A Growth-Based Framework for Leaf Shape Development and Diversity.

How do genes modify cellular growth to create morphological diversity? We study this problem in two related plants with differently shaped leaves: Arabidopsis thaliana (simple leaf shape) and Cardamine hirsuta (complex shape with leaflets). We use live imaging, modeling, and genetics to deconstruct these organ-level differences into their cell-level constituents: growth amount, direction, and differentiation. We show that leaf shape depends on the interplay of two growth modes: a conserved organ-wide growth mode that reflects differentiation; and a local, directional mode that involves the patterning of growth foci along the leaf edge. Shape diversity results from the distinct effects of two homeobox genes on these growth modes: SHOOTMERISTEMLESS broadens organ-wide growth relative to edge-patterning, enabling leaflet emergence, while REDUCED COMPLEXITY inhibits growth locally around emerging leaflets, accentuating shape differences created by patterning. We demonstrate the predictivity of our findings by reconstructing key features of C. hirsuta leaf morphology in A. thaliana. VIDEO ABSTRACT.

Morphomechanical Innovation Drives Explosive Seed Dispersal.

How mechanical and biological processes are coordinated across cells, tissues, and organs to produce complex traits is a key question in biology. Cardamine hirsuta, a relative of Arabidopsis thaliana, uses an explosive mechanism to disperse its seeds. We show that this trait evolved through morphomechanical innovations at different spatial scales. At the organ scale, tension within the fruit wall generates the elastic energy required for explosion. This tension is produced by differential contraction of fruit wall tissues through an active mechanism involving turgor pressure, cell geometry, and wall properties of the epidermis. Explosive release of this tension is controlled at the cellular scale by asymmetric lignin deposition within endocarp b cells-a striking pattern that is strictly associated with explosive pod shatter across the Brassicaceae plant family. By bridging these different scales, we present an integrated mechanism for explosive seed dispersal that links evolutionary novelty with complex trait innovation. VIDEO ABSTRACT.

A CUC1/auxin genetic module links cell polarity to patterned tissue growth and leaf shape diversity in crucifer plants.

How tissue-level information encoded by fields of regulatory gene activity is translated into the patterns of cell polarity and growth that generate the diverse shapes of different species remains poorly understood. Here, we investigate this problem in the case of leaf shape differences between Arabidopsis thaliana, which has simple leaves, and its relative Cardamine hirsuta that has complex leaves divided into leaflets. We show that patterned expression of the transcription factor CUP-SHAPED COTYLEDON1 in C. hirsuta (ChCUC1) is a key determinant of leaf shape differences between the two species. Through inducible genetic perturbations, time-lapse imaging of growth, and computational modeling, we find that ChCUC1 provides instructive input into auxin-based leaf margin patterning. This input arises via transcriptional regulation of multiple auxin homeostasis components, including direct activation of WAG kinases that are known to regulate the polarity of PIN-FORMED auxin transporters. Thus, we have uncovered a mechanism that bridges biological scales by linking spatially distributed and species-specific transcription factor expression to cell-level polarity and growth, to shape diverse leaf forms.

Pan-European study of genotypes and phenotypes in the Arabidopsis relative Cardamine hirsuta reveals how adaptation, demography, and development shape diversity patterns.

We study natural DNA polymorphisms and associated phenotypes in the Arabidopsis relative Cardamine hirsuta. We observed strong genetic differentiation among several ancestry groups and broader distribution of Iberian relict strains in European C. hirsuta compared to Arabidopsis. We found synchronization between vegetative and reproductive development and a pervasive role for heterochronic pathways in shaping C. hirsuta natural variation. A single, fast-cycling ChFRIGIDA allele evolved adaptively allowing range expansion from glacial refugia, unlike Arabidopsis where multiple FRIGIDA haplotypes were involved. The Azores islands, where Arabidopsis is scarce, are a hotspot for C. hirsuta diversity. We identified a quantitative trait locus (QTL) in the heterochronic SPL9 transcription factor as a determinant of an Azorean morphotype. This QTL shows evidence for positive selection, and its distribution mirrors a climate gradient that broadly shaped the Azorean flora. Overall, we establish a framework to explore how the interplay of adaptation, demography, and development shaped diversity patterns of 2 related plant species.

