Allelic variation in cell wall candidate genes affecting solid wood properties in natural populations and land races of Pinus radiata.

Forest trees are ideally suited to association mapping due to their high levels of diversity and low genomic linkage disequilibrium. Using an association mapping approach, single-nucleotide polymorphism (SNP) markers influencing quantitative variation in wood quality were identified in a natural population of Pinus radiata. Of 149 sites examined, 10 demonstrated significant associations (P < 0.05, q < 0.1) with one or more traits after accounting for population structure and experimentwise error. Without accounting for marker interactions, phenotypic variation attributed to individual SNPs ranged from 2 to 6.5%. Undesirable negative correlations between wood quality and growth were not observed, indicating potential to break negative correlations by selecting for individual SNPs in breeding programs. Markers that yielded significant associations were reexamined in an Australian land race. SNPs from three genes (PAL1, PCBER, and SUSY) yielded significant associations. Importantly, associations with two of these genes validated associations with density previously observed in the discovery population. In both cases, decreased wood density was associated with the minor allele, suggesting that these SNPs may be under weak negative purifying selection for density in the natural populations. These results demonstrate the utility of LD mapping to detect associations, even when the power to detect SNPs with small effect is anticipated to be low.

Eucalypt MADS-box genes expressed in developing flowers.

Three MADS-box genes were identified from a cDNA library derived from young flowers of Eucalyptus grandis W. Hill ex Maiden. The three egm genes are single-copy genes and are expressed almost exclusively in flowers. The egm1 and egm3 genes shared strongest homology with other plant MADS-box genes, which mediate between the floral meristem and the organ-identity genes. The egm3 gene was also expressed strongly in the receptacle or floral tube, which surrounds the carpels in the eucalypt flower and bears the sepals, petals, and numerous stamens. There appeared to be a group of genes in eucalypts with strong homology with the 3' region of the egm1 gene. The egm2 gene was expressed in eucalypt petals and stamens and was most homologous to MADS-box genes, which belong to the globosa group of genes, which regulate organogenesis of the second and third floral whorls. The possible role of these three genes in eucalypt floral development is discussed.

Eucalyptus has a functional equivalent of the Arabidopsis floral meristem identity gene LEAFY.

Two genes cloned from Eucalyptus globulus, Eucalyptus LeaFy (ELF1 and ELF2), have sequence homology to the floral meristem identity genes LEAFY from Arabidopsis and FLORICAULA from Antirrhinum. ELF1 is expressed in the developing eucalypt floral organs in a pattern similar to LEAFY while ELF2 appears to be a pseudo gene. ELF1 is expressed strongly in the early floral primordium and then successively in the primordia of sepals, petals, stamens and carpels. It is also expressed in the leaf primordia and young leaves and adult and juvenile trees. The ELF1 promoter coupled to a GUS reporter gene directs expression in transgenic Arabidopsis in a temporal and tissue-specific pattern similar to an equivalent Arabidopsis LEAFY promoter construct. Strong expression is seen in young flower buds and then later in sepals and petals. No expression was seen in rosette leaves or roots of flowering plants or in any non-flowering plants grown under long days. Furthermore, ectopic expression of the ELF1 gene in transgenic Arabidopsis causes the premature conversion of shoots into flowers, as does an equivalent 35S-LFY construct. These data suggest that ELF1 plays a similar role to LFY in flower development and that the basic mechanisms involved in flower initiation and development in Eucalyptus are similar to those in Arabidopsis.

Two pine endo-beta-1,4-glucanases are associated with rapidly growing reproductive structures.

Two cDNA clones encoding endo-beta-1,4-glucanases (EGases) were isolated from a radiata pine (Pinus radiata) cDNA library prepared from immature female strobili. The cDNAs PrCel1 (inus adiata cellulase ) and PrCel2 encode proteins 509 and 515 amino acids in length, respectively, including putative signal peptides. Both proteins contain domains conserved in plant and bacterial EGases. The proteins PRCEL1 and PRCEL2 showed strong similarity to each other (76% amino acid identity), and higher similarity to TPP18 (73 and 67%, respectively), an EGase cloned from tomato (Lycopersicon esculentum) pistils, than to any other reported EGases. Northern-blot analyses indicated that both genes displayed a similar pattern of expression. The only significant difference was in the level of expression. In situ hybridizations were used to demonstrate that, within differentiating pine reproductive structures, PrCel1 expression was greatest in microsporangia in pollen strobili and near the developing ovule in the seed strobili. Expression was also found in vegetative tissues, especially in regions experiencing cell elongation, such as the elongating region of root tips. Both proteins have an ability to degrade carboxymethylcellulose in vitro. Genomic-blot analysis indicated the presence of a family of EGase genes in the radiata pine genome, and that PrCel1 and PrCel2 are transcribed from distinct one-copy genes.