Arabidopsis regeneration from multiple tissues occurs via a root development pathway.

Detlef Weigel

Detlef Weigel

Detlef Weigel

+ -1 Evaluators

Plants can so much more easily regenerate than animals -- but is this a property of all plant cells? This paper beautifully demonstrates that this process does not proceed via simple reprogramming to an undifferentiated state. Rather, the authors show that it occurs through initiation of a root developmental program, most likely from special cells present in many different tissue types.

Christian Hardtke

Christian Hardtke

Christian Hardtke

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In this paper, Sugimoto et al. take a molecular look at callus formation to find, surprisingly, that it basically resembles meristematic root tissue. The formation of callus, a proliferating mass of plant cells that can be produced in cell culture through hormone treatment, is an important tool in plant science and horticulture, for instance for the production of transgenics or novel cultivars. The idea that callus represents undifferentiated cells is a plant science truism and the basis of the claim of plant cell totipotence, since callus can be induced starting from virtually any differentiated plant tissue. Using molecular markers as well as mutant strains, the authors demonstrate that this is actually not the case, even across varying hormone conditions. Rather, callus formation appears to reflect a redirection of fate towards root tip (meristematic) identity. The data fit in well with the previous identification of master regulators of root development, which, if expressed ectopically, can induce root formation in shoot structures {1}.

Renate Schmidt

Renate Schmidt

Renate Schmidt

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This study reports the fascinating but surprising finding that plant calli resemble the tip of a root meristem regardless of their tissue of origin -- very much in contrast to the common notion that each callus represents a mass of undifferentiated cells. Whole plants can be regenerated from almost any differentiated tissue and the formation of a callus is often the first step in such plant in vitro regeneration procedures, which are widely adopted in plant science and biotechnology. Now, gene expression profiling, live cell imaging and mutant analyses have been combined to demonstrate that the ectopic initiation of a root developmental program is the common step in callus formation, even if the callus is formed from aerial organs.

Tomomichi Fujita

Tomomichi Fujita

Tomomichi Fujita

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This exciting paper provides some answers to the questions of what a plant callus is and how a callus initiates from differentiated tissues at the molecular level. Plant cells from differentiated organs display totipotency and are able to regenerate all the different tissues in the whole plant. The molecular mechanisms behind the totipotency, however, remain largely unknown. During the course of regeneration, cells are believed to dedifferentiate and acquire competency to form a callus, which is a mass of unorganized, growing cells. How is a callus induced and how do we define what a callus is at the molecular level? Sugimoto and colleagues demonstrated that the cells that go on to form a callus are directed to the lateral root initiation pathway to differentiate into pericycle-like cells, similar to the founder cells for the lateral root meristem. The results are not consistent with the widely believed idea that a callus is formed from undifferentiated cells. The molecular markers for lateral root development are activated during callus formation, and the gene essential for lateral root initiation is also necessary for this process. Surprisingly, a callus derived from aerial organs (such as cotyledons and petals), whose lineage of cell types is distinct from the one of roots, also required induction of pericycle-like cells. The experiments were done in Arabidopsis thaliana and it will be interesting to find out whether the finding is common to other flowering plants or unique to a few limited species? Plants can also produce other types of calli, root nodules or neoplastic tissues, such as crown galls and genetic tumors, from various organs. Do they form using the same regeneration pathway? This paper provides a clue to address such issues and move towards a greater understanding of the unique plasticity of plants.

Ilha Lee

Ilha Lee

Ilha Lee

+ -1 Evaluators

This paper upsets the unchallenged idea of the callus being undifferentiated tissue by showing that the callus is more like the lateral root primordia. It will be a critical milestone in the field of plant regeneration and plasticity. In contrast to animal cells, plant cells are totipotent because they can regenerate a full array of plant tissues if the proper hormones are provided, regardless of their origin. For in vivo regeneration, plant cells usually undergo callus state, an amorphous cell mass, then the shooting and rooting from the callus are induced by the proper combinations of auxin and cytokinin, the two plant hormones activating cell division {1}. For more than 60 years, it was believed that the callus is dedifferentiated into undifferentiated stem cells, but this paper clearly shows that the callus undergoes the root developmental program. During the callus formation, the explants derived from roots, cotyledons, and petals strongly expressed SCR, SHR, and WOX5, key regulators of the root meristem, and expressed a pericycle-specific marker, J0121. In addition, genome-wide analysis showed that the callus has a similar expression profile to the root meristem. Consistently, any tissue explants derived from a mutant defective of lateral root formation failed to induce a callus, suggesting that the callus follows a similar genetic program to the lateral root primordia. Finally, the paper showed that callus formation starts from the pericycle-like tissues even in aerial organs. Therefore, the next frontier will be the characterization of this tissue.