Uniparental silencing of 5S rRNA genes in plant allopolyploids - insights from Cardamine (Brassicaceae).

While the phenomenon of uniparental silencing of 35S rDNA in interspecific hybrids and allopolyploids is well documented, there is a notable absence of information regarding whether such silencing extends to the 5S RNA component of ribosomes. To address this gap in knowledge, we analyzed the 5S and 35S rDNA expression in Cardamine (Brassicaceae) allopolyploids, namely C. × insueta (2n = 3x = 24, genome composition RRA), C. flexuosa (2n = 4x = 32, AAHH), and C. scutata (2n = 4x = 32, PPAA) which share a common diploid ancestor (AA). We employed high-throughput sequencing of transcriptomes and genomes and phylogenetic analyses of 5S rRNA variants. The genomic organization of rDNA was further scrutinized through clustering and fluorescence in situ hybridization. In the C. × insueta allotriploid, we observed uniparental dominant expression of 5S and 35S rDNA loci. In the C. flexuosa and C. scutata allotetraploids, the expression pattern differed, with the 35S rDNA being expressed from the A subgenome, whereas the 5S rDNA was expressed from the partner subgenome. Both C. flexuosa and C. scutata but not C. × insueta showed copy and locus number changes. We conclude that in stabilized allopolyploids, transcription of ribosomal RNA components occurs from different subgenomes. This phenomenon appears to result in the formation of chimeric ribosomes comprising rRNA molecules derived from distinct parental origins. We speculate that the interplay of epigenetic silencing and rDNA rearrangements introduces an additional layer of variation in multimolecule ribosomal complexes, potentially contributing to the evolutionary success of allopolyploids.

Achromobacter seleniivolatilans sp. nov. and Buttiauxella selenatireducens sp. nov., isolated from the rhizosphere of selenium hyperaccumulator Cardamine hupingshanesis.

Two Gram-stain-negative bacterial strains, R39T and R73T, were isolated from the rhizosphere soil of the selenium hyperaccumulator Cardamine hupingshanesis in China. Strain R39T transformed selenite into elemental and volatile selenium, whereas strain R73T transformed both selenate and selenite into elemental selenium. Phylogenetic and phylogenomic analyses indicated that strain R39T belonged to the genus Achromobacter, while strain R73T belonged to the genus Buttiauxella. Strain R39T (genome size, 6.68 Mb; G+C content, 61.6 mol%) showed the closest relationship to Achromobacter marplatensis LMG 26219T and Achromobacter kerstersii LMG 3441T, with average nucleotide identity (ANI) values of 83.6 and 83.4 %, respectively. Strain R73T (genome size, 5.22 Mb; G+C content, 50.3 mol%) was most closely related to Buttiauxella ferragutiae ATCC 51602T with an ANI value of 86.4 %. Furthermore, strain A111 from the GenBank database was found to cluster with strain R73T within the genus Buttiauxella through phylogenomic analyses. The ANI and digital DNA-DNA hybridization values between strains R73T and A111 were 97.5 and 80.0% respectively, indicating that they belong to the same species. Phenotypic characteristics also differentiated strain R39T and strain R73T from their closely related species. Based on the polyphasic analyses, strain R39T and strain R73T represent novel species of the genera Achromobacter and Buttiauxella, respectively, for which the names Achromobacter seleniivolatilans sp. nov. (type strain R39T=GDMCC 1.3843T=JCM 36009T) and Buttiauxella selenatireducens sp. nov. (type strain R73T=GDMCC 1.3636T=JCM 35850T) are proposed.

Gene identification, expression analysis, and molecular docking of SAT and OASTL in the metabolic pathway of selenium in Cardamine hupingshanensis.

