the immune and hypersensitive responses of Eucalyptus globulus to the rust pathogen Puccinia psidii

The rust Puccinia psidii infects many species in the family Myrtaceae. Native to South America, the pathogen has recently entered Australia which has a rich Myrtaceous flora, including trees of the ecologically and economically important genus Eucalyptus. We studied the genetic basis of variation in rust resistance in Eucalyptus globulus, the main plantation eucalypt in Australia. Quantitative trait loci (QTL) analysis was undertaken using 218 genotypes of an outcross F2 mapping family, phenotyped by controlled inoculation of their open pollinated progeny with the strain of P. psidii found in Australia. QTL analyses were conducted using a binary classification of individuals with no symptoms (immune) versus those with disease symptoms, and in a separate analysis dividing plants with disease symptoms into those exhibiting the hypersensitive response versus those with more severe symptoms. Four QTL were identified, two influencing whether a plant exhibited symptoms (Ppr2 and Ppr3), and two influencing the presence or absence of a hypersensitive reaction (Ppr4 and Ppr5). These QTL mapped to four different linkage groups, none of which overlap with Ppr1, the major QTL previously identified for rust resistance in Eucalyptus grandis. Candidate genes within the QTL regions are presented and possible mechanisms discussed. Together with past findings, our results suggest that P. psidii resistance in eucalypts is quantitative in nature and influenced by the complex interaction of multiple loci of variable effect.

Genetic diversity of stay-green sorghums and their derivatives revealed by microsatellites

The genetic variability of 28 sorghum genotypes of known senescence phenotype was investigated using 66 SSR markers well-distributed across the sorghum genome. The genotypes of a number of lines from breeding programmes for stay-green were also determined. This included lines selected phenotypically for stay-green and also RSG 03123, a marker-assisted backcross progeny of R16 (recurrent parent) and B35 (stay-green donor). A total of 419 alleles were detected with a mean of 6.2 per locus. The number of alleles ranged from one for Xtxp94 to 14 for Xtxp88. Chromosome SBI-10 had the highest mean number of alleles (8.33), while SBI-05 had the lowest (4.17). The PIC values obtained ranged from zero to 0.89 in Xtxp94 and Xtxp88, respectively, with a mean of 0.68. On a chromosome basis, mean PIC values were highest in SBI-10 (0.81) and lowest in SBI-05 (0.53). Most of the alleles from B35 in RSG 03123 were found on chromosomes SBI-01, SBI-02 and SBI-03, confirming the successful introgression of quantitative trait loci associated with stay-green from B35 into the senescent background R16. However, the alternative stay-green genetic sources were found to be distinct based on either all the SSRs employed or using only those associated with the stay-green trait in B35. Therefore, the physiological and biochemical basis of each stay-green source should be evaluated in order to enhance the understanding of the functioning of the trait in the various backgrounds. These genetic sources of stay-green could provide a valuable resource for improving this trait in sorghum breeding programmes.