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Abstract The genus Acacia is a large group of woody legumes containing an enormous amount of morphological diversity in leaf shape. This diversity is at least in part the result of an innovation in leaf development where many Acacia species are capable of developing leaves of both bifacial and unifacial morphologies. While not unique in the plant kingdom, unifaciality is most commonly associated with monocots, and its developmental genetic mechanisms have yet to be explored beyond this group. In this study, we identify an accession of Acacia crassicarpa with high regeneration rates and isolate a clone for genome sequencing. We generate a chromosome-level assembly of this readily transformable clone, and using comparative analyses, confirm a whole-genome duplication unique to Caesalpinoid legumes. This resource will be important for future work examining genome evolution in legumes and the unique developmental genetic mechanisms underlying unifacial morphogenesis in Acacia. 

Age-dependent changes in plant defense against herbivores are widespread, but why these changes exist remains a mystery. We explored this question by examining a suite of traits required for the interaction between swollen thorn acacias (genus Vachellia ) and ants of the genus Pseudomyrmex . In this system, plants provide ants with refuge and food in the form of swollen stipular spines, protein-lipid–rich “Beltian” bodies, and sugar-secreting extrafloral nectaries—the “swollen thorn syndrome.” We show that this syndrome develops at a predictable time in shoot development and is tightly associated with the temporal decline in the microRNAs miR156 and miR157 and a corresponding increase in their targets—the SPL transcription factors. Growth under reduced light intensity delays both the decline in miR156/157 and the development of the swollen thorn syndrome, supporting the conclusion that these traits are controlled by the miR156-SPL pathway. Production of extrafloral nectaries by Vachellia sp. that do not house ants is also correlated with a decline in miR156/157, suggesting that this syndrome evolved by co-opting a preexisting age-dependent program. Along with genetic evidence from other model systems, these findings support the hypothesis that the age-dependent development of the swollen thorn syndrome is a consequence of genetic regulation rather than a passive developmental pattern arising from developmental constraints on when these traits can develop.