KEY MESSAGE: Identification of selenium stress-responsive expression and molecular docking of serine acetyltransferase (SAT) and O-acetyl serine (thiol) lyase (OASTL) in Cardamine hupingshanensis. A complex coupled with serine acetyltransferase (SAT) and O-acetyl serine (thiol) lyase (OASTL) is the key enzyme that catalyzes selenocysteine (Sec) synthesis in plants. The functions of SAT and OASTL genes were identified in some plants, but it is still unclear whether SAT and OASTL are involved in the selenium metabolic pathway in Cardamine hupingshanensis. In this study, genome-wide identification and comparative analysis of ChSATs and ChOASTLs were performed. The eight ChSAT genes were divided into three branches, and the thirteen ChOASTL genes were divided into four branches by phylogenetic analysis and sequence alignment, indicating the evolutionary conservation of the gene structure and its association with other plant species. qRT-PCR analysis showed that the ChSAT and ChOASTL genes were differentially expressed in different tissues under various selenium levels, suggesting their important roles in Sec synthesis. The ChSAT1;2 and ChOASTLA1;2 were silenced by the VIGS system to investigate their involvement in selenium metabolites in C. hupingshanensis. The findings contribute to understanding the gene functions of ChSATs and ChOASTLs in the selenium stress and provide a reference for further exploration of the selenium metabolic pathway in plants.

Coupled enhancer and coding sequence evolution of a homeobox gene shaped leaf diversity.

Here we investigate mechanisms underlying the diversification of biological forms using crucifer leaf shape as an example. We show that evolution of an enhancer element in the homeobox gene REDUCED COMPLEXITY (RCO) altered leaf shape by changing gene expression from the distal leaf blade to its base. A single amino acid substitution evolved together with this regulatory change, which reduced RCO protein stability, preventing pleiotropic effects caused by its altered gene expression. We detected hallmarks of positive selection in these evolved regulatory and coding sequence variants and showed that modulating RCO activity can improve plant physiological performance. Therefore, interplay between enhancer and coding sequence evolution created a potentially adaptive path for morphological evolution.

Evolution of hygrophytic plant species in the Anatolia-Caucasus region: insights from phylogenomic analyses of Cardamine perennials.

BACKGROUND AND AIMS: Southwestern Asia is a significant centre of biodiversity and a cradle of diversification for many plant groups, especially xerophytic elements. In contrast, little is known about the evolution and diversification of its hygrophytic flora. To fill this gap, we focus on Cardamine (Brassicaceae) species that grow in wetlands over a wide altitudinal range. We aimed to elucidate their evolution, assess the extent of presumed historical gene flow between species, and draw inferences about intraspecific structure. METHODS: We applied the phylogenomic Hyb-Seq approach, ecological niche analyses and multivariate morphometrics to a total of 85 Cardamine populations from the target region of Anatolia-Caucasus, usually treated as four to six species, and supplemented them with close relatives from Europe. KEY RESULTS: Five diploids are recognized in the focus area, three of which occur in regions adjacent to the Black and/or Caspian Sea (C. penzesii, C. tenera, C. lazica), one species widely distributed from the Caucasus to Lebanon and Iran (C. uliginosa), and one western Anatolian entity (provisionally C. cf. uliginosa). Phylogenomic data suggest recent speciation during the Pleistocene, likely driven by both geographic separation (allopatry) and ecological divergence. With the exception of a single hybrid (allotetraploid) speciation event proven for C. wiedemanniana, an endemic of southern Turkey, no significant traces of past or present interspecific gene flow were observed. Genetic variation within the studied species is spatially structured, suggesting reduced gene flow due to geographic and ecological barriers, but also glacial survival in different refugia. CONCLUSIONS: This study highlights the importance of the refugial regions of the Black and Caspian Seas for both harbouring and generating hygrophytic species diversity in Southwestern Asia. It also supports the significance of evolutionary links between Anatolia and the Balkan Peninsula. Reticulation and polyploidization played a minor evolutionary role here in contrast to the European relatives.

A novel selenite-tolerant rhizosphere bacterium Wautersiella enshiensis sp. nov., isolated from Chinese selenium hyperaccumulator, Cardamine hupingshanensis.

