The adaptation and survival of boreal and temperate perennials relies on the precise demarcation of the growing season. Seasonal growth and development are defined by day length and temperature ...signals. Under long-day conditions in spring, poplar FLOWERING LOCUS T2 (FT2) systemically induces shoot growth. In contrast, FT2 downregulation induced by autumnal short days triggers growth cessation and bud set. However, the molecular role of FT2 in local and long-range signaling is not entirely understood. In this study, the CRISPR/Cas9 editing tool was used to generate FT2 loss of function lines of hybrid poplar. Results indicate that FT2 is essential to promote shoot apex development and restrict internode elongation under conditions of long days. The application of bioactive gibberellins (GAs) to apical buds in FT2 loss of function lines was able to rescue bud set. Expression analysis of GA sensing and metabolic genes and hormone quantification revealed that FT2 boosts the 13-hydroxylation branch of the GA biosynthesis pathway in the shoot apex. Paclobutrazol treatment of WT leaves led to limited internode growth in the stem elongation zone. In mature leaves, FT2 was found to control the GA 13-hydroxylation pathway by increasing
and reducing
expression, causing reduced GA1 levels. We here show that in poplar, the FT2 signal promotes shoot apex development and restricts internode elongation through the GA 13-hydroxylation pathway.
Poplar trees use photoperiod as a precise seasonal indicator, synchronizing plant phenology with the environment. Daylength cue determines FLOWERING LOCUS T 2 (FT2) daily expression, crucial for ...shoot apex development and establishment of the annual growing period. However, limited evidence exists for the molecular factors controlling FT2 transcription and the conservation with the photoperiodic control of Arabidopsis flowering. We demonstrate that FT2 expression mediates growth cessation response quantitatively, and we provide a minimal data-driven model linking core clock genes to FT2 daily levels. GIGANTEA (GI) emerges as a critical inducer of the FT2 activation window, time-bound by TIMING OF CAB EXPRESSION (TOC1) and LATE ELONGATED HYPOCOTYL (LHY2) repressions. CRISPR/Cas9 loss-of-function lines validate these roles, identifying TOC1 as a long-sought FT2 repressor. Additionally, model simulations predict that FT2 downregulation upon daylength shortening results from a progressive narrowing of this activation window, driven by the phase shift observed in the preceding clock genes. This circadian-mediated mechanism enables poplar to exploit FT2 levels as an accurate daylength-meter.
Perennial species in the boreal and temperate regions are subject to extreme annual variations in light and temperature. They precisely adapt to seasonal changes by synchronizing cycles of growth and ...dormancy with external cues. Annual dormancy-growth transitions and flowering involve factors that integrate environmental and endogenous signals. MADS-box transcription factors have been extensively described in the regulation of
flowering. However, their participation in annual dormancy-growth transitions in trees is minimal. In this study, we investigate the function of
, a
×
(
)-related gene. Our gene expression analysis reveals that
displays lower mRNA levels during the winter than during early spring and mid-spring. Moreover,
activation depends on the fulfillment of the chilling requirement. Hybrid poplars overexpressing
show no differences in growth cessation and bud set, while ecodormant plants display an early bud break, indicating that
overexpression promotes bud growth reactivation. Comparative expression analysis of available bud break-promoting genes reveals that
overexpression downregulates the
(
), a gene involved in gibberellin catabolism. Moreover, the mid-winter to mid-spring RNAseq profiling indicates that
and
show antagonistic expression during bud dormancy release. Our results support
participation in the reactivation of shoot meristem growth during ecodormancy and link
activation and
downregulation within the temporal events that lead to poplar bud break.
In temperate and boreal regions, perennial trees arrest cell division in their meristematic tissues during winter dormancy until environmental conditions become appropriate for their renewed growth. ...Release from the dormant state requires exposure to a period of chilling temperatures similar to the vernalization required for flowering in
. Over the past decade, genomic DNA (gDNA) methylation and transcriptome studies have revealed signatures of chromatin regulation during active growth and winter dormancy. To date, only a few chromatin modification genes, as candidate regulators of these developmental stages, have been functionally characterized in trees. In this work, we summarize the major findings of the chromatin-remodeling role during growth-dormancy cycles and we explore the transcriptional profiling of vegetative apical bud and stem tissues during dormancy. Finally, we discuss genetic strategies designed to improve the growth and quality of forest trees.
