Jorma Vahala

Chilling of Dormant Buds Hyperinduces FLOWERING LOCUS T and Recruits GA-Inducible 1,3-β-Glucanases to Reopen Signal Conduits and Release Dormancy in Populus

This work identifies 10 putative Populus orthologs of Arabidopsis genes that encode structurally different 1,3-β-glucanases and shows that they localize at and around plasmodesmata. These enzymes are differently regulated by daylength, temperature, GA3, and GA4, providing a mechanistic explanation of how cell communication is modulated during the dormancy cycling in synchrony with the seasons. In trees, production of intercellular signals and accessibility of signal conduits jointly govern dormancy cycling at the shoot apex. We identified 10 putative cell wall 1,3-β-glucanase genes (glucan hydrolase family 17 [GH17]) in Populus that could turn over 1,3-β-glucan (callose) at pores and plasmodesmata (PD) and investigated their regulation in relation to FT and CENL1 expression. The 10 genes encode orthologs of Arabidopsis thaliana BG_ppap, a PD-associated glycosylphosphatidylinositol (GPI) lipid-anchored protein, the Arabidopsis PD callose binding protein PDCB, and a birch (Betula pendula) putative lipid body (LB) protein. We found that these genes were differentially regulated by photoperiod, by chilling (5°C), and by feeding of gibberellins GA3 and GA4. GA3 feeding upregulated all LB-associated GH17s, whereas GA4 upregulated most GH17s with a GPI anchor and/or callose binding motif, but only GA4 induced true bud burst. Chilling upregulated a number of GA biosynthesis and signaling genes as well as FT, but not CENL1, while the reverse was true for both GA3 and GA4. Collectively, the results suggest a model for dormancy release in which chilling induces FT and both GPI lipid-anchored and GA3-inducible GH17s to reopen signaling conduits in the embryonic shoot. When temperatures rise, the reopened conduits enable movement of FT and CENL1 to their targets, where they drive bud burst, shoot elongation, and morphogenesis.

Ethylene is an endogenous stimulator of cell division in the cambial meristem of Populus

The plant hormone ethylene is an important signal in plant growth responses to environmental cues. In vegetative growth, ethylene is generally considered as a regulator of cell expansion, but a role in the control of meristem growth has also been suggested based on pharmacological experiments and ethylene-overproducing mutants. In this study, we used transgenic ethylene-insensitive and ethylene-overproducing hybrid aspen (Populus tremula × tremuloides) in combination with experiments using an ethylene perception inhibitor [1-methylcyclopropene (1-MCP)] to demonstrate that endogenous ethylene produced in response to leaning stimulates cell division in the cambial meristem. This ethylene-controlled growth gives rise to the eccentricity of Populus stems that is formed in association with tension wood.

Ethylene Insensitivity Modulates Ozone-Induced Cell Death in Birch

We have used genotypic variation in birch (Betula pendula Roth) to investigate the roles of ozone (O3)-induced ethylene (ET), jasmonic acid, and salicylic acid in the regulation of tissue tolerance to O3. Of these hormones, ET evolution correlated best with O3-induced cell death. Disruption of ET perception by transformation of birch with the dominant negative mutant allele etr1-1 of the Arabidopsis ET receptor gene ETR1 or blocking of ET perception with 1-methylcyclopropene reduced but did not completely prevent the O3-induced cell death, when inhibition of ET biosynthesis with aminooxyacetic acid completely abolished O3 lesion formation. This suggests the presence of an ET-signaling-independent but ET biosynthesis-dependent component in the ET-mediated stimulation of cell death in O3-exposed birch. Functional ET signaling was required for the O3 induction of the gene encoding β-cyanoalanine synthase, which catalyzes detoxification of the cyanide formed during ET biosynthesis. The results suggest that functional ET signaling is required to protect birch from the O3-induced cell death and that a decrease in ET sensitivity together with a simultaneous, high ET biosynthesis can potentially cause cell death through a deficient detoxification of cyanide.

