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A model for Arabidopsis class-1 KNOX gene function

The Arabidopsis class-1 KNOX genes STM, BP/KNAT1, KNAT2 and KNAT6 encode homeodomain transcriptional regulators important for shoot apical meristem (SAM) and carpel development. During vegetative growth, STM is required to establish and maintain the stem cell pool of the SAM, a function replaceable by ectopic expression of BP/KNAT1 but not of other class-1 KNOX genes. We recently demonstrated additional STM roles in the development of the central floral whorl and subsequent formation of carpels, a function replaceable by ectopic KNAT2 expression. However, STM is normally required for BP/KNAT1 and KNAT2 expression, explaining why it is essential for both SAM and carpel development. We propose therefore that STM provides the critical KNOX function in these processes and that the SAM- and carpel-promoting activities of STM are redundantly duplicated in BP/KNAT1 and KNAT2 respectively. Here we show that ectopic KNAT2 expression can restore carpel development to stm mutants, but fails to restore proper development of the central floral whorl, which requires a function analogous to the SAM-promoting activities of STM and BP/KNAT1. Similarly, we show that ectopic KNAT2 expression does not restore normal meristem organization to the SAM. We propose a model for discrete and overlapping class-1 KNOX gene function in Arabidopsis.