Early Cone Setting in Picea abies acrocona Is Associated with Increased Transcriptional Activity of a MADS Box Transcription Factor    

Abstract

Conifers normally go through a long juvenile period, for Norway spruce (Picea abies) around 20 to 25 years, before developing male and female cones. We have grown plants from inbred crosses of a naturally occurring spruce mutant (acrocona). One-fourth of the segregating acrocona plants initiate cones already in their second growth cycle, suggesting control by a single locus. The early cone-setting properties of the acrocona mutant were utilized to identify candidate genes involved in vegetative-toreproductive phase change in Norway spruce. Poly(A + ) RNA samples from apical and basal shoots of cone-setting and noncone-setting plants were subjected to high-throughput sequencing (RNA-seq). We assembled and investigated 33,383 expressed putative protein-coding acrocona transcripts. Eight transcripts were differentially expressed between selected sample pairs. One of these (Acr42124_1) was significantly up-regulated in apical shoot samples from cone-setting acrocona plants, and the encoded protein belongs to the MADS box gene family of transcription factors. Using quantitative real-time polymerase chain reaction with independently derived plant material, we confirmed that the MADS box gene is up-regulated in both needles and buds of cone-inducing shoots when reproductive identity is determined. Our results constitute important steps for the development of a rapid cycling model system that can be used to study gene function in conifers. In addition, our data suggest the involvement of a MADS box transcription factor in the vegetative-to-reproductive phase change in Norway spruce.The two most commonly grown and economically important conifers in Sweden, Scots pine (Pinus sylvestris) and Norway spruce (Picea abies), both go through a long vegetative growth phase before they begin to produce cones; the vegetative growth period is 20 to 25 years and 8 to 20 years for Norway spruce and Scots pine, respectively. Mature Scots pine trees produce cones every year, while mature Norway spruce trees produce cones only every 3rd to 5th year. The timing of cone establishment is largely synchronized within the various spruce populations and is determined by a combination of genetic and environmental factors (Lindgren et al., 1977;Högberg and Eriksson, 1994). The long generation time and the irregular cone setting between different years pose major obstacles for breeding of Norway spruce both with respect to genetic gain and retained diversity. To guarantee a sufficient seed supply, forest companies have to plant and maintain large seed orchards and in addition store large quantities of seeds to cover the demand during years of little or no seed production. Both conifer breeding programs and the production of improved seed for forest regeneration would benefit from methods to control the length of the vegetative period before the trees start to produce cones as well as the possibilities to control cone setting and cone production itself. Despite that, our knowledge of the genetic mechanisms that regulate the transition from vegetati...

Molecular control of normal and acrocona mutant seed cone development in Norway spruce (Picea abies) and the evolution of conifer ovule‐bearing organs

“…Segregating siblings derived from inbred crosses of the acrocona mutant produce cones at a very early age (Uddenberg et al ., ). The early cone‐setting property of the acrocona mutation segregates as a single locus (Acheré et al ., ; Uddenberg et al ., ), but the mutation causing these traits has not yet been identified.…”

A transcriptomics investigation into pine reproductive organ development

“…Some of these orthologues in P . abies have been shown to play key roles in cone development (Carlsbecker et al ., ; Uddenberg et al ., ). We found that the expression of eight MADS ‐ box genes ( PtDAL1 , PtDAL2 , PtDAL3 , PtDAL4 , PtDAL10 , PtDAL14 , PtMADS1 and PtMADS3 ) and PtNDLY were significantly higher in male structures of bisexual cones than in normal male cones (closest to maturity) (Figs , S9).…”

Partial correlation analysis of transcriptomes helps detangle the growth and defense network in spruce

“…We identified spruce MADS‐box genes that are candidates for reproductive maturity or reproductive meristem identity in conifers (annotated as AGL20 (S uppressor of O verexpression of C onstans 1 ), SOC1‐like gene AGL42 and AGL2 ( SEPALLATA1 ) based on sequence homology with Arabidopsis genes) (Melzer et al ., ; Uddenberg et al ., ; Katahata et al ., ) which are paralogs of flowering genes in Arabidopsis . As there are no SEP1 genes in gymnosperms, the identified P. glauca gene WS00823_F11 is likely the ortholog of P. abies DEFICIENS AGAMOUS‐LIKE1 (PgMADS10_DAL1, with proposed function in regulating the transition from juvenile to adult phase in Picea ; Carlsbecker et al ., ), and which is located within the SEP1 sister clade more closely related to AGL6 .…”