How the Tassel Pathway Controls Flower Sex in Maize

The tassel pathway is not a single linear route but a network of interacting genetic and hormonal circuits that together build the male inflorescence of maize and ensure it sheds pollen rather than producing grain. At its core, the network hinges on programmed cell death: every maize floret starts out bisexual, and a set of genes called the tasselseed genes triggers the destruction of female organ primordia in the tassel, leaving only functional stamens behind. Layered on top of that sex-determination step are hormone signaling cascades, microRNA modules, meristem-size controls, and branching regulators that collectively shape the tassel’s final architecture, timing, and pollen output.

Every Maize Floret Starts Bisexual

Unlike many plants that produce inherently male or female flowers from the start, maize begins with bisexual floral meristems. Both the tassel (the male inflorescence at the top of the plant) and the ear (the female inflorescence on a lateral branch) initially develop primordia for stamens and pistils. Unisexuality is achieved after the fact, through selective elimination of the organs that don’t belong. In the tassel, pistil primordia are aborted; in the ear, stamen primordia are arrested. This two-step mechanism, organ initiation followed by organ destruction, is the foundation of the entire tassel pathway.¹

The abortion process in tassel florets follows a precise spatial and temporal pattern. Cell death begins in the subepidermal layers of the pistil primordium and spreads inward until the entire organ collapses. Nuclear degeneration is the hallmark of this process, distinguishing it from simple growth arrest.² The genes responsible for triggering this death are the tasselseed genes, and when they are mutated, pistils survive in the tassel and begin to develop silks, feminizing flowers that would otherwise be strictly male.

Jasmonic Acid Drives Pistil Abortion

Two of the best-characterized sex-determination genes are tasselseed1 (ts1) and tasselseed2 (ts2). The TS2 protein is a short-chain alcohol dehydrogenase required for stage-specific organ abortion in the tassel.³ The TS1 protein turned out to be a plastid-targeted lipoxygenase, an enzyme that feeds into the biosynthesis of jasmonic acid. When ts1 is knocked out, lipoxygenase activity disappears and jasmonic acid levels drop in developing inflorescences. Applying jasmonic acid externally to mutant tassels rescues normal stamen development and restores pistil abortion, confirming that jasmonic acid is the key hormonal signal for making tassel florets male.⁴

This is a satisfying finding because it connects a classical genetic mutant to a well-known plant hormone. Jasmonic acid is involved in wound responses and defense in many plants, but in maize it has been co-opted for a completely different job: killing off the wrong floral organs. The TS1-TS2 pathway essentially converts a stress-related signal into a developmental switch.

A MicroRNA Layer Reinforces Male Identity

Sex determination in the tassel doesn’t rely on jasmonic acid alone. A third gene, tasselseed4 (ts4), encodes a microRNA called miR172 that targets a class of transcription factors related to APETALA2, a gene family involved in floral organ identity across flowering plants. One critical target of ts4 is a gene called indeterminate spikelet1 (ids1), which normally promotes spikelet meristem growth. When ts4 is mutated, IDS1 protein accumulates more broadly than it should, and the result is the same feminized tassel phenotype seen in ts1 and ts2 mutants, along with branching defects. Conversely, loss of ids1 suppresses those defects, proving that IDS1 overexpression is what causes the problem.⁵

A related mutant, Tasselseed6 (Ts6), carries mutations in the miR172 binding site of ids1 itself, making the transcript resistant to microRNA-mediated silencing. The result phenocopies ts4. Together, these findings show that sexual identity in the tassel is partly acquired by capping floral growth: miR172 keeps APETALA2-like factors in check, preventing pistil primordia from maturing past the point of no return.

SILKLESS1 Protects Pistils in the Ear

If the tasselseed pathway is the executioner, then the SILKLESS1 (SK1) protein is the bodyguard. SK1 protects pistil primordia in the ear from the same cell-death machinery that eliminates them in the tassel. In the ear, SK1 is active and pistils survive to become silks and kernels. In the tassel, SK1 expression is suppressed, leaving pistils vulnerable to tasselseed-mediated death.⁶

The interplay between these systems has been demonstrated with genetic experiments. When silkless1 is mutated, it suppresses silk growth both in the ear and in the feminized tassels of jasmonic-acid-deficient and APETALA2-pathway mutants, showing that the protection mechanism and the death mechanism converge on shared molecular targets.⁷ Brassinosteroids have also been implicated as an additional layer of pistil suppression in the tassel that operates independently of jasmonic acid, suggesting the pathway has more redundancy than was originally appreciated.