Circadian clock performance during winter dormancy has been investigated in chestnut by using as marker genes CsTOC1 and CsLHY, which are homologous to essential components of the central circadian ...oscillator in Arabidopsis. During vegetative growth, mRNA levels of these two genes in chestnut seedlings and adult plants cycled daily, as expected. However, during winter dormancy, CsTOC1 and CsLHY mRNA levels were high and did not oscillate, indicating that the circadian clock was altered. A similar disruption was induced by chilling chestnut seedlings (to 4°C). Normal cycling resumed when endodormant or cold-treated plants were returned to 22°C. The behavior of CsTOC1 and CsLHY during a cold response reveals a relevant aspect of clock regulation not yet encountered in Arabidopsis.
Woody perennials adapt their genetic traits to local climate conditions. Day length plays an essential role in the seasonal growth of poplar trees. When photoperiod falls below a given critical day ...length, poplars undergo growth cessation and bud set. A leaf-localized mechanism of photoperiod measurement triggers the transcriptional modulation of a long distance signaling molecule, FLOWERING LOCUS T (FT). This molecule targets meristem function giving rise to these seasonal responses. Studies over the past decade have identified conserved orthologous genes involved in photoperiodic flowering in Arabidopsis that regulate poplar vegetative growth. However, phenological and molecular examination of key photoperiod signaling molecules reveals functional differences between these two plant model systems suggesting alternative components and/or regulatory mechanisms operating during poplar vegetative growth. Here, we review current knowledge and provide new data regarding the molecular components of the photoperiod measuring mechanism that regulates annual growth in poplar focusing on main achievements and new perspectives.
Cold acclimation in woody plants may have special features compared to similar processes in herbaceous plants. Recent studies have shown that circadian clock behavior in the chestnut tree (Castanea ...sativa) is disrupted by cold temperatures and that the primary oscillator feedback loop is not functional at 4 degrees C or in winter. In these conditions, CsTOC1 and CsLHY genes are constantly expressed. Here, we show that this alteration also affects CsPRR5, CsPRR7 and CsPRR9. These genes are homologous to the corresponding Arabidopsis PSEUDO-RESPONSE REGULATOR genes, which are also components of the circadian oscillator feedback network. The practically constant presence of mRNAs of the 5 chestnut genes at low temperature reveals an unknown aspect of clock regulation and suggests a mechanism regulating the transcription of oscillator genes as a whole.
Sylleptic branching in trees may increase significantly branch number, leaf area and the general growth of the tree, particularly in its early years. Although this is a very important trait, so far ...little is known about the genes that control this process.
This article characterizes the Castanea sativa RAV1 gene, homologous to Arabidopsis TEM genes, by analyzing its circadian behavior and examining its winter expression in chestnut stems and buds. Transgenic hybrid poplars over-expressing CsRAV1 or showing RNA interference down-regulated PtaRAV1 and PtaRAV2 expression were produced and analyzed.
Over-expression of the CsRAV1 gene induces the early formation of sylleptic branches in hybrid poplar plantlets during the same growing season in which the lateral buds form. Only minor growth differences and no changes in wood anatomy are produced.
The possibility of generating trees with a greater biomass by manipulating the CsRAV1 gene makes CsRAV1 transgenic plants promising candidates for bioenergy production.
Precise control of gene expression is essential to synchronize plant development with the environment. In perennial plants, transcriptional regulation remains poorly understood, mainly due to the ...long time required to perform functional studies. Transcriptional reporters based on luciferase have been useful to study circadian and diurnal regulation of gene expression, both by transcription factors and chromatin remodelers. The high mobility group proteins are considered transcriptional chaperones that also modify the chromatin architecture. They have been found in several species, presenting in some cases a circadian expression of their mRNA or protein.
Transactivation experiments have been shown as a powerful and fast method to obtain information about the potential role of transcription factors upon a certain reporter. We designed and validated a luciferase transcriptional reporter using the 5' sequence upstream ATG of
gene. We showed the robustness of this reporter line under long day and continuous light conditions. Moreover, we confirmed that
activity reproduces the accumulation of
mRNA. We performed transactivation studies by transient expression, using the reporter line as a genetic background, unraveling a new function of a high mobility group protein in poplar, which can activate the
promoter in a gate-dependent manner. We also showed PtaHMGB2/3 needs darkness to produce that activation and exhibits an active degradation after dawn, mediated by the 26S proteasome.
We generated a stable luciferase reporter poplar line based on the circadian clock gene
which can be used to investigate transcriptional regulation and signal transduction pathway. Using this reporter line as a genetic background, we established a methodology to rapidly assess potential regulators of diurnal and circadian rhythms. This tool allowed us to demonstrate that PtaHMGB2/3 promotes the transcriptional activation of our reporter in a gate-dependent manner. Moreover, we added new information about the PtaHMGB2/3 protein regulation along the day. This methodology can be easily adapted to other transcription factors and reporters.
PDF
