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Even though individuals with a deviant morphology have been elemental in genetics of model species, they have thus far been largely ignored in the studies of forest trees. Here we studied the inheritance of curly-grained and brown-figured wood phenotype in curly birch (Betula pendula var. carelica (Mercklin) Hamet-Ahti). In addition of the figured wood, curly birches display reduced and aberrant growth, indicating that the causative locus (loci) is (are) vital for normal tree development. To explore the genetic basis of this mutation, we studied the inheritance of the curly birch phenotype in a progeny trial (crosses between curly birch parent trees, between curly and normal phenotypes, and from selfings of curly trees). Based on the external morphology, the phenotypes of 11-year-old progeny trees were scored as either curly or wild. Based on the phenotypic segregation ratios, we postulate a simple Mendelian inheritance model for curliness: (i) a one-locus, two-allele model in which the allele coding for curly phenotype is dominant over the allele coding for normal phenotype and (ii) the semi-dominant curly allele is lethal when homozygous. We expect that further studies on the molecular genetic basis of the curly birch phenotype will provide valuable information on the developmental pathways involved in wood formation. Key words: Betula, curly birch, figured wood, genetics, wood quality. Meme si les individus dont la morphologie est anormale ont ete incontournables en genetique des especes modeles, ils ont jusqu'a maintenant ete largement ignores dans les etudes portant sur les arbres forestiers. Nous avons etudie la transmission du phenotype au bois figure de couleur brune ou madre chez le bouleau madre (Betula pendula var. carelica (Mercklin) Hamet-Ahti). En plus du bois figure, les bouleaux madres ont une croissance reduite et aberrante, indiquant que le locus (ou loci) responsables est (ou sont) essentiels au developpement normal de l'arbre. Afin d'explorer les fondements genetiques de cette mutation, la transmission du phenotype du bouleau madre a ete etudiee a l'aide d'un test de descendances (croisements entre des bouleaux madres, entre des phenotypes normaux et madres, et auto-croisements de bouleaux madres). A partir de la morphologie externe, le phenotype de descendants ages de 11 ans a ete classe madre ou sauvage. En se basant sur les rapports de segregation des phenotypes, un modele mendelien simple de transmission du phenotype madre a ete postule : (i) un modele a un locus et deux alleles oU l'allele codant pour le phenotype madre est dominant par rapport a l'allele codant pour le phenotype normal et (ii) l'allele semi-dominant codant pour le bois madre est letal lorsque homozygote. Nous esperons que des etudes additionnelles sur les fondements genetiques moleculaires du phenotype du bouleau madre fourniront des informations precieuses sur les sentiers de developpement qui contribuent a la formation du bois. [Traduit par la Redaction] Mots-cles : Betula, bouleau madre, bois figure, genetique, qualite du bois., Introduction Wood represents a renewable source of lignocellulosic biomass with immense commercial value and the genetic regulation of wood development is an important target for tree breeding efforts. Formation of [...]

Due to their long lifespan, trees and bushes develop higher order of branches in a perennial manner. In contrast to a tall tree, with a clearly defined main stem and branching order, a bush is shorter and has a less apparent main stem and branching pattern. To address the developmental basis of these two forms, we studied several naturally occurring architectural variants in silver birch ( Betula pendula ). Using a candidate gene approach, we identified a bushy kanttarelli variant with a loss-of-function mutation in the BpMAX1 gene required for strigolactone (SL) biosynthesis. While kanttarelli is shorter than the wild type (WT), it has the same number of primary branches, whereas the number of secondary branches is increased, contributing to its bush-like phenotype. To confirm that the identified mutation was responsible for the phenotype, we phenocopied kanttarelli in transgenic BpMAX1::RNAi birch lines. SL profiling confirmed that both kanttarelli and the transgenic lines produced very limited amounts of SL. Interestingly, the auxin (IAA) distribution along the main stem differed between WT and BpMAX1::RNAi . In the WT, the auxin concentration formed a gradient, being higher in the uppermost internodes and decreasing toward the basal part of the stem, whereas in the transgenic line, this gradient was not observed. Through modeling, we showed that the different IAA distribution patterns may result from the difference in the number of higher-order branches and plant height. Future studies will determine whether the IAA gradient itself regulates aspects of plant architecture., Competing Interests: Competing interests statement:The authors declare no competing interest.

Tree architecture has evolved to support a top-heavy above-ground biomass, but this integral feature poses a weight-induced challenge to trunk stability. Maintaining an upright stem is expected to require vertical proprioception through feedback between sensing stem weight and responding with radial growth. Despite its apparent importance, the principle by which plant stems respond to vertical loading forces remains largely unknown. Here, by manipulating the stem weight of downy birch (Betula pubescens) trees, we show that cambial development is modulated systemically along the stem. We carried out a genetic study on the underlying regulation by combining an accelerated birch flowering program with a recessive mutation at the ELIMÄKI locus (EKI), which causes a mechanically defective response to weight stimulus resulting in stem collapse after just 3 months. We observed delayed wood morphogenesis in eki compared with WT, along with a more mechanically elastic cambial zone and radial compression of xylem cell size, indicating that rapid tissue differentiation is critical for cambial growth under mechanical stress. Furthermore, the touch-induced mechanosensory pathway was transcriptionally misregulated in eki, indicating that the ELIMÄKI locus is required to integrate the weight-growth feedback regulation. By studying this birch mutant, we were able to dissect vertical proprioception from the gravitropic response associated with reaction wood formation. Our study provides evidence for both local and systemic responses to mechanical stimuli during secondary plant development., Competing Interests: Declaration of Interests The authors declare no competing interest, (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Silver birch (Betula pendula) is a pioneer boreal tree that can be induced to flower within 1 year. Its rapid life cycle, small (440-Mb) genome, and advanced germplasm resources make birch an attractive model for forest biotechnology. We assembled and chromosomally anchored the nuclear genome of an inbred B. pendula individual. Gene duplicates from the paleohexaploid event were enriched for transcriptional regulation, whereas tandem duplicates were overrepresented by environmental responses. Population resequencing of 80 individuals showed effective population size crashes at major points of climatic upheaval. Selective sweeps were enriched among polyploid duplicates encoding key developmental and physiological triggering functions, suggesting that local adaptation has tuned the timing of and cross-talk between fundamental plant processes. Variation around the tightly-linked light response genes PHYC and FRS10 correlated with latitude and longitude and temperature, and with precipitation for PHYC. Similar associations characterized the growth-promoting cytokinin response regulator ARR1, and the wood development genes KAK and MED5A.