Further Development 22.2: A Lateral Root Way to Be Call(o)us

Roots can be “callous”; their regenerative programs don’t require the stem cell niche. Arabidopsis thaliana mutants with defects in the PLT-SHR-SCR gene network fail to regenerate the QC following ablation, demonstrating that these genes are essential to rebuilding the niche (Xu et al. 2006). Surprisingly, despite the lack of the QC in this condition, the root tip can still regenerate following its own excision (Sena et al. 2009). This is regeneration without a functional meristematic niche! This phenomenon speaks to the competence of the cells surrounding the injury to fuel organ regeneration. As mentioned above, “partially differentiated cells” are competent to respond to the injury by re-entering the cell cycle and either dedifferentiating or transdifferentiating into the missing cells and tissues for complete regeneration. For instance, xylem pole-specific pericycle cells have been demonstrated to exhibit extreme cellular plasticity, particularly for the initiation of lateral root formation, as well as for the development of callus and shoot cell identities (Malamy and Benfey 1997; De Smet et al. 2006; Sugimoto et al. 2010; Kareem et al. 2015, 2016a, b).

More recent genetic analysis has started to suggest a modest distinction between embryonic and regenerative developmental programs in plants (Kareem et al. 2016a,b). Embryogenesis occurs normally in the A. thaliana aberrant lateral root formation 4 (alf4) mutant and in a triple plt mutant, but both mutants fail to form totipotent callus or to properly regenerate organs (Celenza et al. 1995; Prasad et al. 2011; Kareem et al. 2015). Thus, during plant regeneration, the callus may not be reverting to a true undifferentiated embryonic state, but rather transitioning to something more like a lateral root primordium identity (Sugimoto et al. 2010).

Literature Cited

Celenza Jr., J. L., P. L. Grisafi and G. R. Fink. 1995. A pathway for lateral root formation in Arabidopsis thaliana. Genes Dev. 9: 2131–2142.

De Smet, I., S. Vanneste, D. Inze and T. Beeckman. 2006. Lateral root initiation or the birth of a new meristem. Plant Mol. Biol. 60: 871–887.

Kareem, A., and 10 others. 2015. PLETHORA genes control regeneration by a two-step mechanism. Curr. Biol. 25: 1017–1030.

Kareem, A., D. Radhakrishnan, Y. Sondhi, M. Aiyaz, M. V. Roy, K. Sugimoto and K. Prasad. 2016a. De novo assembly of plant body plan: A step ahead of Deadpool. Regeneration 3: 182–197.

Kareem, A., D. Radhakrishnan, X. Wang, S. Bagavathiappan, Z. B. Trivedi, K. Sugimoto, J. Xu, A. P. Mähönen and K. Prasad. 2016b. Protocol: A method to study the direct reprogramming of lateral root primordia to fertile shoots. Plant Methods 12: 27.

Malamy, J. E. and P. N. Benfey. 1997. Organization and cell differentiation in lateral roots of Arabidopsis thaliana. Development 124: 33–44.

Prasad, K. and 17 others. 2011. Arabidopsis PLETHORA transcription factors control phyllotaxis. Curr. Biol. 21: 1123–1128.

Sena, G., X. Wang, H. Y. Liu, H. Hofhuis and K. D. Birnbaum. 2009. Organ regeneration does not require a functional stem cell niche in plants. Nature 457: 1150–1153.

Sugimoto, K., Y. Jiao and E. M. Meyerowitz. 2010. Arabidopsis regeneration from multiple tissues occurs via a root development pathway. Dev. Cell, 18: 463–471.

Xu, J., H. Hofhuis, R. Heidstra, M. Sauer, J. Friml and B. Scheres. 2006. A molecular framework for plant regeneration. Science 311: 385–388.

 

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