Differential Effects of Elevated Ozone on Two Hybrid Aspen Genotypes Predisposed to Chronic Ozone Fumigation. Role of Ethylene and Salicylic Acid

The role of ethylene (ET) signaling in the responses of two hybrid aspen (Populus tremula L. × P.tremuloides Michx.) clones to chronic ozone (O3; 75 nL L−1) was investigated. The hormonal responses differed between the clones; the O3-sensitive clone 51 had higher ET evolution than the tolerant clone 200 during the exposure, whereas the free salicylic acid concentration in clone 200 was higher than in clone 51. The cellular redox status, measured as glutathione redox balance, did not differ between the clones suggesting that the O3 lesions were not a result of deficient antioxidative capacity. The buildup of salicylic acid during chronic O3 exposure might have prevented the up-regulation of ET biosynthesis in clone 200. Blocking of ET perception with 1-methylcyclopropene protected both clones from the decrease in net photosynthesis during chronic exposure to O3. After a pretreatment with low O3 for 9 d, an acute 1.5-fold O3 elevation caused necrosis in the O3-sensitive clone 51, which increased substantially when ET perception was blocked. The results suggest that in hybrid aspen, ET signaling had a dual role depending on the severity of the stress. ET accelerated leaf senescence under low O3, but under acute O3 elevation, ET signaling seemed to be required for protection from necrotic cell death.

Molecular evidence suggests that Ceratobasidium bicorne has an anamorph known as a conifer pathogen

In Finland and Norway, a uninucleate Rhizoctonia sp. is causing a root dieback disease on nursery-grown Norway spruce and Scots pine seedlings. This Rhizoctonia can be fruited under laboratory conditions and the basidial characters fit well in the species concept of Ceratobasidium bicorne, a species originally described as a moss parasite under forest conditions. Further comparison using traditional methods (cultural morphology, nuclear condition, anastomosis) has not been possible as the forest population of C. bicorne has apparently never been cultured. In the present study, we isolated DNA from a herbarium sample of C. bicorne grown on the moss Polytrichastrum formosum. Sequence analysis of the PCR-amplified rDNA region containing the internal transcribed spacer (ITS) regions and the 5.8S rDNA gene was used to examine the conspecificity of the herbarium sample and the uninucleate Rhizoctonia sp. The nucleotide sequence of the 5.8S rDNA gene was identical between the herbarium sample and five sequenced uninucleate Rhizoctonia strains. Within the uninucleate Rhizoctonia sp., the sequence identity ranged from 96.1 to 100% in ITS1 and from 99.6 to 100% in ITS2. The sequence from the herbarium sample fits well within these limits, strongly suggesting that the uninucleate Rhizoctonia sp. and C. bicorne are conspecific. Interestingly, two of the uninucleate Rhizoctonia strains produced two ITS alleles: the genetic implications are also discussed.

Genome sequencing and population genomic analyses provide insights into the adaptive landscape of silver birch

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.

Axillary buds are dwarfed shoots that tightly regulate GA pathway and GA-inducible 1,3-β-glucanase genes during branching in hybrid aspen

Highlight Axillary buds uniquely regulate gibberellin (GA) pathway genes, enabling them to stay inhibited but simultaneously poised for growth. Decapitation promotes expression of GA-inducible 1,3-β-glucanase genes that function to reinvigorate symplasmic connections to the stem.

Long and short photoperiod buds in hybrid aspen share structural development and expression patterns of marker genes

Highlight Short photoperiod and apical dominance trigger a shared developmental bud programme at terminal and axillary positions, while the capacity to establish photoperiod-induced dormancy is lost in maturing para-dormant axillary buds.

Ethylene signaling via Ethylene Response Factors (ERFs) modifies wood development in hybrid aspen

Background The phytohormone ethylene (ET) has the potential to regulate secondary growth of plants and wood formation in trees. Application of exogenous ethylene or its in planta precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), to wood forming tissues of hybrid aspen (Populus tremula x Populus tremuloides) enhances xylem growth [1]. In the same study it was demonstrated that stimulation of enhanced xylem formation (tension wood, TW) at the upper side of leaning stems is mediated by endogenous ET. The production of endogenous ET in TW forming tissues is further supported by the increase of ACC oxidase gene transcript and enzyme activity on the TW side [2]. The ET perception and signal transmission cascade in Arabidopsis has been linked to the transcriptional activation of Ethylene Response Factors (ERFs) [3,4]. As transcription factors, ERFs regulate the expression of various specific downstream target genes by binding to cis-elements in their promoters [5]. We hypothesize that ERFs participate in xylem development through ethylene signaling and that they are involved in ET responses during TW formation.