Selenium (Se) is a dietary essential trace element for humans with various physiological functions and it could also be accumulated by some plant species, like Astragalus bisulcatus, Stanleya pinnata, and Cardamine hupinshanensis. A novel Gram-stain-negative, facultatively anaerobic, selenite-tolerant bacterium, designated strain YLX-1T , was isolated from the rhizosphere of a Se hyperaccumulating plant, Cardamine hupingshanensis in Enshi, China. Phylogenetic analysis based on 16 S rRNA gene sequences indicated that strain YLX-1T is a potential new species in the genus Wautersiella. Strain YLX-1T could grow in the temperature range of 4-37°C (optimally at 28°C) and in the pH range of 5-9 (optimum pH 7), which also could tolerate Se up to 6000 mg Se/L via producing extracellular red nano-Se with 100-300 nm size. However, it could predominantly accumulate selenocystine (SeCys2 ) in the cell under lower Se stress (1.5 mg Se/L). These results would help broaden our knowledge about the Se accumulation and transformation mechanism involved in rhizosphere bacteria like strain YLX-1T in C. hupingshanensis. Based on polyphasic data, we propose the creation of the new species Wautersiella enshiensis sp. nov., strain YLX-1T ( = CCTCC M 2013671) which will be promising to produce nano-Se as fertilizer, food additives or medicine.

Alternate wiring of a KNOXI genetic network underlies differences in leaf development of A. thaliana and C. hirsuta.

Two interrelated problems in biology are understanding the regulatory logic and predictability of morphological evolution. Here, we studied these problems by comparing Arabidopsis thaliana, which has simple leaves, and its relative, Cardamine hirsuta, which has dissected leaves comprising leaflets. By transferring genes between the two species, we provide evidence for an inverse relationship between the pleiotropy of SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS (BP) homeobox genes and their ability to modify leaf form. We further show that cis-regulatory divergence of BP results in two alternative configurations of the genetic networks controlling leaf development. In C. hirsuta, ChBP is repressed by the microRNA164A (MIR164A)/ChCUP-SHAPED COTYLEDON (ChCUC) module and ChASYMMETRIC LEAVES1 (ChAS1), thus creating cross-talk between MIR164A/CUC and AS1 that does not occur in A. thaliana. These different genetic architectures lead to divergent interactions of network components and growth regulation in each species. We suggest that certain regulatory genes with low pleiotropy are predisposed to readily integrate into or disengage from conserved genetic networks influencing organ geometry, thus rapidly altering their properties and contributing to morphological divergence.

Photoreceptor Activity Contributes to Contrasting Responses to Shade in Cardamine and Arabidopsis Seedlings.

Plants have evolved two major ways to deal with nearby vegetation or shade: avoidance and tolerance. Moreover, some plants respond to shade in different ways; for example, Arabidopsis (Arabidopsis thaliana) undergoes an avoidance response to shade produced by vegetation, but its close relative Cardamine hirsuta tolerates shade. How plants adopt opposite strategies to respond to the same environmental challenge is unknown. Here, using a genetic strategy, we identified the C. hirsuta slender in shade1 mutants, which produce strongly elongated hypocotyls in response to shade. These mutants lack the phytochrome A (phyA) photoreceptor. Our findings suggest that C. hirsuta has evolved a highly efficient phyA-dependent pathway that suppresses hypocotyl elongation when challenged by shade from nearby vegetation. This suppression relies, at least in part, on stronger phyA activity in C. hirsuta; this is achieved by increased ChPHYA expression and protein accumulation combined with a stronger specific intrinsic repressor activity. We suggest that modulation of photoreceptor activity is a powerful mechanism in nature to achieve physiological variation (shade tolerance versus avoidance) for species to colonize different habitats.

The evolutionary history of Cardamine bulbifera shows a successful rapid postglacial Eurasian range expansion in the absence of sexual reproduction.

