The control of compound inflorescences: insights from grasses and legumes

Inflorescences, with remarkable variation in their branching patterns, are a great system to investigate the development and evolution of biological complexity. Specialized inflorescence-like structures in some plant lineages, such as spikelets in grasses and secondary inflorescences (I2s) in legumes, self-repeat as fundamental units and form compound inflorescences, representing structural innovations towards the development and evolution of inflorescence complexity and diversity. The TFL1-FUL/AGL79-AP2 module harbors conserved function in the regulation of identity and/or indeterminacy of inflorescence meristems. The identity of the higher-order repetitive inflorescence-like modules are specified by both conserved and divergent factors in grasses and legumes. Variation is found within the fundamental units between species in grasses and legumes and regulated by distinct molecules. A major challenge in biology is to understand how organisms have increased developmental complexity during evolution. Inflorescences, with remarkable variation in branching systems, are a fitting model to understand architectural complexity. Inflorescences bear flowers that may become fruits and/or seeds, impacting crop productivity and species fitness. Great advances have been achieved in understanding the regulation of complex inflorescences, particularly in economically and ecologically important grasses and legumes. Surprisingly, a synthesis is still lacking regarding the common or distinct principles underlying the regulation of inflorescence complexity. Here, we synthesize the similarities and differences in the regulation of compound inflorescences in grasses and legumes, and propose that the emergence of novel higher-order repetitive modules is key to the evolution of inflorescence complexity. A major challenge in biology is to understand how organisms have increased developmental complexity during evolution. Inflorescences, with remarkable variation in branching systems, are a fitting model to understand architectural complexity. Inflorescences bear flowers that may become fruits and/or seeds, impacting crop productivity and species fitness. Great advances have been achieved in understanding the regulation of complex inflorescences, particularly in economically and ecologically important grasses and legumes. Surprisingly, a synthesis is still lacking regarding the common or distinct principles underlying the regulation of inflorescence complexity. Here, we synthesize the similarities and differences in the regulation of compound inflorescences in grasses and legumes, and propose that the emergence of novel higher-order repetitive modules is key to the evolution of inflorescence complexity. similarities due to independent origins, may be caused by similar (also known as parallelism) and different (a narrow definition of convergence sensu stricto) mechanisms. an inflorescence is determinate if its continuous growth is terminated by the formation of a terminal flower or inflorescence-like structure. genes that are similar by sequence identity and derived from a common ancestor. an indeterminate inflorescence has an undifferentiated shoot apical meristem. a flower-bearing structure in flowering plants. a module consists of a set of independent units that can form a more complex structure. genes that share a common ancestry due to speciation. homologous genes that are derived from a duplication event. the principal axis of an inflorescence that has a main shoot apical meristem in the top. also known as pseudoraceme and fascicle in legumes, an I2 is a specialized inflorescence-like structure that emerges from the primary inflorescence and bears flowers directly along its axis. An I2 functions as a basic repetitive unit of the compound inflorescences in legumes. the very tip of a shoot that houses a pool of stem cells that literally give rise to the bulk of the aerial organs of a plant. It is called inflorescence shoot apical meristem (IM) after transition from vegetative to reproductive growth. a little spike in grasses [i.e., a specialized short-branch inflorescence that contains one to several small flowers (i.e., florets)].