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Invariant floral forms are important for reproductive success and robust to natural perturbations. Petal number, for example, is invariant in Arabidopsis thaliana flowers. However, petal number varies in the closely related species Cardamine hirsuta , and the genetic basis for this difference between species is unknown. Here we show that divergence in the pleiotropic floral regulator APETALA1 ( AP1 ) can account for the species-specific difference in petal number robustness. This large effect of AP1 is explained by epistatic interactions: A. thaliana AP1 confers robustness by masking the phenotypic expression of quantitative trait loci controlling petal number in C. hirsuta . We show that C. hirsuta AP1 fails to complement this function of A. thaliana AP1 , conferring variable petal number, and that upstream regulatory regions of AP1 contribute to this divergence. Moreover, variable petal number is maintained in C. hirsuta despite sufficient standing genetic variation in natural accessions to produce plants with four-petalled flowers., Competing Interests: MM, BP, SM, AR, DK, RS, AH No competing interests declared, (© 2018, Monniaux et al.)

Four petals characterize the flowers of most species in the Brassicaceae family, and this phenotype is generally robust to genetic and environmental variation. A variable petal number distinguishes the flowers of Cardamine hirsuta from those of its close relative Arabidopsis ( Arabidopsis thaliana ), and allelic variation at many loci contribute to this trait. However, it is less clear whether C. hirsuta petal number varies in response to seasonal changes in environment. To address this question, we assessed whether petal number responds to a suite of environmental and endogenous cues that regulate flowering time in C. hirsuta We found that petal number showed seasonal variation in C. hirsuta , such that spring flowering plants developed more petals than those flowering in summer. Conditions associated with spring flowering, including cool ambient temperature, short photoperiod, and vernalization, all increased petal number in C. hirsuta Cool temperature caused the strongest increase in petal number and lengthened the time interval over which floral meristems matured. We performed live imaging of early flower development and showed that floral buds developed more slowly at 15°C versus 20°C. This extended phase of floral meristem formation, coupled with slower growth of sepals at 15°C, produced larger intersepal regions with more space available for petal initiation. In summary, the growth and maturation of floral buds is associated with variable petal number in C. hirsuta and responds to seasonal changes in ambient temperature., (© 2017 American Society of Plant Biologists. All Rights Reserved.)

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., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

The transition to flowering in many plant species, including Arabidopsis (Arabidopsis thaliana), is marked by the elongation of internodes to make an inflorescence upon which lateral branches and flowers are arranged in a characteristic pattern. Inflorescence patterning relies in part on the activities of two three-amino-acid loop-extension homeodomain transcription factors: BREVIPEDICELLUS (BP) and PENNYWISE (PNY) whose interacting products also promote meristem function. We examine here the genetic interactions between BP-PNY whose expression is up-regulated in stems at the floral transition, and the lateral organ boundary genes BLADE-ON-PETIOLE1 (BOP1) and BOP2, whose expression is restricted to pedicel axils. Our data show that bp and pny inflorescence defects are caused by BOP1/2 gain of function in stems and pedicels. Compatible with this, inactivation of BOP1/2 rescues these defects. BOP expression domains are differentially enlarged in bp and pny mutants, corresponding to the distinctive patterns of growth restriction in these mutants leading to compacted internodes and clustered or downward-oriented fruits. Our data indicate that BOP1/2 are positive regulators of KNOTTED1-LIKE FROM ARABIDOPSIS THALIANA6 expression and that growth restriction in BOP1/2 gain-of-function plants requires KNOTTED1-LIKE FROM ARABIDOPSIS THALIANA6. Antagonism between BOP1/2 and BP is explained in part by their reciprocal regulation of gene expression, as evidenced by the identification of lignin biosynthetic genes that are repressed by BP and activated by BOP1/2 in stems. These data reveal BOP1/2 gain of function as the basis of bp and pny inflorescence defects and reveal how antagonism between BOP1/2 and BP-PNY contributes to inflorescence patterning in a model plant species.

Acyl-CoA Synthetase (ACOS) genes are related to 4-coumarate:CoA ligase (4CL) but have distinct functions. The Arabidopsis thaliana ACOS5 protein is in clade A of Arabidopsis ACOS proteins, the clade most closely related to 4CL proteins. This clade contains putative nonperoxisomal ACOS enzymes conserved in several angiosperm lineages and in the moss Physcomitrella patens. Although its function is unknown, ACOS5 is preferentially expressed in the flowers of all angiosperms examined. Here, we show that an acos5 mutant produced no pollen in mature anthers and no seeds by self-fertilization and was severely compromised in pollen wall formation apparently lacking sporopollenin or exine. The phenotype was first evident at stage 8 of anther development and correlated with maximum ACOS5 mRNA accumulation in tapetal cells at stages 7 to 8. Green fluorescent protein-ACOS5 fusions showed that ACOS5 is located in the cytoplasm. Recombinant ACOS5 enzyme was active against oleic acid, allowing kinetic constants for ACOS5 substrates to be established. Substrate competition assays indicated broad in vitro preference of the enzyme for medium-chain fatty acids. We propose that ACOS5 encodes an enzyme that participates in a conserved and ancient biochemical pathway required for sporopollenin monomer biosynthesis that may also include the Arabidopsis CYP703A2 and MS2 enzymes.

The Arabidopsis BLADE-ON-PETIOLE 1 (BOP1) and BOP2 genes encode redundant transcription factors that promote morphological asymmetry during leaf and floral development. Loss-of-function bop1 bop2 mutants display a range of developmental defects, including a loss of floral organ abscission. Abscission occurs along specialised cell files, called abscission zones (AZs) that develop at the junction between the leaving organ and main plant body. We have characterized the bop1 bop2 abscission phenotype to determine how BOP1 and BOP2 contribute to the known abscission developmental framework. Histological analysis and petal breakstrength measurements of bop1 bop2 flowers show no differentiation of floral AZs. Furthermore, vestigial cauline leaf AZs are also undifferentiated in bop1 bop2 mutants, suggesting that BOP proteins are essential to establish AZ cells in different tissues. In support of this hypothesis, BOP1/BOP2 activity is required for both premature floral organ abscission and the ectopic abscission of cauline leaves promoted by the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) gene under the control of the constitutive CaMV 35S promoter. Expression of several abscission-related marker genes, including IDA, is relatively unperturbed in bop1 bop2 mutants, indicating that these AZ genes respond to positional cues that are independent of BOP1/BOP2 activity. We also show that BOP1 and BOP2 promote growth of nectary glands, which normally develop at the receptacle adjacent to developing AZs. Taken together, these data suggest that BOP1/BOP2 activity is required for multiple cell differentiation events in the proximal regions of inflorescence lateral organs.