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Background Lagerstroemia speciosa ‘Zijuan’ is a new variety of L. speciosa with highly petaloid stamens that exhibit ornamental value but sterility. To investigate the mechanism of stamen petaloidy, this study compared the morphological structures and transcriptomic profiles of stamens between L. speciosa and L. speciosa ‘Zijuan’. Results Stamen petaloidy in L. speciosa ‘Zijuan’ initiates when bud diameters reached approximately 3 mm. During three developmental stages—stamen primordium (buds 1–2 mm), anther formation (buds 2–3 mm), and anther development (buds 3–4 mm)—8122, 8058, and 7242 differentially expressed genes (DEGs) were identified between the two varieties, respectively. GO enrichment analysis revealed that most DEGs related to tapetum development and photoperiod sensing in L. speciosa ‘Zijuan’ were upregulated during stamen development compared to L. speciosa . KEGG pathway analysis identified activated auxin, gibberellins, and jasmonic acid response factors in hormone signaling pathways. Expression differences in A-, B-, and C-class genes were observed across the three stages: A- and B-class genes showed minimal changes, while C-class gene expression was suppressed. Weighted gene co-expression network analysis (WGCNA) analysis of C-class co-expression networks suggested potential associations between AGAMOUS and its homolog MADS3 with SMD1B . Marked expression disparities in these genes were confirmed between L. speciosa and L. speciosa ‘Zijuan’ flower buds. RT-qPCR results corroborated transcriptome data, validating reliability. Conclusions This study proposes novel regulatory mechanisms for stamen petaloidy, providing technical insights for future investigations into stamen development, petaloidy mechanisms, and candidate gene selection in L. speciosa .

Abstract Background and Aims Salvia is the largest genus within Lamiaceae, with about 980 species currently recognized. East Asia, with approx. 100 species, is one of the three major biodiversity centres of Salvia. However, relationships within this lineage remain unclear, and the staminal lever mechanism, which may represent a key innovation within the genus, has been understudied. By using six genetic markers and nearly comprehensive taxon sampling, this study attempts to elucidate relationships and examine evolutionary trends of staminal development within the East Asia (EA) Salvia clade. Methods Ninety-one taxa of EA Salvia were sampled and 34 taxa representing all other major lineages of Salvia were included for analysis. Two nuclear [internal transcribed spacer (ITS) and external transcribed spacer (ETS)] and four chloroplast (psbA–trnH, ycf1–rps15, trnL–trnF and rbcL) DNA markers were used for phylogenetic analysis employing maximum parsimony (MP), maximum likelihood (ML) and BEAST, with the latter also used to estimate divergence times. Key Results All Salvia species native to East Asia form a clade, and eight major subclades (A–G) were recognized. Subclade A, comprising two limestone endemics (S. sonchifolia and S. petrophila), is sister to the remainder of EA Salvia. Six distinct stamen types were observed within the EA clade. Stamen type A, with two fully fertile posterior thecae, only occurs in S. sonchifolia and may represent the ancestral stamen type within EA Salvia. Divergence time estimates showed that the crown of EA Salvia began to diversify approx. 17.4 million years ago. Conclusions This study supports the adoption of a broadly defined Salvia and treats EA Salvia as a subgenus, Glutinaria, recognizing eight sections within this subgenus. Stamen type A is ostensibly plesiomorphic within EA Salvia, and the other five types may have been derived from it. Staminal morphology has evolved in parallel within the EA Salvia, and staminal structure alone is inadequate to delimit infrageneric categories.

