DELLA proteins restrict cell divisions by distinct mechanisms

by Jens Sundström
Swedish University of Agricultural Sciences

Breeding cereal crops with reduced stem length, in so-called semi-dwarf varieties, greatly contributed to yield increases associated with the green revolution. However, the semi-dwarf genotype was also associated with reduced inflorescence size. Now, researchers at the John Innes Centre in Norwich, UK, have demonstrated that these traits are regulated by distinct pathways (Serrano-Mislata et al., 2017).  This finding opens up new venues for breeding of semi-dwarf crops without compromising yields by reducing inflorescence size.

Breeding efforts between 1960 and 1985, largely carried out at international public goods institutions such as the International Maize and Wheat Improvement Centre in Mexico (CIMMYT), contributed to a massive increase in crop yields (Pingali, 2012). For instance, yields for wheat in many developing countries have increased almost 200% since the mid -1960s. One of the key traits introgressed in many high yielding varieties is the semi-dwarf genotype. Reduced stem elongation aids reduction of lodging and allows more resources to be allocated to other parts of the plant. Typically, varieties harbouring this trait are mutated in genes affecting responses to the plant hormone gibberellin (GA) (Daviere and Achard, 2013).

Jens blog Dec 2017 3


Fig. 1. Rye usually grow shoulder high. Field of rye-wheat in which the semi-dwarf genotype found in modern wheat varieties has been introgressed. The work by Serrano-Mislata et al. indicates that the semi-dwarf trait can be uncoupled from the reduction of inflorescence size.

Genetic analyses of GA response mutants, primarily carried out in the model species Arabidopsis thaliana have contributed to a working model for GA activity, in which GA acts as an “inhibitor of an inhibitor” (Harberd et al., 2009). DELLA-proteins, which belong to a sub-family of the plant specific GRAS family, act as key repressors of GA responses (Daviere and Achard, 2013). In the absence of GA, DELLA-proteins bind other transcription factors and inhibit their activity. In the presence of GA, the DELLA proteins are degraded and transcription of GA-responsive genes can occur. Plants with mutated DELLA-proteins have pleiotropic phenotypes; for example, reduced seed germination and reduced stem length.

In a recent report, Serrano-Mislata and co-workers (2017) demonstrated that DELLA proteins inhibit shoot growth by negatively regulating cell division rather than cell expansion. The authors provided evidence for this by expressing a stabilized form of DELLA proteins in either the internodes of a stem or in the apical segment of an inflorescence (Fig. 1).

In both cases, expression of the stabilized DELLA-protein resulted in fewer dividing cells as compared to the wild type. These results suggested that DELLA-proteins act as inhibitors of genes involved in cell-cycle regulation or cell division. To test this hypothesis, the authors performed a chromatin immunoprecipitation (ChIP) experiment that allowed them to identify promoters to which the DELLA proteins bind. One of the candidate genes identified in the ChIP experiment encodes a protein belonging to a family of cell cycle inhibitors. Next, the authors made a cross between a knock-out mutant of the cell cycle inhibitor and the line expressing the stabilized form of the DELLA-proteins. Interestingly, the resulting line retained the semi-dwarf phenotype, but the number of cells in the shoot apical meristem was similar to that in the wild type. Hence, cell division were inhibited in the stem internodes but unaffected in the shoot apical meristem, suggesting that DELLA proteins, at least in part, control growth through the activity of cell cycle inhibitors and that this regulation occurs through distinct pathways in different parts of the plant.

While the mechanistic and genetic insights revealed by Serrano-Mislata et al., (2017) are based on work done in Arabidopsis, their study also provides evidence for the presence of  conserved mechanisms in cereals. Hence, their findings may provide future tools for breeding high yielding semi-dwarf cereal varieties, without compromising growth in the seed-bearing parts of the plants.

References
Serrano-Mislata A, Bencivenga S, Bush M, Schiessl K, Boden S, Sablowski R. (2017). DELLA genes restrict inflorescence meristem function independently of plant height. Nature Plants 3(9):749-754. doi:10.1038/s41477-017-0003-y

Pingali PL. (2012). Green revolution: impacts, limits, and the path ahead. Proccedings of the Natural Academy of Sciences, USA 109(31):12302-12308. doi:10.1073/pnas.0912953109

Daviere JM and Achard P. (2013). Gibberellin signaling in plants. Development 140(6):1147-1151. doi: 10.1242/dev.087650

Harberd NP, Belfield E, Yasumura Y. (2009). The angiosperm gibberellin-GID1-DELLA growth regulatory mechanism: how an “inhibitor of an inhibitor” enables flexible response to fluctuating environments. The Plant Cell 21(5):1328-1339. https://doi.org/10.1105/tpc.109.066969

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