BACKGROUND AND AIMS: Sexual reproduction is known to drive plant diversification and adaptation. Here we investigate the evolutionary history and spatiotemporal origin of a dodecaploid (2n = 12x = 96) Eurasian deciduous woodland species, Cardamine bulbifera, which reproduces and spreads via vegetative bulb-like structures only. The species has been among the most successful range-expanding understorey woodland plants in Europe, which raises the question of the genetic architecture of its gene pool, since its hexaploid (2n = 6x = 48) but putatively outcrossing closest relative, C. quinquefolia, displays a smaller distribution range in Eastern Europe towards the Caucasus region. Cardamine bulbifera belongs to a small monophyletic clade of four species comprising also C. abchasica (2n = 2x = 16) and C. bipinnata (unknown ploidy) from the Caucasus region. METHODS: We sequenced the genomes of the two polyploids and their two putative ancestors using Illumina short-read sequencing technology (×7-8 coverage). Covering the entire distribution range, genomic data were generated for 67 samples of the two polyploids (51 samples of C. bulbifera, 16 samples of C. quinquefolia) and 6 samples of the putative diploid taxa (4 samples of C. abchasica, 2 samples of C. bipinnata) to unravel the evolutionary origin of the polyploid taxa using phylogenetic reconstructions of biparentally and maternally inherited genetic sequence data. Ploidy levels of C. bulbifera and C. quinquefolia were analysed by comparative chromosome painting. We used genetic assignment analysis (STRUCTURE) and approximate Bayesian computation (ABC) modelling to test whether C. bulbifera represents genetically differentiated lineages and addressed the hypothesis of its hybrid origin. Comparative ecological modelling was applied to unravel possible niche differentiation among the two polyploid species. KEY RESULTS: Cardamine bulbifera was shown to be a non-hybridogenous, auto-dodecaploid taxon of early Pleistocene origin, but with a history of past gene flow with its hexaploid sister species C. quinquefolia, likely during the last glacial maximum in shared refuge areas in Eastern Europe towards Western Turkey and the Crimean Peninsula region. The diploid Caucasian endemic C. abchasica is considered an ancestral species, which also provides evidence for the origin of the species complex in the Caucasus region. Cardamine bulbifera successfully expanded its distribution range postglacially towards Central and Western Europe accompanied by a transition to exclusively vegetative propagation. CONCLUSIONS: A transition to vegetative propagation in C. bulbifera is hypothesized as the major innovation to rapidly expand its distribution range following postglacially progressing woodland vegetation throughout Europe. Preceding and introgressive gene flow from its sister species C. quinquefolia in the joint refuge area is documented. This transition and ecological differentiation may have been triggered by preceding introgressive gene flow from its sister species in the joint East European refuge areas.

Integration analysis of PacBio SMRT- and Illumina RNA-seq reveals candidate genes and pathway involved in selenium metabolism in hyperaccumulator Cardamine violifolia.

BACKGROUND: Cardamine violifolia, native to China, is one of the selenium (Se) hyperaccumulators. The mechanism of Se metabolism and tolerance remains unclear, and only limited genetic information is currently available. Therefore, we combined a PacBio single-molecule real-time (SMRT) transcriptome library and the Illumina RNA-seq data of sodium selenate (Na2SeO4)-treated C. violifolia to further reveal the molecular mechanism of Se metabolism. RESULTS: The concentrations of the total, inorganic, and organic Se in C. violifolia seedlings significantly increased as the Na2SeO4 treatment concentration increased. From SMRT full-length transcriptome of C. violifolia, we obtained 26,745 annotated nonredundant transcripts, 14,269 simple sequence repeats, 283 alternative splices, and 3407 transcription factors. Fifty-one genes from 134 transcripts were identified to be involved in Se metabolism, including transporter, assimilatory enzyme, and several specific genes. Analysis of Illumina RNA-Seq data showed that a total of 948 differentially expressed genes (DEGs) were filtered from the four groups with Na2SeO4 treatment, among which 11 DEGs were related to Se metabolism. The enrichment analysis of KEGG pathways of all the DEGs showed that they were significantly enriched in five pathways, such as hormone signal transduction and plant-pathogen interaction pathways. Four genes related to Se metabolism, adenosine triphosphate sulfurase 1, adenosine 5'-phosphosulfate reductase 3, cysteine (Cys) desulfurase 1, and serine acetyltransferase 2, were regulated by lncRNAs. Twenty potential hub genes (e.g., sulfate transporter 1;1, Cys synthase, methionine gamma-lyase, and Se-binding protein 1) were screened and identified to play important roles in Se accumulation and tolerance in C. violifolia as concluded by weighted gene correlation network analysis. Based on combinative analysis of expression profiling and annotation of genes as well as Se speciation and concentration in C. violifolia under the treatments with different Na2SeO4 concentrations, a putative Se metabolism and assimilation pathway in C. violifolia was proposed. CONCLUSION: Our data provide abundant information on putative gene transcriptions and pathway involved in Se metabolism of C. violifolia. The findings present a genetic resource and provide novel insights into the mechanism of Se hyperaccumulation in C. violifolia.