Gibberellin Suppresses Stamens in the Ear

While jasmonic acid eliminates pistils in the tassel, a different hormone handles the complementary job of suppressing stamens in the ear. Gibberellic acid (GA) determines whether stamens develop or are arrested in the ear florets.⁸ Recent single-cell transcriptomic work has revealed something unexpected about how GA acts: the genes known to suppress ear stamen development, such as ANTHER EAR1 and DWARF3, are barely expressed in the stamen cells themselves. Instead, they are enriched in the pith and cortex tissues surrounding the florets, suggesting that part of the GA signal travels from neighboring tissues rather than acting within the stamen primordium directly.⁹

This non-cell-autonomous signaling model is an active area of research. It means that stamen fate in the ear isn’t decided by the stamen primordia alone but by the hormonal environment created by surrounding tissues, adding a spatial dimension to the sex-determination circuit.

Auxin and the Birth of Lateral Meristems

Before the tassel can produce florets at all, it needs to generate the axillary meristems from which those florets will arise. This is where auxin, the hormone best known for controlling plant growth direction, enters the story. Auxin transport creates concentration peaks at specific positions on the flanks of the inflorescence meristem, and each peak marks the spot where a new branch or spikelet meristem will form.

The BARREN STALK1 (BA1) protein is required for creating these auxin concentration maxima. In ba1 mutant tassels, the periodic auxin peaks that normally appear at the inflorescence meristem flanks simply don’t form, and the result is a tassel with few or no branches and spikelets.¹⁰ A related gene, BARREN STALK2 (BA2), encodes a protein that physically interacts with BA1. Mutants of ba2 show fewer paired spikelets and more single spikelets, consistent with defective axillary meristem initiation.¹¹

Metabolomic studies have added nuance to this picture. Elevated levels of indole-3-acetamide, an auxin precursor, have been linked to reduced branching, possibly through downregulation of BARREN STALK1 expression. This suggests that too much auxin in the wrong place can actually suppress the very meristems that auxin concentration peaks are needed to initiate, a balancing act rather than a simple “more hormone, more growth” relationship.¹²

Keeping the Meristem the Right Size

The inflorescence meristem that gives rise to the tassel must maintain a careful balance between self-renewal and differentiation. If it grows too large, the tassel becomes abnormally thick and dense. The gene thick tassel dwarf1 (td1) encodes a receptor-like kinase that limits meristem size, functioning as the maize version of the CLAVATA1 receptor known from other plants. When td1 is mutated, tassels pack spikelets more densely, ears develop extra rows of kernels, and individual florets can produce extra stamens.¹³

Another gene in this system, fasciated ear2 (fea2), encodes a CLAVATA2-like protein. Double mutants of td1 and fea2 show more severe phenotypes than either single mutant, including twice as many kernel rows, indicating that the two proteins don’t simply work as one exclusive receptor complex. The maize signaling network for meristem maintenance appears to be more branched and redundant than the relatively clean CLAVATA pathway described in other model plants.¹⁴

What Controls Tassel Branch Number

Tassel branch number is one of the most visible and agronomically relevant aspects of tassel architecture. Breeders have long selected for smaller tassels because large, heavily branched tassels shade the leaves below them and can reduce grain yield.¹⁵ Several genetic pathways converge on branch number.

The RAMOSA pathway is one of the best-studied branching regulators in maize inflorescences. A transcription factor network that includes RAMOSA2 (RA2) and TASSELSHEATH4 (TSH4) restricts the number and position of branches by controlling when branch meristems shift to spikelet identity. Recent work has connected this pathway to the circadian clock component ZmELF3.1, which physically interacts with the RA2-TSH4 module to repress branching.¹⁶ The fact that a clock gene participates in branching hints that tassel architecture is sensitive to environmental light cues, not just to internal developmental signals.

Another layer of branching control involves miR156, a different microRNA from the miR172 involved in sex determination. MiR156 targets a family of SPL transcription factors, including TSH4, UB2, and UB3, which restrict lateral primordia initiation and allow stem cells to be maintained in the meristem.¹⁷ A classical mutant called Tp2 was recently mapped to a miR156 family member, and both overexpression of miR156h and knockout of its target ZmSBP13 significantly reduced tassel branch number, confirming the miR156-SPL axis as a key determinant of branching.¹⁸

Flowering Time Sets the Stage

The tassel can’t develop until the plant transitions from vegetative growth to reproductive growth, and that transition is governed by its own signaling network. In maize, the florigen signal that triggers flowering is encoded by ZCN8, a gene whose transcript accumulates in leaves before the shoot apical meristem commits to making an inflorescence. In photoperiod-sensitive tropical maize lines, ZCN8 transcription is strongly upregulated under short days and shows pronounced daily cycling. In day-neutral temperate lines, ZCN8 expression is more constant and less responsive to daylength.¹⁹

This matters for the tassel pathway because the timing of floral transition determines how many leaves the plant produces before the tassel begins to differentiate. Tropical germplasm moved to temperate latitudes (where days are longer during summer) often flowers very late and produces abnormally large tassels, a direct consequence of the photoperiod sensitivity of ZCN8 regulation. Conversely, artificial microRNA silencing of ZCN8 causes late flowering, while ectopic expression in shoot tips promotes early flowering.

