Genetic factors acting prior to dormancy in sour cherry influence bloom time the following spring.

Understanding the process of Prunus species floral development is crucial for developing strategies to manipulate bloom time and prevent crop loss due to climate change. Here, we present a detailed examination of flower development from initiation until bloom for early- and late-blooming sour cherries (Prunus cerasus) from a population segregating for a major bloom time QTL on chromosome 4. Using a new staging system, we show floral buds from early-blooming trees were persistently more advanced than those from late-blooming siblings. A gDNA coverage analysis revealed the late-blooming haplotype of this QTL, k, is located on a subgenome originating from the late-blooming P. fruticosa progenitor. Transcriptome analyses identified many genes within this QTL as differentially expressed between early- and late-blooming trees during the vegetative-to-floral transition. From these, we identified candidate genes for the late bloom phenotype, including multiple transcription factors homologous to REproductive Meristem (REM) B3 domain-containing proteins. Additionally, we determined the basis of k in sour cherry is likely separate from candidate genes found in sweet cherry - suggesting several major regulators of bloom time are located on Prunus chromosome 4.

Defying Gravity: WEEP promotes negative gravitropism in peach trees by establishing asymmetric auxin gradients.

Trees with weeping shoot architectures are valued for their beauty and are a resource for understanding how plants regulate posture control. The peach (Prunus persica) weeping phenotype, which has elliptical downward arching branches, is caused by a homozygous mutation in the WEEP gene. Little is known about the function of WEEP despite its high conservation throughout Plantae. Here, we present the results of anatomical, biochemical, biomechanical, physiological, and molecular experiments that provide insight into WEEP function. Our data suggest that weeping peach trees do not have defects in branch structure. Rather, transcriptomes from the adaxial (upper) and abaxial (lower) sides of standard and weeping branch shoot tips revealed flipped expression patterns for genes associated with early auxin response, tissue patterning, cell elongation, and tension wood development. This suggests that WEEP promotes polar auxin transport toward the lower side during shoot gravitropic response, leading to cell elongation and tension wood development. In addition, weeping peach trees exhibited steeper root systems and faster lateral root gravitropic response. This suggests that WEEP moderates root gravitropism and is essential to establishing the set-point angle of lateral roots from the gravity vector. Additionally, size-exclusion chromatography indicated that WEEP proteins self-oligomerize, like other proteins with sterile alpha motif (SAM) domains. Collectively, our results from weeping peach provide insight into polar auxin transport mechanisms associated with gravitropism and lateral shoot and root orientation.

Peach LAZY1 and DRO1 protein-protein interactions and co-expression with PRAF/RLD family support conserved gravity-related protein interactions across plants.

IGT/LAZY proteins play a central role in determining gravitropic set point angle and orientation of lateral organs across plant species. Recent work in model systems has demonstrated that interactions between IGT/LAZY proteins and BREVIS RADIX (BRX)-domain containing proteins, such as PH, RCC1, AND FYVE/RCC1-LIKE DOMAIN (PRAF/RLD), and BREVIS RADIX LIKE (BRXL) family members, are mechanistically important for setting gravitropic set point angle. Here, we identified peach PRAF/RLD proteins as interactors of the peach IGT/LAZY proteins PpeLAZY1 and DEEPER ROOTING 1 (PpeDRO1) from a yeast-two-hybrid screen. We also show that the BRX domains of these interacting proteins have high sequence similarity with PRAF/RLD and BRX family proteins from rice and Arabidopsis. Further, PpeLAZY1 and the peach PRAF/RLD interactors are all expressed at relatively high levels in leaf, meristem, and shoot tip tissues. Together, this evidence supports the importance and conservation of IGT/LAZY-BRX-domain interactions, which underlie setting gravitropic set point angle across angiosperms.

Genome of tetraploid sour cherry (Prunus cerasus L.) 'Montmorency' identifies three distinct ancestral Prunus genomes.