Variation in Below-to Aboveground Systemic Induction of Glucosinolates Mediates Plant Fitness Consequences under Herbivore Attack.

Plants defend themselves against herbivore attack by constitutively producing toxic secondary metabolites, as well as by inducing them in response to herbivore feeding. Induction of secondary metabolites can cross plant tissue boundaries, such as from root to shoot. However, whether the potential for plants to systemically induce secondary metabolites from roots to shoots shows genetic variability, and thus, potentially, is under selection conferring fitness benefits to the plants is an open question. To address this question, we induced 26 maternal plant families of the wild species Cardamine hirsuta belowground (BG) using the wound-mimicking phytohormone jasmonic acid (JA). We measured resistance against a generalist (Spodoptera littoralis) and a specialist (Pieris brassicae) herbivore species, as well as the production of glucosinolates (GSLs) in plants. We showed that BG induction increased AG resistance against the generalist but not against the specialist, and found substantial plant family-level variation for resistance and GSL induction. We further found that the systemic induction of several GSLs tempered the negative effects of herbivory on total seed set production. Using a widespread natural system, we thus confirm that BG to AG induction has a strong genetic component, and can be under positive selection by increasing plant fitness. We suggest that natural variation in systemic induction is in part dictated by allocation trade-offs between constitutive and inducible GSL production, as well as natural variation in AG and BG herbivore attack in nature.

Trait-dependent resemblance of the flowering phenology and floral morphology of the allopolyploid Cardamine flexuosa to those of the parental diploids in natural habitats.

Allopolyploids possess complete sets of genomes derived from different parental species and exhibit a range of variation in various traits. Reproductive traits may play a key role in the reproductive isolation between allopolyploids and their parental species, thus affecting the thriving of allopolyploids. However, empirical data, especially in natural habitats, comparing reproductive trait variation between allopolyploids and their parental species remain rare. Here, we documented the flowering phenology and floral morphology of the allopolyploid wild plant Cardamine flexuosa and its diploid parents C. amara and C. hirsuta in their native range in Switzerland. The flowering of C. flexuosa started at an intermediate time compared with those of the parents and the flowering period of C. flexuosa overlapped with those of the parents. Cardamine flexuosa resembled C. hirsuta in the size of flowers and petals and the length/width ratio of petals, while it resembled C. amara in the length/width ratio of flowers. These results provide empirical evidence of the trait-dependent variation of allopolyploid phenotypes in natural habitats at the local scale. They also suggest that the variation in some reproductive traits in C. flexuosa is associated with self-fertilization. Therefore, it is helpful to consider the mating system in furthering the understanding of the processes that may have shaped trait variation in polyploids in nature.

Antagonistic roles for KNOX1 and KNOX2 genes in patterning the land plant body plan following an ancient gene duplication.

Neofunctionalization following gene duplication is thought to be one of the key drivers in generating evolutionary novelty. A gene duplication in a common ancestor of land plants produced two classes of KNOTTED-like TALE homeobox genes, class I (KNOX1) and class II (KNOX2). KNOX1 genes are linked to tissue proliferation and maintenance of meristematic potentials of flowering plant and moss sporophytes, and modulation of KNOX1 activity is implicated in contributing to leaf shape diversity of flowering plants. While KNOX2 function has been shown to repress the gametophytic (haploid) developmental program during moss sporophyte (diploid) development, little is known about KNOX2 function in flowering plants, hindering syntheses regarding the relationship between two classes of KNOX genes in the context of land plant evolution. Arabidopsis plants harboring loss-of-function KNOX2 alleles exhibit impaired differentiation of all aerial organs and have highly complex leaves, phenocopying gain-of-function KNOX1 alleles. Conversely, gain-of-function KNOX2 alleles in conjunction with a presumptive heterodimeric BELL TALE homeobox partner suppressed SAM activity in Arabidopsis and reduced leaf complexity in the Arabidopsis relative Cardamine hirsuta, reminiscent of loss-of-function KNOX1 alleles. Little evidence was found indicative of epistasis or mutual repression between KNOX1 and KNOX2 genes. KNOX proteins heterodimerize with BELL TALE homeobox proteins to form functional complexes, and contrary to earlier reports based on in vitro and heterologous expression, we find high selectivity between KNOX and BELL partners in vivo. Thus, KNOX2 genes confer opposing activities rather than redundant roles with KNOX1 genes, and together they act to direct the development of all above-ground organs of the Arabidopsis sporophyte. We infer that following the KNOX1/KNOX2 gene duplication in an ancestor of land plants, neofunctionalization led to evolution of antagonistic biochemical activity thereby facilitating the evolution of more complex sporophyte transcriptional networks, providing plasticity for the morphological evolution of land plant body plans.

