Functional analysis of cell cycle inhibitors and crosstalk between auxin and drought stress during leaf development of Arabidopsis thaliana

Datum: 30 oktober 2019

Locatie: Campus Groenenborger, V.008 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger)

Tijdstip: 14 uur

Organisatie / co-organisatie: Departement Biologie

Promovendus: Bulelani Sizani

Promotor: Gerrit Beemster

Korte beschrijving: Doctoraatsverdediging Bulelani Sizani - Faculteit Wetenschappen, Departement Biologie



Abstract

Plant growth and development are controlled by coordinated cell proliferation and cell expansion activities. Cell proliferation, differentiation and subsequently expansion requires activation of kinase complexes consisting of a cyclin-dependent (CDK) bound to a cyclin (CYCs), which in turn are downstream regulated by CDK-inhibitors (CKIs). In plants, two families of CKIs have been identified. KIP-RELATED PROTEIN (ICK/KRPs) family shares high sequence similarity with p53, an animal KIP-type CDK-inhibitor and SIAMESE/SIAMESE-RELATED (SIM/SMRs) family shares no homology with the mammalian genome.

To functionally characterize these KRPs and SIM/SMRs, we studied the effect of mutations leading to loss of function of one or more of these genes. We showed that downregulation of KRPs promote cell division and endoreduplication, resulting in organs with an increased number of cells and ploidy levels, but smaller cell size. In addition to the increase in number of cells, concurrent silencing of KRP genes induce seed abortion resulting in large seeds, presumably due to extra resources available for the remaining seeds. We demonstrated that the increased leaf size in the krp4/6/7 mutant is largely due to this increased seed size as there was a close correlation between seed size and leaf area.

Similar to the effects of concurrent mutation of multiple KRPs, mutation of SMR2 promotes cell proliferation during leaf growth. Contrary to KRPSs, mutation of SMRs delay endocycle onset and collectively reduce endoreduplication in sim-smr1/2 triple mutant. Although CKIs regulate cell cycle progression, our results showed no significant growth response in matured leaf size of CKI mutants grown in abiotic stress.

The inhibition of cell division plays a key role in the response to adverse environmental conditions. Using NAA to increase auxin levels and mannitol as osmotic stress, we provided evidence that auxin signalling plays a functional role in the response of Arabidopsis to mild osmotic stress. Our results showed that in the presence of 0.1 µM NAA, leaf expansion is hypersensitive to mannitol (that is, the addition of NAA exacerbates the effects of osmotic stress on leaf growth). Finally, we show that the sensitivity in leaf expansion to osmotic stress remains, but hypersensitivity to osmotic stress in presence of NAA is diminished in several auxin mutants.

Together these results demonstrate a key role for cell division in the regulation of leaf growth under control and adverse conditions. While it is clear that CKIs are key players, altering their expression in a constitutive fashion does not lead to increased drought tolerance.



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