Sour cherry (Prunus cerasus L.) is a valuable fruit crop in the Rosaceae family and a hybrid between progenitors closely related to extant Prunus fruticosa (ground cherry) and Prunus avium (sweet cherry). Here we report a chromosome-scale genome assembly for sour cherry cultivar Montmorency, the predominant cultivar grown in the USA. We also generated a draft assembly of P. fruticosa to use alongside a published P. avium sequence for syntelog-based subgenome assignments for 'Montmorency' and provide compelling evidence P. fruticosa is also an allotetraploid. Using hierarchal k-mer clustering and phylogenomics, we show 'Montmorency' is trigenomic, containing two distinct subgenomes inherited from a P. fruticosa-like ancestor (A and A') and two copies of the same subgenome inherited from a P. avium-like ancestor (BB). The genome composition of 'Montmorency' is AA'BB and little-to-no recombination has occurred between progenitor subgenomes (A/A' and B). In Prunus, two known classes of genes are important to breeding strategies: the self-incompatibility loci (S-alleles), which determine compatible crosses, successful fertilization, and fruit set, and the Dormancy Associated MADS-box genes (DAMs), which strongly affect dormancy transitions and flowering time. The S-alleles and DAMs in 'Montmorency' and P. fruticosa were manually annotated and support subgenome assignments. Lastly, the hybridization event 'Montmorency' is descended from was estimated to have occurred less than 1.61 million years ago, making sour cherry a relatively recent allotetraploid. The 'Montmorency' genome highlights the evolutionary complexity of the genus Prunus and will inform future breeding strategies for sour cherry, comparative genomics in the Rosaceae, and questions regarding neopolyploidy.

Defying Gravity: WEEP promotes negative gravitropism in Prunus persica (peach) shoots and roots by establishing asymmetric auxin gradients.

Trees with weeping shoot architectures are valued for their beauty and serve as tremendous resources for understanding how plants regulate posture control. The Prunus persica (peach) weeping phenotype, which has elliptical downward arching branches, is caused by a homozygous mutation in the WEEP gene. Until now, little was known about the function of WEEP protein despite its high conservation throughout Plantae. Here, we present the results of anatomical, biochemical, biomechanical, physiological, and molecular experiments that provide insight into WEEP function. Our data suggest that weeping peach does not have defects in branch structure. Rather, transcriptomes from the adaxial (upper) and abaxial (lower) sides of standard and weeping branch shoot tips revealed flipped expression patterns for genes associated with early auxin response, tissue patterning, cell elongation, and tension wood development. This suggests that WEEP promotes polar auxin transport toward the lower side during shoot gravitropic response, leading to cell elongation and tension wood development. In addition, weeping peach trees exhibited steeper root systems and faster root gravitropic response, just as barley and wheat with mutations in their WEEP homolog EGT2. This suggests that the role of WEEP in regulating lateral organ angles and orientations during gravitropism may be conserved. Additionally, size-exclusion chromatography indicated that WEEP proteins self-oligomerize, like other SAM-domain proteins. This oligomerization may be required for WEEP to function in formation of protein complexes during auxin transport. Collectively, our results from weeping peach provide new insight into polar auxin transport mechanisms associated with gravitropism and lateral shoot and root orientation.

Arabidopsis weep mutants exhibit narrow root angles.

Peach ( Prunus persica ) trees with a mutation in the weep gene exhibit a weeping branch phenotype. In contrast, Arabidopsis ( Arabidopsis thaliana ) weep mutants do not have a shoot architecture phenotype. A recent report revealed that barley ( Hordeum vulgare ) and wheat ( Triticum aestivum ) with mutations in EGT2, a WEEP homolog, have steeper root angles than standard varieties. We investigated the root architecture of three Arabidopsis weep mutant lines. All three lines exhibited steeper root angles and a smaller convex hull area, indicating that the total area explored by the root system is reduced. These results reveal WEEP is important for regulating lateral root angles in a dicot.

Preanthesis changes in freeze resistance, relative water content, and ovary growth preempt bud phenology and signify dormancy release of sour cherry floral buds.