Heteranthery, the presence of distinct stamen types within a flower, is commonly explained as functional adaptation to alleviate the “pollen dilemma,” defined as the dual and conflicting function of pollen as pollinator food resource and male reproductive agent. A single primary hypothesis, “division of labor,” has been central in studies on heteranthery. This hypothesis postulates that one stamen type functions in rewarding pollen-collecting pollinators and the other in reproduction, thereby minimizing pollen loss. Only recently, alternative functions (i.e., staggered pollen release), were proposed, but comparative and experimental investigations are lagging behind. Here, we used 63 species of the tribe Merianieae (Melastomataceae) to demonstrate that, against theory, heteranthery occurs in flowers offering rewards other than pollen, such as staminal food bodies or nectar. Although shifts in reward type released species from the “pollen dilemma,” heteranthery has evolved repeatedly de novo in food-body-rewarding, passerine-pollinated flowers. We used field investigations to show that foraging passerines discriminated between stamen types and removed large stamens more quickly than small stamens. Passerines removed small stamens on separate visits towards the end of flower anthesis. We propose that the staggered increase in nutritive content of small stamens functions to increase chances for outcross-pollen transfer.

Recent advances in molecular phylogenetics and a series of important palaeobotanical discoveries have revolutionized our understanding of angiosperm diversification. Yet, the origin and early evolution of their most characteristic feature, the flower, remains poorly understood. In particular, the structure of the ancestral flower of all living angiosperms is still uncertain. Here we report model-based reconstructions for ancestral flowers at the deepest nodes in the phylogeny of angiosperms, using the largest data set of floral traits ever assembled. We reconstruct the ancestral angiosperm flower as bisexual and radially symmetric, with more than two whorls of three separate perianth organs each (undifferentiated tepals), more than two whorls of three separate stamens each, and more than five spirally arranged separate carpels. Although uncertainty remains for some of the characters, our reconstruction allows us to propose a new plausible scenario for the early diversification of flowers, leading to new testable hypotheses for future research on angiosperms. The fossil record of flowers is limited, necessitating other approaches to understanding floral evolution. Here, Sauquet and colleagues reconstruct the characteristics and diversification of ancient angiosperm flowers by combining models of flower evolution with an extensive database of extant floral traits.

Animal-pollinated plants have to get pollen to a conspecific stigma while protecting it from getting eaten. Touch-sensitive stamens, which are found in hundreds of flowering plants, are thought to function in enhancing pollen export and reducing its loss, but experimental tests are scarce. Stamens of Berberis and Mahonia are inserted between paired nectar glands and when touched by an insect’s tongue rapidly snap forward so that their valvate anthers press pollen on the insect’s tongue or face. We immobilized the stamens in otherwise unmodified flowers and studied pollen transfer in the field and under enclosed conditions. On flowers with immobilized stamens, the most common bee visitor stayed up to 3.6× longer, yet removed 1.3× fewer pollen grains and deposited 2.1× fewer grains on stigmas per visit. Self-pollen from a single stamen hitting the stigma amounted to 6% of the grains received from single bee visits. Bees discarded pollen passively placed on their bodies, likely because of its berberine content; nectar has no berberine. Syrphid flies fed on both nectar and pollen, taking more when stamens were immobilized. Pollen-tracking experiments in two Berberis species showed that mobile-stamen-flowers donate pollen to many more recipients. These results demonstrate another mechanism by which plants simultaneously meter out their pollen and reduce pollen theft.

In a genetic screen in a drnl-2 background, we isolated a loss-of-function allele in miR319a (miR319a¹²⁹). Previously, miR319a has been postulated to play a role in leaf development based on the dramatic curled-leaf phenotype of plants that ectopically express miR319a (jaw-D). miR319a¹²⁹ mutants exhibit defects in petal and stamen development; petals are narrow and short, and stamens exhibit defects in anther development. The miR319a¹²⁹ loss-of-function allele contains a single-base change in the middle of the encoded miRNA, which reduces the ability of miR319a to recognize targets. Analysis of the expression patterns of the three members of the miR319 gene family (miR319a, miR319b, and miR319c) indicates that these genes have largely non-overlapping expression patterns suggesting that these genes have distinct developmental functions. miR319a functions by regulating the TCP transcription factors TCP2, TCP3, TCP4, TCP10, and TCP24; the level of RNA expression of these TCP genes is down-regulated in jaw-D and elevated in miR319a¹²⁹. Several lines of evidence demonstrate that TCP4 is a key target of miR319a. First, the tcp4soj⁶ mutant, which contains a mutation in the TCP4 miRNA-binding site complementary to the miR319a¹²⁹ mutation, suppresses the flower phenotype of miR319a¹²⁹. Second, expression of wild-type TCP4 in petals and stamens (i.e., AP3:TCP4) has no effect on flower development; by contrast, a miRNA-resistant version of TCP4, when expressed in petals and stamens (i.e., pAP3:mTCP4) causes these organs not to develop. Surprisingly, when AP3:TCP4 is present in a miR319a¹²⁹ background, petal and stamen development is severely disrupted, suggesting that proper regulation by miR319a of TCP4 is critical in these floral organs.

