Abstract
Grasses have varying inflorescence shapes; however, little is known about the genetic mechanisms specifying such shapes among tribes. Here, we identify the grass-specific TCP transcription factor COMPOSITUM 1 (COM1) expressing in inflorescence meristematic boundaries of different grasses. COM1 specifies branch-inhibition in barley (Triticeae) versus branch-formation in non-Triticeae grasses. Analyses of cell size, cell walls and transcripts reveal barley COM1 regulates cell growth, thereby affecting cell wall properties and signaling specifically in meristematic boundaries to establish identity of adjacent meristems. COM1 acts upstream of the boundary gene Liguleless1 and confers meristem identity partially independent of the COM2 pathway. Furthermore, COM1 is subject to purifying natural selection, thereby contributing to specification of the spike inflorescence shape. This meristem identity pathway has conceptual implications for both inflorescence evolution and molecular breeding in Triticeae.
Highlights
Grasses have varying inflorescence shapes; little is known about the genetic mechanisms specifying such shapes among tribes
The triple mound (TM) corresponds to three spikelets meristems (SMs): one central (CSM) and two lateral (LSM) SMs
As the com[1] phenotype resembles that of com[2] (Fig. 8a–c), we developed and characterized BW-near-isogenic lines (NILs)(com1./com2.g) double mutants (e.g., DM, or com1./com2.g double mutant) to study their interactions in regulating branch inhibition in barley
Summary
Grasses have varying inflorescence shapes; little is known about the genetic mechanisms specifying such shapes among tribes. Current grass inflorescences are proposed to originate from a primitive ancestral shape exhibiting “a relatively small panicle-like branching system made up of primary and secondary paracladia (branches), each one standing single at the nodes”[2] (Fig. 1a). E.g., Oryzeae (rice) and Andropogoneae (maize and sorghum), still display ancestral and complex compound shapes, keeping true-lateral long primary and secondary branches Other grasses, such as Brachypodium distachyon, show lower inflorescence complexity with branch length and number reduced to lateral, small pedicels ending in only one multi-floretted spikelet (Fig. 1a–c). A valuable toolkit to further explore such genetic determinants regulating inflorescence patterning in Triticeae is a collection of morphological barley mutants, induced by physical and chemical mutagens[11] This collection includes compositum (com) barley mutants displaying branched spikes and their corresponding near-isogenic lines (NILs)[12]. The underlying gene of com[2] encodes an AP2/ERF transcription factor orthologous to maize BRANCHED SILKLESS
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