MAIN CONCLUSION: Growing degree hours (GDH) predicted floral bud development of 'Montmorency' sour cherry and explained changes in lethal temperatures (LT50) that preempted any visible changes in bud phenology. The gradual warming during late winter and early spring promotes floral bud development and, concomitantly, the de-acclimation of Prunus sp. flowers. In fact, once ecodormancy releases, an approximate 20 °C loss of hardiness occurs prior to any distinguishable changes in external bud phenology. The aim of the following work was to characterize the physiological changes of 'Montmorency' sour cherry floral buds as they transition from endo- and ecodormancy and resume growth, and to determine whether physiological and anatomical characteristics within the buds preempt or signify dormancy release to enable a better prediction of freeze susceptibility. Here, we present a developmental timeline of the preanthesis changes of 'Montmorency' floral buds, ovaries and anthers over 2 years following their completion of chilling and relate these changes to growing degree hours (GDH) and the lethal temperature (LT50) of flowers. Changes in bud dry weight (DW), fresh weight (FW), volume, and external phenology stage including the percentage of green color development of bud scales were predicted by heat accumulation but were not early predictors of the increasing freeze susceptibility of pistils. Between endodormancy and green tip stage, ovary volume increased nearly threefold and relative water content (RWC) increased from ~ 45 to 70% in both years. A linear mixed regression model indicated that RWC and the interaction between RWC and ovary growth were significant predictors of LT50. Importantly, the loss of ~ 20 °C of freeze resistance occurred between 45 and 57% RWC and preceded any detectable changes in bud phenology. Microsporogenesis was observed after dormancy release when measurable changes in the ovary and bud RWC had already occurred. A GDH model estimated freeze sensitivity of pistils and explained 93% of the variation in LT50 during preanthesis development. A simple GDH model to predict critical freeze temperature of pistils should aid producers to manage frost protection.

There is more to flowering than those DAM genes: the biology behind bloom in rosaceous fruit trees.

The regulation of bloom time in deciduous fruit trees is an area of increasing interest due to the negative impact of climate change on fruit production. Although flower development has been well-studied in model species, there are many knowledge gaps about this process in perennial fruit trees, whose floral development spans the four seasons and includes many temperature-driven transitions. To develop solutions for minimizing crop loss, a comprehensive research strategy is needed to understand flower development and bloom time in deciduous fruit trees. This approach must incorporate genetic, physiological, and phenological strategies which include morphological and molecular analyses. Here, we describe key floral development events for rosaceae family fruit trees, highlight recent molecular and genetic discoveries, and discuss future directions for this field.

Opposing influences of TAC1 and LAZY1 on Lateral Shoot Orientation in Arabidopsis.

TAC1 and LAZY1 are members of a gene family that regulates lateral shoot orientation in plants. TAC1 promotes outward orientations in response to light, while LAZY1 promotes upward shoot orientations in response to gravity via altered auxin transport. We performed genetic, molecular, and biochemical assays to investigate possible interactions between these genes. In Arabidopsis they were expressed in similar tissues and double mutants revealed the wide-angled lazy1 branch phenotype, indicating it is epistatic to the tac1 shoot phenotype. Surprisingly, the lack of TAC1 did not influence gravitropic shoot curvature responses. Combined, these results suggest TAC1 might negatively regulate LAZY1 to promote outward shoot orientations. However, additional results revealed that TAC1- and LAZY1 influence on shoot orientation is more complex than a simple direct negative regulatory pathway. Transcriptomes of Arabidopsis tac1 and lazy1 mutants compared to wild type under normal and gravistimulated conditions revealed few overlapping differentially expressed genes. Overexpression of each gene did not result in major branch angle differences. Shoot tip hormone levels were similar between tac1, lazy1, and Col, apart from exceptionally elevated levels of salicylic acid in lazy1. The data presented here provide a foundation for future study of TAC1 and LAZY1 regulation of shoot architecture.

Branching out: new insights into the genetic regulation of shoot architecture in trees.

Directional growth in all plants involves both phototropic and gravitropic responses. Accordingly, mechanisms controlling shoot architecture throughout the plant kingdom are likely similar. However, as forms vary between species due in part to gene copy number and functional divergence, some aspects of how plants predetermine and regulate architecture can differ. This is especially true when comparing annual herbaceous species (e.g. model plants) to woody perennials such as trees. In the past decade, inexpensive genomic sequencing and technological advances enabled gene discovery and functional analyses in trees. This led to the identification of genes associated with tree shoot architecture control. Here, we present recent discoveries on the regulation of shoot architectures for which causative genes have been identified, including dwarf, weeping, columnar, and pillar growth habits. We also discuss potential applications of these findings.

Alteration of TAC1 expression in Prunus species leads to pleiotropic shoot phenotypes.

