Abstract
The formation and maintenance of the shoot apical meristem (SAM) are critical for plant development. However, the underlying molecular mechanism of regulating meristematic cell activity is poorly understood in the model legume Medicago truncatula. Using forward genetic approaches, we identified HEADLESS (HDL), a homolog of Arabidopsis WUSCHEL, required for SAM maintenance and leaf development in M. truncatula. Disruption of HDL led to disorganized specification and arrest of the SAM and axillary meristems, resulting in the hdl mutant being locked in the vegetative phase without apparent stem elongation. hdl mutant leaves are shorter in the proximal-distal axis due to reduced leaf length elongation, which resulted in a higher blade width/length ratio and altered leaf shape, uncovering novel phenotypes undescribed in the Arabidopsis wus mutant. HDL functions as a transcriptional repressor by recruiting MtTPL through its conserved WUS-box and EAR-like motif. Further genetic analysis revealed that HDL and STENOFOLIA (STF), a key regulator of M. truncatula lamina outgrowth, act independently in leaf development although HDL could recruit MtTPL in the same manner as STF does. Our results indicate that HDL has conserved and novel functions in regulating shoot meristems and leaf shape in M. truncatula, providing new avenues for understanding meristem biology and plant development.
Highlights
The shoot apical meristem (SAM) is made of pluripotent stem cells located at the shoot apex, which is responsible for self-maintenance and producing lateral organ primordia that develop into post-embryonic aerial organs (Barton, 2010)
Disruption of HDL led to disorganized specification and arrest of the SAM and axillary meristems, resulting in the hdl mutant being locked in the vegetative phase without apparent stem elongation. hdl mutant leaves are shorter in the proximal–distal axis due to reduced leaf length elongation, which resulted in a higher blade width/length ratio and altered leaf shape, uncovering novel phenotypes undescribed in the Arabidopsis wus mutant
We described an M. truncatula stemless and bushy mutant named headless, in which shoot meristem maintenance is disrupted by Tnt1 retrotransposon insertion in the exons of the HDL gene
Summary
The shoot apical meristem (SAM) is made of pluripotent stem cells located at the shoot apex, which is responsible for self-maintenance and producing lateral organ primordia that develop into post-embryonic aerial organs (Barton, 2010). The homeobox gene, WUSCHEL (WUS), which is expressed in the organizing center of the SAM, plays a central role in shoot stem cell identity and is required for non-cell-autonomous induction and maintenance of stem cell fate in Arabidopsis (Laux et al, 1996; Mayer et al, 1998; Lenhard et al, 2001; Yadav et al, 2010). In Arabidopsis wus mutant plants, the defect is exhibited at all developmental stages, and the shoot stem cells are misspecified, resulting in the premature termination of SAMs and floral meristems, and leading to an aberrant flat morphology (Laux et al, 1996; Mayer et al, 1998).The CLE peptide CLAVATA3 (CLV3) acts as a negative signal for shoot stem cell proliferation (Fletcher et al, 1999; Kondo et al, 2006; Ohyama et al, 2009). Recent studies in Arabidopsis using computational and experimental approaches revealed that WUS activates CLV3 only in the absence of HAIRY MERISTEM (HAM) proteins, and an apical–basal gradient of HAM defines the expression pattern of CLV3 in the initiating meristems (Zhou et al, 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