Stamens are the plant male reproductive organs essential for plant fertility. Proper development of stamens is modulated by environmental cues and endogenous hormone signals. Deficiencies in biosynthesis or perception of the phytohormone jasmonate (JA) attenuate stamen development, disrupt male fertility, and abolish seed production in Arabidopsis thaliana. This study revealed that JA-mediated stamen development and seed production are regulated by a bHLH-MYB complex. The IIIe basic helix-loop-helix (bHLH) transcription factor MYC5 acts as a target of JAZ repressors to function redundantly with other IIIe bHLH factors such as MYC2, MYC3, and MYC4 in the regulation of stamen development and seed production. The myc2 myc3 myc4 myc5 quadruple mutant exhibits obvious defects in stamen development and significant reduction in seed production. Moreover, these IIIe bHLH factors interact with the MYB transcription factors MYB21 and MYB24 to form a bHLH-MYB transcription complex and cooperatively regulate stamen development. We speculate that the JAZ proteins repress the bHLH-MYB complex to suppress stamen development and seed production, while JA induces JAZ degradation and releases the bHLH-MYB complex to subsequently activate the expression of downstream genes essential for stamen development and seed production.

The molecular and genetic networks underlying the determination of floral organ identity are well studied, but much less is known about how the flower is partitioned into four developmentally distinct whorls. The SUPERMAN gene is required for proper specification of the boundary between stamens in whorl 3 and carpels in whorl 4, as superman mutants exhibit supernumerary stamens but usually lack carpels. However, it has remained unclear whether extra stamens in superman mutants originate from an organ identity change in whorl 4 or the overproliferation of whorl 3. Using live confocal imaging, we show that the extra stamens in superman mutants arise from cells in whorl 4, which change their fate from female to male, while floral stem cells proliferate longer, allowing for the production of additional stamens.

Tree peony ( Paeonia ostii ) is an economically important ornamental plant native to China. It is also notable for its seed oil, which is abundant in unsaturated fatty acids such as α-linolenic acid (ALA). Here, we report chromosome-level genome assembly (12.28 Gb) of P. ostii . In contrast to monocots with giant genomes, tree peony does not appear to have undergone lineage-specific whole-genome duplication. Instead, explosive LTR expansion in the intergenic regions within a short period (~ two million years) may have contributed to the formation of its giga-genome. In addition, expansion of five types of histone encoding genes may have helped maintain the giga-chromosomes. Further, we conduct genome-wide association studies (GWAS) on 448 accessions and show expansion and high expression of several genes in the key nodes of fatty acid biosynthetic pathway, including SAD , FAD2 and FAD3 , may function in high level of ALAs synthesis in tree peony seeds. Moreover, by comparing with cultivated tree peony ( P. suffruticosa ), we show that ectopic expression of class A gene AP1 and reduced expression of class C gene AG may contribute to the formation of petaloid stamens. Genomic resources reported in this study will be valuable for studying chromosome/genome evolution and tree peony breeding. Tree peony ( Paeonia ostii ) has the largest chromosome of any sequenced plants to date. Here, the authors assemble its genome and reveal the association of a list of candidate genes with fatty acid biosynthesis and the possible contribution of transposon and histone expansion to maintain the giga-chromosomes.