Phenotypic and genetic charatcterization of a hybrid aspen (Populus × canescens)

Youran Fan, Purdue University

Abstract

The value of wood is based not only on the species from which it was derived, but also on its aesthetic qualities. Figure, which can be defined as any design, pattern, or marking on the longitudinal surfaces of wood, is one of the characteristics that enhances the beauty of wood. In the research reported here, an attempt was made to elucidate the mechanism by which figure is produced by a male hybrid aspen, ‘Curly Poplar’ (P. × canescens), which is also known as ‘Curly Aspen’ or ‘Grober’. To determine whether figure was reproducible, wood produced by rooted propagules of ‘Curly Poplar’ was compared with that of various straight-grained aspens: ‘A502’ (P. alba), ‘4877’ (P. alba), ‘Ca-2-75’ (P. × canescens ), ‘Crandon’ (P. alba × P. grandidentata ), and ‘Sherrill’ (P. alba × P. grandidentata ). More than 80 ‘Curly Poplar’ propagules were sampled at various times (seven, 11, 12, and 60 months) after being grown under two environmental regimes (field vs. greenhouse) and following two treatments (pruned and unpruned). All ‘Curly Poplar’ propagules exhibited figure, represented by an undulating pattern on radial surfaces, which was visible after splitting stems through their pith. No figure was seen in any of the other groups, whose radial surfaces were all smooth and straight. Moreover, the veneer sliced from a 26-year-old ‘Curly Poplar’ tree revealed a type of figure that had not yet been described; thus, it was named ‘Scattered Moiré’. To investigate the mechanism of figure formation, thin sections from 51 vegetative propagules of three genotypes–‘Curly Poplar’, ‘4877’, and ‘Ca-2-75’–grown under greenhouse and field conditions, were used to make permanent microscope slides. In addition to a variety of anatomical measurements, basic specific gravity (BSG), volumetric shrinkage (VS), and field-grown-tree height (TH) were also determined. Our results showed that environmental conditions had an effect on tangential vessel diameter (TVD), fiber length (FL), and vessel length (VL). Ray height in cells (RHC), which is strongly influenced by genetic background, was lower in ‘Curly Poplar’ than in the other genotypes; this property may be associated with figure. Vessel number per mm2 (VMM2) and TVD are unlikely to be related to figure, as they were significantly different only for ‘4877’, regardless of growing conditions. In the greenhouse, ‘Curly Poplar’ had maximal FL and VL. Environmental conditions affected BSG, but not VS. Growth rate in the field was not significant different for any of the aspen genotypes evaluated. Cell-wall sugar composition was also analyzed, but no differences were detected among figured and non-figured trees as shown by an F-test (p>0.05). In an attempt to produce figured wood, the putative orthologs of two Arabidopsis thaliana genes (TUA4 and TUA6 ) were cloned from hybrid poplar clone INRA 353-38 (P. tremula x P. tremuloides) and used to assemble over-expression binary vectors. Dominant negative mutations of those genes have been shown to cause left-handed helical growth and clockwise twisting in elongating organs. Site-directed mutagenesis was used to cause the substitution of serine with phenylalanine at amino acid 180 in proteins encoded by the poplar genes. Constructs that over-expressed mutagenized versions of the poplar genes were transformed in INRA 353-38 via Agrobacterium tumefaciens. Only one transgenic poplar plant exhibited a visual phenotype: shortened internodes in vitro, and twisted petioles in the greenhouse. Quantitative real-time polymerase chain reaction (qRT-PCR) revealed a seven-fold increase in TUA5 expression of this transgenic line, as compared to the wild type, when grown in a greenhouse. The mutagenized poplar TUA3 and TUA7 genes were also over-expressed in Arabidopsis thaliana (Col-0). Only transgenic plants expressing TUA3 showed a phenotype similar to that seen in lefty mutants of A. thaliana. RNA interference constructs were also made using the putative Populus (INRA 353-38) orthologs of the A. thaliana TWD1 gene, and then transformed into INRA 353-38. However, transgenic poplar plants did not show any clear phenotype when compared to wild-type INRA 353-38. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Meilan, Purdue University.

Subject Area

Plant biology|Plant sciences|Wood sciences

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