The regular placement of leaves and flowers around the plant stem (phyllotaxis) is driven by a self-organizing feedback loop in which the hormone auxin, transported by the polarly localized efflux carrier PIN-FORMED 1 (PIN1), accumulates at future organ sites, and this accumulation in turn reorients PIN1 polarity in neighboring cells. Bhatia et al. (2016) established that the auxin-responsive transcription factor MONOPTEROS (MP/ARF5) is a central component of this feedback: local MP activity orients PIN1 polarity non-cell-autonomously toward MP-expressing cells, and sub-epidermal MP activity stabilizes auxin maxima, but they left the underlying molecular machinery open. A decade of follow-up work has advanced parts of the question — the MP target TMO5 was shown to mediate PIN1 polarity convergence and organogenesis, and a TMK-kinase-based cell-surface auxin signaling circuit was shown to modulate PIN1 phosphorylation and polarization — yet the molecular identity and mode of action of the intercellular cue that reorients PIN1 toward sites of high MP activity, the biochemical basis of the stabilizing auxin depletion in sub-epidermal tissues, and the modulation of the module across species all remain unresolved. This problem asks for the completion of that molecular account.

Background

Flowering plants produce their leaves and flowers one after another from the shoot apical meristem (SAM), a dome of stem cells at the growing tip, in strikingly regular geometric arrangements called phyllotaxis. The plant hormone auxin is the key positional signal: it is transported cell to cell in a directional manner by PIN-FORMED 1 (PIN1), an efflux carrier that sits asymmetrically on one face of each cell, so that the polarity of PIN1 in each cell determines the direction of auxin flow. Auxin accumulates at future organ initiation sites on the meristem periphery, where it triggers outgrowth (Reinhardt et al., 2003; Heisler et al., 2005). Because organs arise at new positions as the meristem grows, phyllotaxis must be self-organizing: the leading idea, supported by transport-feedback models, is that auxin somehow feeds back on the polarity of its own transporter, so that PIN1 convergence points — and hence auxin maxima and organ positions — emerge and are spaced by the transport pattern itself.

Inside cells, auxin is perceived by the TIR1/AFB family of intracellular receptors, which promote the degradation of repressor proteins and thereby activate AUXIN RESPONSE FACTORs (ARFs), transcription factors that switch on auxin-responsive genes. MONOPTEROS (MP, also called ARF5) is one such ARF. Bhatia et al. (2016) showed that in the SAM, MP expression follows the auxin distribution, and that MP acts upstream of PIN1 polarity: perturbing MP activity globally abolishes localized PIN1 polarity convergence patterns, while small clones of MP-expressing cells reorient PIN1 in surrounding cells toward the clone. This reorientation is non-cell-autonomous — the MP-expressing cell changes the behavior of its neighbors — meaning that some signal must pass from the MP-expressing cell to its neighbors, where it redirects PIN1 to a particular membrane face. When MP was confined to the epidermis alone, PIN1 convergence patterns still formed but the auxin maxima became unstable and drifted over the tissue, implying that additional MP-regulated processes in sub-epidermal tissues, modeled as auxin-triggered auxin depletion, anchor the spacing pattern to the underlying cells. An alternative, not mutually exclusive class of mechanisms is mechanical: auxin-induced cell wall loosening changes the tensile stress pattern in shared walls, and cells were proposed to target PIN1 toward highly stressed walls; Bhatia et al. found their clone data consistent with such a mechanical stress feedback model, without excluding purely biochemical (transport- or ligand-based) mechanisms. The paper closed by identifying the open agenda: identify MP target genes to reach a molecular understanding of the signaling processes, and of how these are modulated in different species and tissue contexts to generate morphological diversity.

Several terms used below: TMO5 (TARGET OF MONOPTEROS 5) is a basic helix-loop-helix transcription factor, an established direct target of MP first characterized in the embryo and vasculature, where it acts with its dimerization partner LHW and promotes cytokinin biosynthesis; cytokinin is a second plant hormone. TMKs (TRANSMEMBRANE KINASEs) are receptor-like kinases at the plasma membrane that participate in noncanonical, cell-surface auxin perception of extracellular auxin, together with ABP1 (AUXIN BINDING PROTEIN 1) and ABP1-like proteins. Ca2+^{2+} here refers to transient cytoplasmic calcium increases that can act as rapid second messengers downstream of mechanical stimulation.

Problem Statement

Complete the molecular account of the MONOPTEROS–PIN1 feedback module that patterns phyllotaxis, as posed at the close of Bhatia et al. (2016). The single scientific objective is a causal, mechanistic explanation of how localized MP/ARF5-mediated auxin signaling reorients PIN1 polarity in neighboring meristem cells, jointly supported by the following linked components: (1) identify the molecular pathway from MP through its downstream targets (of which the TMO5 transcription factor family is the currently established mediator) to the intercellular signal — chemical or mechanical — whose receipt in a neighboring cell redirects PIN1 to the membrane face toward the MP-active cell; (2) establish the biochemical mechanism of the auxin-dependent auxin depletion in sub-epidermal tissues that stabilizes auxin maxima relative to the underlying cells, including whether that depletion is directly MP-regulated; and (3) determine how this feedback module is modulated in different plant species and tissue contexts to generate morphological diversity. Component (1) is the load-bearing molecular core of the module as the source defines it, including whether the intercellular signal is chemical or mechanical in nature; components (2)–(3) complete the same mechanistic account — the stabilization and the evolutionary deployment — and cannot be substituted by a narrower proxy such as a purely computational pattern or a single identified gene without the causal link to PIN1 reorientation. All three components are jointly required: a solution must address (1) together with (2) and (3), with component (1) the central molecular core but not by itself a resolution of the objective.

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