Prunus persica (peach) trees carrying the "Pillar" or "Broomy" trait (br) have vertically oriented branches caused by loss-of-function mutations in a gene called TILLER ANGLE CONTROL 1 (TAC1). TAC1 encodes a protein in the IGT gene family that includes LAZY1 and DEEPER ROOTING 1 (DRO1), which regulate lateral branch and root orientations, respectively. Here we found that some of the native TAC1 alleles in the hexaploid plum species Prunus domestica, which has a naturally more upright stature, contained a variable length trinucleotide repeat within the same exon 3 region previously found to be disrupted in pillar peach trees. RNAi silencing of TAC1 in plum resulted in trees with severely vertical branch orientations similar to those in pillar peaches but with an even narrower profile. In contrast, PpeTAC1 overexpression in plum led to trees with wider branch angles and more horizontal branch orientations. Pillar peach trees and transgenic plum lines exhibited pleiotropic phenotypes, including differences in trunk and branch diameter, stem growth, and twisting branch phenotypes. Expression profiling of pillar peach trees revealed differential expression of numerous genes associated with biotic and abiotic stress, hormone responses, plastids, reactive oxygen, secondary, and cell wall metabolism. Collectively, the data provide important clues for understanding TAC1 function and show that alteration of TAC1 expression may have broad applicability to agricultural and ornamental tree industries.

Loss of a highly conserved sterile alpha motif domain gene (WEEP) results in pendulous branch growth in peach trees.

Plant shoots typically grow upward in opposition to the pull of gravity. However, exceptions exist throughout the plant kingdom. Most conspicuous are trees with weeping or pendulous branches. While such trees have long been cultivated and appreciated for their ornamental value, the molecular basis behind the weeping habit is not known. Here, we characterized a weeping tree phenotype in Prunus persica (peach) and identified the underlying genetic mutation using a genomic sequencing approach. Weeping peach tree shoots exhibited a downward elliptical growth pattern and did not exhibit an upward bending in response to 90° reorientation. The causative allele was found to be an uncharacterized gene, Ppa013325, having a 1.8-Kb deletion spanning the 5' end. This gene, dubbed WEEP, was predominantly expressed in phloem tissues and encodes a highly conserved 129-amino acid protein containing a sterile alpha motif (SAM) domain. Silencing WEEP in the related tree species Prunus domestica (plum) resulted in more outward, downward, and wandering shoot orientations compared to standard trees, supporting a role for WEEP in directing lateral shoot growth in trees. This previously unknown regulator of branch orientation, which may also be a regulator of gravity perception or response, provides insights into our understanding of how tree branches grow in opposition to gravity and could serve as a critical target for manipulating tree architecture for improved tree shape in agricultural and horticulture applications.

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.

A brachytic dwarfism trait (dw) in peach trees is caused by a nonsense mutation within the gibberellic acid receptor PpeGID1c.

Little is known about the genetic factors controlling tree size and shape. Here, we studied the genetic basis for a recessive brachytic dwarfism trait (dw) in peach (Prunus persica) that has little or no effect on fruit development. A sequencing-based mapping strategy positioned dw on the distal end of chromosome 6. Further sequence analysis and fine mapping identified a candidate gene for dw as a non-functional allele of the gibberellic acid receptor GID1c. Expression of the two GID1-like genes found in peach, PpeGID1c and PpeGID1b, was analyzed. GID1c was predominantly expressed in actively growing vegetative tissues, whereas GID1b was more highly expressed in reproductive tissues. Silencing of GID1c in plum via transgenic expression of a hairpin construct led to a dwarf phenotype similar to that of dw/dw peaches. In general, the degree of GID1c silencing corresponded to the degree of dwarfing. The results suggest that PpeGID1c serves a primary role in vegetative growth and elongation, whereas GID1b probably functions to regulate gibberellic acid perception in reproductive organs. Modification of GID1c expression could provide a rational approach to control tree size without impairing fruit development.

Molecular basis of angiosperm tree architecture.

The architecture of trees greatly impacts the productivity of orchards and forestry plantations. Amassing greater knowledge on the molecular genetics that underlie tree form can benefit these industries, as well as contribute to basic knowledge of plant developmental biology. This review describes the fundamental components of branch architecture, a prominent aspect of tree structure, as well as genetic and hormonal influences inferred from studies in model plant systems and from trees with non-standard architectures. The bulk of the molecular and genetic data described here is from studies of fruit trees and poplar, as these species have been the primary subjects of investigation in this field of science.

Floral Transcriptomes in Woodland Strawberry Uncover Developing Receptacle and Anther Gene Networks.

