When to branch: seasonal control of shoot architecture in trees

Long-lived perennial plants optimize their shoot architecture by responding to seasonal cues. The main strategy used by plants of temperate and boreal regions with respect to surviving the extremely unfavourable conditions of winter comprises the protection of their apical and lateral meristematic tissues. This involves myriads of transcriptional, translational and metabolic changes in the plants because shoot architecture is controlled by multiple pathways that regulate processes such as bud formation and flowering, small RNAs, environmental factors (especially light quality, photoperiod and temperature), hormones, and sugars. Recent studies have begun to reveal how these pathways are recruited for the seasonal adaptation and regulation of shoot architecture in perennial plants, including the role of a regulatory module consisting of antagonistic players terminal flower 1 (TFL1) and like-ap1 (LAP1) in the hybrid aspen. Here, we review recent progress in our understanding of the genetic control of shoot architecture in perennials compared to in annuals

A hundred years after: endodormancy and the chilling requirement in subtropical trees

Endodormancy and the related chilling requirement synchronize the seasonal development of trees from the boreal and temperate regions under the climatic conditions prevailing at their native growing sites. The phenomenon of endodormancy has been known at the whole-plant level for 100 years, and in the last couple of decades, insights into the physiological and molecular basis of endodormancy and its release have also been obtained. Intriguingly, recent studies have shown experimentally that subtropical trees also show endodormancy and a chilling requirement. Motivated by the climatic differences between the subtropical and more northern zones, here we address the similarities and differences in endodormancy between trees growing in the subtropical zone and those growing in more northern zones

A molecular timetable for apical bud formation and dormancy induction in poplar

The growth of perennial plants in the temperate zone alternates with periods of dormancy that are typically initiated during bud development in autumn. In a systems biology approach to unravel the underlying molecular program of apical bud development in poplar (Populus tremula 3 Populus alba), combined transcript and metabolite profiling were applied to a high-resolution time course from short-day induction to complete dormancy. Metabolite and gene expression dynamics were used to reconstruct the temporal sequence of events during bud development. Importantly, bud development could be dissected into bud formation, acclimation to dehydration and cold, and dormancy. To each of these processes, specific sets of regulatory and marker genes and metabolites are associated and provide a reference frame for future functional studies. Light, ethylene, and abscisic acid signal transduction pathways consecutively control bud development by setting, modifying, or terminating these processes. Ethylene signal transduction is positioned temporally between light and abscisic acid signals and is putatively activated by transiently low hexose pools. The timing and place of cell proliferation arrest (related to dormancy) and of the accumulation of storage compounds (related to acclimation processes) were established within the bud by electron microscopy. Finally, the identification of a large set of genes commonly expressed during the growth-to-dormancy transitions in poplar apical buds, cambium, or Arabidopsis thaliana seeds suggests parallels in the underlying molecular mechanisms in different plant organs

Mechanochemical feedback mediates tissue bending required for seedling emergence

Tissue bending is vital to plant development, as exemplified by apical hook formation during seedling emergence by bending of the hypocotyl. How tissue bending is coordinated during development remains poorly understood, especially in plants where cells are attached via rigid cell walls. Asymmetric distribution of the plant hormone auxin underlies differential cell elongation during apical hook formation. Yet the underlying mechanism remains unclear. Here, we demonstrate spatial correlation between asymmetric auxin distribution, methylesterified homogalacturonan (HG) pectin, and mechanical properties of the epidermal layer of the hypocotyl in Arabidopsis. Genetic and cell biological approaches show that this mechanochemical asymmetry is essential for differential cell elongation. We show that asymmetric auxin distribution underlies differential HG methylesterification, and conversely changes in HG methylesterification impact the auxin response domain. Our results suggest that a positive feedback loop between auxin distribution and HG methylesterification underpins asymmetric cell wall mechanochemical properties to promote tissue bending and seedling emergence

Katanin-Dependent Microtubule Ordering in Association with ABA Is Important for Root Hydrotropism

Root hydrotropism refers to root directional growth toward soil moisture. Cortical microtubule arrays are essential for determining the growth axis of the elongating cells in plants. However, the role of microtubule reorganization in root hydrotropism remains elusive. Here, we demonstrate that the well-ordered microtubule arrays and the microtubule-severing protein KATANIN (KTN) play important roles in regulating root hydrotropism in Arabidopsis. We found that the root hydrotropic bending of the ktn1 mutant was severely attenuated but not root gravitropism. After hydrostimulation, cortical microtubule arrays in cells of the elongation zone of wild-type (WT) Col-0 roots were reoriented from transverse into an oblique array along the axis of cell elongation, whereas the microtubule arrays in the ktn1 mutant remained in disorder. Moreover, we revealed that abscisic acid (ABA) signaling enhanced the root hydrotropism of WT and partially rescued the oryzalin (a microtubule destabilizer) alterative root hydrotropism of WT but not ktn1 mutants. These results suggest that katanin-dependent microtubule ordering is required for root hydrotropism, which might work downstream of ABA signaling pathways for plant roots to search for water

Assembly of a gene sequence tag microarray by reversible biotin-streptavidin capture for transcript analysis of Arabidopsis thaliana