Heterochrony underpins natural variation in Cardamine hirsuta leaf form.

A key problem in biology is whether the same processes underlie morphological variation between and within species. Here, by using plant leaves as an example, we show that the causes of diversity at these two evolutionary scales can be divergent. Some species like the model plant Arabidopsis thaliana have simple leaves, whereas others like the A. thaliana relative Cardamine hirsuta bear complex leaves comprising leaflets. Previous work has shown that these interspecific differences result mostly from variation in local tissue growth and patterning. Now, by cloning and characterizing a quantitative trait locus (QTL) for C. hirsuta leaf shape, we find that a different process, age-dependent progression of leaf form, underlies variation in this trait within species. This QTL effect is caused by cis-regulatory variation in the floral repressor ChFLC, such that genotypes with low-expressing ChFLC alleles show both early flowering and accelerated age-dependent changes in leaf form, including faster leaflet production. We provide evidence that this mechanism coordinates leaf development with reproductive timing and may help to optimize resource allocation to the next generation.

Conservation vs divergence in LEAFY and APETALA1 functions between Arabidopsis thaliana and Cardamine hirsuta.

A conserved genetic toolkit underlies the development of diverse floral forms among angiosperms. However, the degree of conservation vs divergence in the configuration of these gene regulatory networks is less clear. We addressed this question in a parallel genetic study between the closely related species Arabidopsis thaliana and Cardamine hirsuta. We identified leafy (lfy) and apetala1 (ap1) alleles in a mutant screen for floral regulators in C. hirsuta. C. hirsuta lfy mutants showed a complete homeotic conversion of flowers to leafy shoots, mimicking lfy ap1 double mutants in A. thaliana. Through genetic and molecular experiments, we showed that AP1 activation is fully dependent on LFY in C. hirsuta, by contrast to A. thaliana. Additionally, we found that LFY influences heteroblasty in C. hirsuta, such that loss or gain of LFY function affects its progression. Overexpression of UNUSUAL FLORAL ORGANS also alters C. hirsuta leaf shape in an LFY-dependent manner. We found that LFY and AP1 are conserved floral regulators that act nonredundantly in C. hirsuta, such that LFY has more obvious roles in floral and leaf development in C. hirsuta than in A. thaliana.

A developmental framework for dissected leaf formation in the Arabidopsis relative Cardamine hirsuta.

The developmental basis for the generation of divergent leaf forms is largely unknown. Here we investigate this problem by studying processes that distinguish development of two related species: Arabidopsis thaliana, which has simple leaves, and Cardamine hirsuta, which has dissected leaves with individual leaflets. Using genetics, expression studies and cell lineage tracing, we show that lateral leaflet formation in C. hirsuta requires the establishment of growth foci that form after leaf initiation. These growth foci are recruited at the leaf margin in response to activity maxima of auxin, a hormone that polarizes growth in diverse developmental contexts. Class I KNOTTED1-like homeobox (KNOX) proteins also promote leaflet initiation in C. hirsuta, and here we provide evidence that this action of KNOX proteins is contingent on the ability to organize auxin maxima via the PINFORMED1 (PIN1) auxin efflux transporter. Thus, differential deployment of a fundamental mechanism polarizing cellular growth contributed to the diversification of leaf form during evolution.