Axillary Meristem Formation in Rice Requires the WUSCHEL Ortholog TILLERS ABSENT1

Abstract

Axillary shoot formation is a key determinant of plant architecture. Formation of the axillary shoot is regulated by initiation of the axillary meristem or outgrowth of the axillary bud. Here, we show that rice (Oryza sativa) TILLERS ABSENT1 (TAB1; also known as Os WUS), an ortholog of Arabidopsis thaliana WUS, is required to initiate axillary meristem development. We found that formation of the axillary meristem in rice proceeds via a transient state, which we term the premeristem, characterized by the expression of OSH1, a marker of indeterminate cells in the shoot apical meristem. In the tab1-1 (wus-1) mutant, however, formation of the axillary meristem is arrested at various stages of the premeristem zone, and OSH1 expression is highly reduced. TAB1/WUS is expressed in the premeristem zone, where it shows a partially overlapping pattern with OSH1. It is likely, therefore, that TAB1 plays an important role in maintaining the premeristem zone and in promoting the formation of the axillary meristem by promoting OSH1 expression. Temporal expression patterns of WUSCHEL-RELATED HOMEOBOX4 (WOX4) indicate that WOX4 is likely to regulate meristem maintenance instead of TAB1 after establishment of the axillary meristem. Lastly, we show that the prophyll, the first leaf in the secondary axis, is formed from the premeristem zone and not from the axillary meristem.

Cytokinin Signaling Activates WUSCHEL Expression during Axillary Meristem Initiation

“…Nevertheless, it is possible that the general effect of WUS on SAM function has an indirect role on AM initiation in the leaf axil. Recent studies also showed that formation of functional tiller buds in rice (Oryza sativa) requires the rice ortholog of WUS (Lu et al, 2015;Tanaka et al, 2015). In Arabidopsis wus mutants, leaf-like structures or even-terminal branches can still form in a portion of leaf axils (23% in wus-1; Figure 1C).…”

CLAVATA-WUSCHEL signaling in the shoot meristem

“…A mutant in OsWUS was identified as tillers absent1 (tab1), although tab1 mutant phenotypes are specific to axillary meristems, which arrest at various stages of the premeristem zone (Tanaka et al, 2015). TAB1 is expressed transiently in the pre-meristem zone, and not in the axillary meristems once they form (Tanaka et al, 2015). However, it is unclear whether TAB1 is expressed in the SAM, and OSH1, a SAM marker, accumulates normally in tab1 mutants, suggesting that another gene plays the role of WUS in the rice SAM.…”

Systems model‐guided rice yield improvements based on genes controlling source, sink, and flow

“…However, these genes often have pleiotropic effects on other source, sink, and flow properties. For example, many genes controlling tillering, such as LAX1 (Komatsu et al ; Komatsu et al ), LAX2/Gnp4 (Tabuchi et al ; Zhang et al ), MOC1 (Li et al ), and MOC3/TAB1 (Lu et al ; Tanaka et al ), also influence the panicle morphology, as they are related to initiation of the rachis primordium.…”

“…Furthermore, each of these genes often influences more than one component of rice yield. For example, genes influencing the panicle also impact tillering, including LAX1 (Komatsu et al ; Komatsu et al ), LAX2/Gnp4 (Tabuchi et al ; Zhang et al ), and MOC1 (Li et al ) and MOC3/TAB1 (Lu et al ; Tanaka et al ), through their influence on panicle morphology. As a result, genes controlling source, sink, and flow form a complex interaction network, in which biochemical reactions and signaling transduction processes coexist.…”