Positive autoregulation of a KNOX gene is essential for shoot meristem maintenance in rice
Katsutoshi Tsuda, Yukihiro Ito, Yutaka Sato, Nori Kurata
Plant Cell: December 29, 2011, Published on line ahead of print. DOI: http://dx.doi.org/10.1105/tpc.111.090050
In contrast to animals that complete their organogenesis during embryogenesis, flowering plants continue to form organs such as leaves, stems and flowers from the shoot apical meristem (SAM) throughout the life cycle. Therefore, self-maintenance of the SAM is crucial for the body plan of flowering plants. Previous studies in Arabidopsis and maize have shown that Class I Knotted1-like homeobox (KNOX) genes are specifically expressed in the SAM and play an essential role for SAM maintenance. However, despite their indispensable roles, the mechanism that positively regulates their expression is entirely unknown. In this study, we found that the expression of KNOX genes was positively regulated by a phytohormone cytokinin (CK) and by KNOX genes themselves. In the double mutant of rice KNOX gene OSH1 and OSH15, the expression of other three KNOX genes was reduced. On the other hand, overexpression of OSH1 or CK treatment increased their expression. Given that KNOX genes have been reported to activate CK biosynthesis, these results suggest the existence of a positive autoregulatory loop of KNOX genes via CK. Furthermore, we showed that OSH1 protein directly and positively regulates five KNOX genes through the evolutionarily conserved cis-elements, and that this direct positive autoregulation is essential for its own expression and for SAM maintenance. Thus, the positive autoregulation of a KNOX gene is a novel mechanism for the unique body plan of flowering plants.
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(A) Expression of OSH1 (left). Mutation in OSH1 binding site on OSH1 locus resulted in the loss of its expression (right).
(B) A model of the regulatory mechanism of KNOX genes in the SAM.
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