Positive Autoregulation of aKNOXGene Is Essential for Shoot Apical Meristem Maintenance in Rice  

Abstract

Self-maintenance of the shoot apical meristem (SAM), from which aerial organs are formed throughout the life cycle, is crucial in plant development. Class I Knotted1-like homeobox (KNOX) genes restrict cell differentiation and play an indispensable role in maintaining the SAM. However, the mechanism that positively regulates their expression is unknown. Here, we show that expression of a rice (Oryza sativa) KNOX gene, Oryza sativa homeobox1 (OSH1), is positively regulated by direct autoregulation. Interestingly, loss-of-function mutants of OSH1 lose the SAM just after germination but can be rescued to grow until reproductive development when they are regenerated from callus. Double mutants of osh1 and d6, a loss-of-function mutant of OSH15, fail to establish the SAM both in embryogenesis and regeneration. Expression analyses in these mutants reveal that KNOX gene expression is positively regulated by the phytohormone cytokinin and by KNOX genes themselves. We demonstrate that OSH1 directly binds to five KNOX loci, including OSH1 and OSH15, through evolutionarily conserved cis-elements and that the positive autoregulation of OSH1 is indispensable for its own expression and SAM maintenance. Thus, the maintenance of the indeterminate state mediated by positive autoregulation of a KNOX gene is an indispensable mechanism of self-maintenance of the SAM.

Regulatory modules controlling maize inflorescence architecture

“…5B), which is rich in conserved noncoding sequences involved in kn1 repression (Inada et al 2003). KN1 was shown to bind and auto-regulate itself through the same intron (Tsuda et al 2011;Bolduc et al 2012). Although expression of kn1 was not significantly altered in ra1 mutants in our experiments, regulation could be restricted to a small number of cells, since the ra1 expression domain is much smaller than that of kn1.…”

RiceLHS1/OsMADS1Controls Floret Meristem Specification by Coordinated Regulation of Transcription Factors and Hormone Signaling Pathways

“…Interactions between cytokinin signaling and homeodomain transcription factors govern meristem maintenance in Arabidopsis, maize, and rice (Gordon et al, 2009;Tsuda et al, 2011;Bolduc et al, 2012). In OsMADS1 knockdown panicles, we detected an overall increased activity of cytokinin pathway genes, examples being LONELY GUY (LOG) family members, a predicted receptor, HKL1/CRL4, and several response regulator (RR) genes (Supplemental Table S4).…”

KNOX Protein OSH15 Induces Grain Shattering by Repressing Lignin Biosynthesis Genes

“…In the bp mutant, transcript levels of lignin biosynthesis genes are increased (Mele et al, 2003). KNOX genes are expressed preferentially in the indeterminate cells around the SAM and are necessary for its formation and maintenance (Hake et al, 2004;Tsuda et al, 2011). These observations suggest that KNOX proteins function in maintaining the SAM and repressing lignin deposition during its development.…”

“…The rice genome has five functional class I KNOX genes: OSH1, OSH6, OSH15, OSH43, and OSH71 (Tsuda et al, 2011). They are preferentially expressed in the indeterminate cells around the shoot apical meristem (SAM) and are necessary for the formation and maintenance of that SAM (Hake et al, 2004;Tsuda et al, 2011;Luo et al, 2012).…”

“…They are preferentially expressed in the indeterminate cells around the shoot apical meristem (SAM) and are necessary for the formation and maintenance of that SAM (Hake et al, 2004;Tsuda et al, 2011;Luo et al, 2012). The osh1 null mutant has a defect in the SAM and shows arrested development at the three-leaf stage (Sato et al, 1996;Tsuda et al, 2011).…”