AT-hook motif nuclear localized (AHL) transcription factors MtAHL1 and MtAHL2 were identified in 2023 as essential activators of nodule-specific cysteine-rich (NCR) peptide genes in Medicago truncatula: loss of either factor abolishes NCR expression and blocks bacteroid differentiation, and the same factors drive expression of an introduced NCR169 gene in soybean and Lotus japonicus, which lack NCR genes. Unknown is the full scope of this regulatory module: the complete set of genes it regulates during nodule development, whether its essentiality for nitrogen fixation extends beyond NCR gene activation, whether MtAHL proteins form multimeric regulatory hubs with other AHL family members and other transcription factors, whether AHLs repress NCR or other genes outside nodules, and whether AHL control of symbiont differentiation operates in legumes with different nodule developmental programmes. This problem asks for an experimentally grounded definition of the AHL regulatory module — its target repertoire, essential functions, mechanism, and evolutionary conservation.

Background

Legumes (Fabaceae) form root nodules that host nitrogen-fixing rhizobium bacteria. Inside nodule cells, rhizobia differentiate into 'bacteroids', which may be reversible (as in soybean) or terminal. Terminal bacteroid differentiation — cell enlargement, genome amplification and loss of reproductive viability — is imposed by the plant, improves nitrogen-fixation efficiency, and evolved repeatedly in the legume family, most prominently in the inverted repeat-lacking clade (IRLC), which includes Medicago truncatula and pea. Its plant effectors are nodule-specific cysteine-rich (NCR) peptides, a family of several hundred small secreted antimicrobial-like peptides; loss of a single NCR such as NCR169 (the dnf7 mutant) arrests bacteroid maturation late and abolishes nitrogen fixation (Horváth et al. 2015). Because NCR genes occur only in IRLC legumes (with analogous peptides in the dalbergioid clade), it was assumed that their expression required NCR-specific, clade-restricted transcription factors. Zhang et al. (2023) overturned this assumption: a short (436 bp) NCR169 promoter fragment drives nodule-specific expression even in soybean and L. japonicus, which have no NCR genes. The authors identified two closely related AHL transcription factors, MtAHL1 and MtAHL2, that bind a 37-bp AT-rich element (called H0) within this promoter; this element is conserved in the promoters of close to 300 M. truncatula NCR genes. AHL proteins carry one or two AT-hook DNA-binding motifs (short peptide motifs that bind the minor groove of AT-rich DNA) and a Plant and Prokaryote Conserved (PPC/DUF296) protein-interaction domain; in various plants they form homo- and heterotrimeric complexes, induce or repress genes by binding AT-rich DNA, and have documented roles in organ development, flowering, and immunity, in some cases via histone modifications. In M. truncatula, mutation of either MtAHL1 or MtAHL2 eliminates NCR expression and blocks bacteroid differentiation completely — an earlier and more severe arrest than in dnf7 — implying these factors regulate many NCR genes and possibly other genes. Strikingly, the corresponding L. japonicus mutants lose NCR169 expression yet fix nitrogen normally, consistent with L. japonicus having no NCR genes; the paper notes this as a preliminary observation. M. truncatula has more than 25 AHL genes, at least 12 of them expressed in nodules in distinct zonal patterns, and the two factors show complementary expression dynamics (MtAHL2 high in young roots, MtAHL1 rising strongly during nodule development). These results established a conserved transcriptional entry point into the NCR-based symbiont-control machinery and left open a coherent set of questions about its scope, mechanism, and conservation.

Problem Statement

The problem is to define the MtAHL1/MtAHL2 (and more broadly AHL-family) regulatory module that controls nodule-specific gene expression and bacteroid differentiation in legumes, through four experimentally grounded deliverables: (1) Target repertoire — identify the genes, beyond the NCR family, that MtAHL1 and MtAHL2 regulate during Medicago truncatula nodule development (distinguishing direct from indirect regulation where the evidence allows), and determine whether they regulate symbiosis-related genes in M. truncatula that are not targeted in Lotus japonicus. (2) Essentiality — determine whether the essential role of these AHLs in M. truncatula nitrogen fixation is limited to NCR gene activation (as implied by the L. japonicus mutant phenotype, where loss of the AHLs eliminates NCR169 expression without a symbiotic defect) or extends to other genes required for symbiosis. (3) Mechanism and non-nodule roles — determine whether MtAHL proteins form multimeric complexes with other nodule-expressed AHL family members (at least 12 AHL genes are expressed in nodules) and act as regulatory hubs that integrate root- and nodule-specific transcription factors to induce or repress gene expression; and determine whether MtAHL2, which is highly expressed in non-nodulated roots, functions in root development or in repressing NCR genes outside nodules, including whether other AHLs expressed in non-nodule tissues bind the same H0-like promoter elements to suppress NCR169 expression in non-symbiotic organs. (4) Conservation — determine whether these AHL transcription factors function in determinate nodules with terminally differentiated bacteroids (for example Aeschynomene and Arachis) and in indeterminate nodules without terminal differentiation (for example Leucaena), and how their activity relates to the different nodule developmental programmes and the presence or absence of NCR peptides. For this deliverable the required result is a general conclusion about function across legume nodule developmental programmes, for which the named species are illustrative examples rather than an exhaustive list. The four deliverables are jointly required for a complete resolution of the problem; because they are independent, an experimentally established result for any single deliverable is assessed as a partial contribution to the module definition and does not by itself resolve the problem.

References

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