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Engagement with beneficial microorganisms is a regulated allocation decision: plants combine partner signals with nutrient demand, immune control and whole-plant feedback, then adjust investment according to the association's performance. Recognition, entry, organ development and continued nutrient exchange are separate control points. A compatible signal is evidence of partner identity or capability, not a guarantee of future benefit.

Legume–rhizobial symbiosis illustrates early selectivity. Root-exuded flavonoids can induce production of rhizobial Nod factors. Compatible plant receptors recognize these lipochitooligosaccharides and initiate signaling that permits infection and root nodule development. Structural specificity of the signals and the host receptor repertoire constrain compatibility. Meanwhile, plant immunity must be coordinated with accommodation: unrestricted entry would be costly, but an indiscriminate defense response would prevent the mutualism. This is a conditional developmental program, not a complete abolition of defense.

Early symbiotic signaling includes nuclear calcium signatures and activation of CCaMK within the common symbiosis signaling pathway. Kinase activation connects early perception to downstream transcription and developmental responses. Deregulated CCaMK can induce spontaneous nodules, demonstrating that downstream developmental machinery can be activated without the normal upstream sequence. A spontaneous nodule is nevertheless not evidence of bacterial entry or effective nitrogen fixation; the latter requires additional coordinated functions.^[ref]

Arbuscular mycorrhiza uses a partly shared signaling apparatus but a different exchange structure and different partners. Plant strigolactones released into the rhizosphere stimulate fungal branching, increasing the potential for contact. Isolation of a root-exuded branching factor and chemical identification as a strigolactone connected a specific plant molecule with fungal behavior.^[ref] Subsequent recognition and intracellular accommodation depend on host signaling and cell remodeling. Sharing a calcium-linked pathway does not mean that fungi are recognized by the same Nod-factor receptors as rhizobia, or that every beneficial root bacterium uses this pathway.

Nutritional context changes the expected return. A plant with adequate available nitrogen has less reason to spend photosynthate on additional nitrogen-fixing nodules. Similarly, high phosphate availability commonly reduces investment in arbuscular mycorrhiza. Hormone and nutrient pathways can alter signal release, developmental competence and allocation. Carbon availability matters as well: the plant must supply carbon to sustain the association. These controls act on potential benefit and cost rather than classifying one microorganism as beneficial under all conditions. A symbiont can be compatible yet provide little net growth advantage in a particular environment.

Local permissiveness must also be coordinated across a branched root system. Autoregulation of nodulation provides systemic negative feedback. Signals from an initiating symbiosis include root-produced CLE peptides; shoot receptor-kinase pathways involving HAR1 in Lotus japonicus contribute to limiting further engagement. A shoot-to-root arm involves miR2111, which represses the root negative regulator TML. Reduced delivery of this permissive microRNA allows the brake to rise. Thus stopping further infection can be achieved by withdrawing repression of a repressor, rather than by sending a simple mobile toxin to all root tips. Experiments established shoot-to-root movement of miR2111 and its role in controlling infection through TML.^[ref] Particular receptor and peptide names vary among legumes; the Lotus mechanism should not be transferred gene-for-gene to every plant.

Systemic quantity control is distinct from assessment of an established partner's performance. In soybean, experimentally preventing an otherwise mutualistic rhizobium from fixing nitrogen reduced its reproductive success. Nodule oxygen measurements implicated reduced oxygen supply as a possible sanction mechanism. Because cooperation was changed by removing molecular nitrogen from the atmosphere, the comparison helped separate fixation performance from unrelated differences between bacterial strains. It still does not show that every inefficient symbiont is perfectly detected or that oxygen supply is the only possible control.^[ref]

In mycorrhizal exchange, resource allocation can be conditional on both sides. Experiments demonstrated increased plant carbon allocation toward more cooperative fungal partners and increased fungal nutrient allocation in response to carbon supplied. This supports reciprocal reward in symbiosis, rather than a model of unlimited plant command over a passive fungus.^[ref] Host sanctions, partner choice before entry and reciprocal rewards are therefore related but distinguishable routes to maintaining benefit.

Plants regulate engagement through successive filters: compatibility and defense at entry, developmental and nutritional permission, systemic feedback on quantity, and conditional support of established partners. Experiments should score infection, nodule or arbuscule abundance, actual nutrient transfer and host growth separately. A greater colonized area or larger nodule count alone does not prove a more beneficial association.