Flowers are reproductive organs and precursors to fruits and seeds. While the basic tenets of the ABCE model of flower development are conserved in angiosperms, different flowering plants exhibit different and sometimes unique characteristics. A distinct feature of strawberry (Fragaria spp.) flowers is the development of several hundreds of individual apocarpous (unfused) carpels. These individual carpels are arranged in a spiral pattern on the subtending stem tip, the receptacle. Therefore, the receptacle is an integral part of the strawberry flower and is of significant agronomic importance, being the precursor to strawberry fruit. Taking advantage of next-generation sequencing and laser capture microdissection, we generated different tissue- and stage-transcriptomic profiling of woodland strawberry (Fragaria vesca) flower development. Using pairwise comparisons and weighted gene coexpression network analysis, we identified modules of coexpressed genes and hub genes of tissue-specific networks. Of particular importance is the discovery of a developing receptacle-specific module exhibiting similar molecular features to those of young floral meristems. The strawberry homologs of a number of meristem regulators, including LOST MERISTEM and WUSCHEL, are identified as hub genes operating in the developing receptacle network. Furthermore, almost 25% of the F-box genes in the genome are transiently induced in developing anthers at the meiosis stage, indicating active protein degradation. Together, this work provides important insights into the molecular networks underlying strawberry's unique reproductive developmental processes. This extensive floral transcriptome data set is publicly available and can be readily queried at the project Web site, serving as an important genomic resource for the plant biology research community.

SGR: an online genomic resource for the woodland strawberry.

BACKGROUND: Fragaria vesca, a diploid strawberry species commonly known as the alpine or woodland strawberry, is a versatile experimental plant system and an emerging model for the Rosaceae family. An ancestral F. vesca genome contributed to the genome of the octoploid dessert strawberry (F. ×ananassa), and the extant genome exhibits synteny with other commercially important members of the Rosaceae family such as apple and peach. To provide a molecular description of floral organ and fruit development at the resolution of specific tissues and cell types, RNAs from flowers and early developmental stage fruit tissues of the inbred F. vesca line YW5AF7 were extracted and the resulting cDNA libraries sequenced using an Illumina HiSeq2000. To enable easy access as well as mining of this two-dimensional (stage and tissue) transcriptome dataset, a web-based database, the Strawberry Genomic Resource (SGR), was developed. DESCRIPTION: SGR is a web accessible database that contains sample description, sample statistics, gene annotation, and gene expression analysis. This information can be accessed publicly from a web-based interface at http://bioinformatics.towson.edu/strawberry/Default.aspx. The SGR website provides user friendly search and browse capabilities for all the data stored in the database. Users are able to search for genes using a gene ID or description or obtain differentially expressed genes by entering different comparison parameters. Search results can be downloaded in a tabular format compatible with Microsoft excel application. Aligned reads to individual genes and exon/intron structures are displayed using the genome browser, facilitating gene re-annotation by individual users. CONCLUSIONS: The SGR database was developed to facilitate dissemination and data mining of extensive floral and fruit transcriptome data in the woodland strawberry. It enables users to mine the data in different ways to study different pathways or biological processes during reproductive development.

PpeTAC1 promotes the horizontal growth of branches in peach trees and is a member of a functionally conserved gene family found in diverse plants species.

Trees are capable of tremendous architectural plasticity, allowing them to maximize their light exposure under highly competitive environments. One key component of tree architecture is the branch angle, yet little is known about the molecular basis for the spatial patterning of branches in trees. Here, we report the identification of a candidate gene for the br mutation in Prunus persica (peach) associated with vertically oriented growth of branches, referred to as 'pillar' or 'broomy'. Ppa010082, annotated as hypothetical protein in the peach genome sequence, was identified as a candidate gene for br using a next generation sequence-based mapping approach. Sequence similarity searches identified rice TAC1 (tiller angle control 1) as a putative ortholog, and we thus named it PpeTAC1. In monocots, TAC1 is known to lead to less compact growth by increasing the tiller angle. In Arabidopsis, an attac1 mutant showed more vertical branch growth angles, suggesting that the gene functions universally to promote the horizontal growth of branches. TAC1 genes belong to a gene family (here named IGT for a shared conserved motif) found in all plant genomes, consisting of two clades: one containing TAC1-like genes; the other containing LAZY1, which contains an EAR motif, and promotes vertical shoot growth in Oryza sativa (rice) and Arabidopsis through influencing polar auxin transport. The data suggest that IGT genes are ancient, and play conserved roles in determining shoot growth angles in plants. Understanding how IGT genes modulate branch angles will provide insights into how different architectural growth habits evolved in terrestrial plants.