The SHOOT MERISTEMLESS gene is required for maintenance of undifferentiated cells in Arabidopsis shoot and floral meristems and acts at a different regulatory level than the meristem genes WUSCHEL and ZWILLE

Abstract

The function of the SHOOT MERISTEMLESS (STM) gene in shoot and floral meristems throughout Arabidopsis development has been analyzed. The results show that STM plays a major role in maintaining shoot and floral meristems. In an allelic series of stm mutants the shoot meristem was either reduced or completely absent in mature embryos and mutant seedling cotyledons showed partial fusion, indicating that the STM gene affects embryonic shoot meristem development and spacing of cotyledons. Postembryonically, stm mutants initiated adventitious shoot development at a position corresponding to the shoot meristem in wild-type. Repetitively initiated defective mutant shoot and floral meristems were consumed during primordia formation and typically terminated prematurely in fused ectopic primordia, indicating that STM is required for continuous shoot and floral meristem function. Analogous defects were observed in stm embryonic and postembryonic development suggesting that similar mechanisms are employed in embryonic and postembryonic organ primordia initiation. Allelic combination suggest different thresholds for STM requirement during plant development. STM requirement could not be bypassed by standard growth factor regimes or by shoot regeneration from calli. The results suggest that STM functions by preventing incorporation of cells in the meristem center into differentiating organ primordia and that this role can completely account for all defects observed in stm mutants. Mutations in the WUSCHEL (WUS) and ZWILLE (ZLL) genes result in defective organization and premature termination of shoot meristems. Genetic interactions between STM, WUS and ZLL were analyzed and the results indicate that STM acts upstream of WUS and ZLL. Therefore, while STM appears to function in keeping central meristem cells undifferentiated, WUS and ZLL seem to be subsequently required for proper function of these cells.

Interaction ofKNAT6andKNAT2withBREVIPEDICELLUSandPENNYWISEinArabidopsisInflorescences

“…KNAT class I contains four members: SHOOT MERISTEMLESS (STM), BREVIPEDICELLUS (BP)/KNAT1, KNAT2, and KNAT6. STM is required for the initiation of the meristem during embryogenesis and its maintenance during postembryonic development (Clark et al, 1996;Endrizzi et al, 1996;Long et al, 1996). BP contributes with STM to SAM maintenance since the loss of function of BP reduces the residual meristematic activity of the weak allele stm-2 (Byrne et al, 2002(Byrne et al, , 2003.…”

“…PNY also interacts with BP to regulate inflorescence patterning and fruit development (Smith and Hake, 2003;Alonso-Cantabrana et al, 2007). A role for STM in inflorescence growth also is suggested as weak alleles of STM show aberrant organ position, fusion defects, and abnormal flowers (Endrizzi et al, 1996;Kanrar et al, 2006). However, these defects may be indirect effects resulting from the abnormal structure of the SAM.…”

Different subcellular localization and trafficking properties of KNOX class 1 homeodomain proteins from rice

“…Also included is the KN1 antagonist identified by Kragler et al (2000). premature termination of the SAM (Endrizzi et al, 1996;Kerstetter et al, 1997). Mutant analysis of KNAT1 (BREVIPEDICELLUS) in Arabidopsis and of OSH15 in rice, both in the subclass with the peripheral type of expression, showed these genes to be involved in stem development in addition to SAM maintenance Mele et al, 2003).…”

“…The complex process of SAM organization requires the interaction of a large number of transcription factors and other regulatory proteins to fine-tune activities of cell division, specification and differentiation. Transcription factors involved in SAM organization include the KNOX class-1 homeodomain proteins, which have been named after the maize protein KNOTTED1 (KN1) and are involved in maintenance of the undifferentiated state of stem cells Endrizzi et al, 1996;Brand et al, 2002). It has been proposed that the process of SAM maintenance depends on inhibition of gibberellin (GA) biosynthesis through down-regulation of GA20-oxidase by KNOX proteins (Kusaba et al, 1998;Sakamoto et al, 2001;Hay et al, 2002Hay et al, , 2004.…”

Meristemas: fontes de juventude e plasticidade no desenvolvimento vegetal

“…Este gene codifica um fator de transcrição encontrado em toda a extensão do MAC onde não há diferenciação celular (figura 1), e sua expressão parece ser essencial para o correto funcionamento meristemático, uma vez que plantas mutantes com distúrbios deste fator de transcrição falham na manutenção da organização e atividade do MAC (Clark et al 1996, Endrizzi et al 1996, Long et al 1996.…”