How the Tassel Pathway Shaped Domestication

Maize was domesticated from teosinte, a wild grass with dramatically different architecture. Teosinte plants are highly branched with many tillers, and their lateral inflorescences are bisexual rather than purely female. One of the most important domestication loci, teosinte branched1 (tb1), suppresses lateral branching and influences inflorescence sex. Maize plants carrying loss-of-function tb1 mutations revert toward a teosinte-like architecture, with extensive secondary and tertiary axillary branches.²⁰

In situ hybridization showed that tb1 is expressed in axillary meristems and in stamens of ear primordia, consistent with a role in suppressing growth of these tissues. Double-mutant experiments with anther ear1 (a GA-pathway gene) and tasselseed2 showed that the sex of the inflorescence on a lateral branch wasn’t correlated with its length, meaning tb1’s growth-suppression function operates independently of its effects on sexual identity.²¹ The tb1 locus has effects on both branch architecture and inflorescence sex, making it a point where the tassel pathway intersects with the broader domestication story of maize.²²

Chromatin Accessibility Distinguishes Ear and Tassel

Beyond the individual genes themselves, researchers have begun to ask how the genome is physically organized differently in ear versus tassel tissue. Using techniques that map which stretches of DNA are accessible (unwound and available for gene activation) versus compacted and silent, one study identified over 50,000 open chromatin regions in developing ear and tassel primordia. Integrating that chromatin map with data on DNA methylation, histone modifications, and transcription factor binding revealed coordinated epigenetic differences that help explain why the same genome produces such different structures in the two inflorescences.²³

A separate line of work has shown that the chromatin-remodeling factor ZmSSRP1 modulates accessibility at the ZmBRD1 locus, a gene involved in brassinosteroid synthesis. Loss of ZmSSRP1 function reduces chromatin accessibility across thousands of genomic regions, many of which are enriched for hormone-related genes. Because brassinosteroids themselves participate in pistil suppression in the tassel, this creates a link between chromatin state and sex-determination outcomes.²⁴

Heat, Drought, and Pollen Failure

The tassel pathway is not insulated from the environment. Heat and drought stress can severely disrupt tassel development at multiple stages. Tassel growth and spikelet development are considerably suppressed under stress, pollen viability drops, and starch granules inside pollen grains are depleted at anthesis. Combined heat and drought stress is worse than either stress alone.²⁵ The underlying mechanism involves disrupted sugar metabolism, particularly the enzyme sucrose synthase, which fails to provision developing pollen with the carbohydrates it needs to remain viable. For farmers, this translates directly into poor pollination and reduced kernel set during extreme weather, a vulnerability with growing relevance as heat waves become more frequent.

Why Breeders Manipulate the Tassel Pathway

Understanding the tassel pathway has practical payoffs for hybrid maize breeding. Hybrid seed production requires controlled pollination: one parent line must contribute pollen and the other must not self-pollinate. Traditionally, this is accomplished by manually or mechanically removing tassels from the female parent rows, a labor-intensive process called detasseling. Genetic male sterility offers an alternative. Dozens of genic male-sterility genes have been identified in maize, many of them affecting anther and pollen development steps within the tassel pathway, and biotechnology-based male-sterility systems built around these genes can streamline hybrid seed production.²⁶

Tassel size itself is a breeding target. Large tassels shade the uppermost leaves of the canopy, reducing the photosynthesis available for grain filling. Removing tassels experimentally increases grain yield in some hybrid backgrounds, particularly under high plant densities.²⁷ Modern commercial hybrids already carry smaller tassels than their predecessors, a change driven by decades of selection. The genetic tools emerging from tassel-pathway research, such as the miR156-SPL module for branch number and the RAMOSA-ELF3 connection, give breeders increasingly precise targets for fine-tuning tassel architecture without sacrificing pollen output. The goal is a tassel just large enough to ensure pollination but small enough to stay out of the canopy’s way.