BACKGROUND: Transcriptional profiling using microarrays has developed into a key molecular tool for the elucidation of gene function and gene regulation. Microarray platforms based on either oligonucleotides or purified amplification products have been utilised in parallel to produce large amounts of data. Irrespective of platform examined, the availability of genome sequence or a large number of representative expressed sequence tags (ESTs) is, however, a pre-requisite for the design and selection of specific and high-quality microarray probes. This is of great importance for organisms, such as Arabidopsis thaliana, with a high number of duplicated genes, as cross-hybridisation signals between evolutionary related genes cannot be distinguished from true signals unless the probes are carefully designed to be specific. RESULTS: We present an alternative solid-phase purification strategy suitable for efficient preparation of short, biotinylated and highly specific probes suitable for large-scale expression profiling. Twenty-one thousand Arabidopsis thaliana gene sequence tags were amplified and subsequently purified using the described technology. The use of the arrays is exemplified by analysis of gene expression changes caused by a four-hour indole-3-acetic (auxin) treatment. A total of 270 genes were identified as differentially expressed (120 up-regulated and 150 down-regulated), including several previously known auxin-affected genes, but also several previously uncharacterised genes. CONCLUSIONS: The described solid-phase procedure can be used to prepare gene sequence tag microarrays based on short and specific amplified probes, facilitating the analysis of more than 21 000 Arabidopsis transcripts

Constraining the nuclear equation of state at subsaturation densities

Only one third of the nucleons in $^{208}$Pb occupy the saturation density area. Consequently nuclear observables related to average properties of nuclei, such as masses or radii, constrain the equation of state (EOS) not at saturation density but rather around the so-called crossing density, localised close to the mean value of the density of nuclei: $\rho\simeq$0.11 fm$^{-3}$. This provides an explanation for the empirical fact that several EOS quantities calculated with various functionals cross at a density significantly lower than the saturation one. The third derivative M of the energy at the crossing density is constrained by the giant monopole resonance (GMR) measurements in an isotopic chain rather than the incompressibility at saturation density. The GMR measurements provide M=1110 $\pm$ 70 MeV (6% uncertainty), whose extrapolation gives K$_\infty$=230 $\pm$ 40 MeV (17% uncertainty).Comment: 4 pages, 4 figure

A genetic network mediating the control of bud break in hybrid aspen

In boreal and temperate ecosystems, temperature signal regulates the reactivation of growth (bud break) in perennials in the spring. Molecular basis of temperature-mediated control of bud break is poorly understood. Here we identify a genetic network mediating the control of bud break in hybrid aspen. The key components of this network are transcription factor SHORT VEGETATIVE PHASE-LIKE (SVL), closely related to Arabidopsis floral repressor SHORT VEGETATIVE PHASE, and its downstream target TCP18, a tree homolog of a branching regulator in Arabidopsis. SVL and TCP18 are downregulated by low temperature. Genetic evidence demonstrates their role as negative regulators of bud break. SVL mediates bud break by antagonistically acting on gibberellic acid (GA) and abscisic acid (ABA) pathways, which function as positive and negative regulators of bud break, respectively. Thus, our results reveal the mechanistic basis for temperature-cued seasonal control of a key phenological event in perennial plants

The chromatin-modifying protein HUB2 is involved in the regulation of lignin composition in xylem vessels

PIRIN2 (PRN2) was earlier reported to suppress syringyl (S)-type lignin accumulation of xylem vessels of Arabidopsis thaliana. In the present study, we report yeast two-hybrid results supporting the interaction of PRN2 with HISTONE MONOUBIQUITINATION2 (HUB2) in Arabidopsis. HUB2 has been previously implicated in several plant developmental processes, but not in lignification. Interaction between PRN2 and HUB2 was verified by β-galactosidase enzymatic and co-immunoprecipitation assays. HUB2 promoted the deposition of S-type lignin in the secondary cell walls of both stem and hypocotyl tissues, as analysed by pyrolysis-GC/MS. Chemical fingerprinting of individual xylem vessel cell walls by Raman and Fourier transform infrared microspectroscopy supported the function of HUB2 in lignin deposition. These results, together with a genetic analysis of the hub2 prn2 double mutant, support the antagonistic function of PRN2 and HUB2 in deposition of S-type lignin. Transcriptome analyses indicated the opposite regulation of the S-type lignin biosynthetic gene FERULATE-5-HYDROXYLASE1 by PRN2 and HUB2 as the underlying mechanism. PRN2 and HUB2 promoter activities co-localized in cells neighbouring the xylem vessel elements, suggesting that the S-type lignin-promoting function of HUB2 is antagonized by PRN2 for the benefit of the guaiacyl (G)-type lignin enrichment of the neighbouring xylem vessel elements

EARLY BUD-BREAK 1 and EARLY BUD-BREAK 3 control resumption of poplar growth after winter dormancy

Bud-break is an economically and environmentally important process in trees and shrubs from boreal and temperate latitudes, but its molecular mechanisms are poorly understood. Here, we show that two previously reported transcription factors, EARLY BUD BREAK 1 (EBB1) and SHORT VEGETATIVE PHASE-Like (SVL) directly interact to control bud-break. EBB1 is a positive regulator of bud-break, whereas SVL is a negative regulator of bud-break. EBB1 directly and negatively regulates SVL expression. We further report the identification and characterization of the EBB3 gene. EBB3 is a temperature-responsive, epigenetically-regulated, positive regulator of bud-break that provides a direct link to activation of the cell cycle during bud-break. EBB3 is an AP2/ERF transcription factor that positively and directly regulates CYCLIND3.1 gene. Our results reveal the architecture of a putative regulatory module that links temperature-mediated control of bud-break with activation of cell cycle. An AP2/ERF family gene EBB1 and a MADS-box gene SVL encode two regulators of poplar bud break. Here, the authors report another AP2/ERF transcription factor EBB3, which functions together with EBB1, SVL, and cell cycle progression promoter CYCD3.1 to regulate poplar bud break