A CENH3 mutation promotes meiotic exit and restores fertility in SMG7-deficient Arabidopsis

Abstract

Meiosis in angiosperm plants is followed by mitotic divisions to form multicellular haploid gametophytes. Termination of meiosis and transition to gametophytic development is, in Arabidopsis, governed by a dedicated mechanism that involves SMG7 and TDM1 proteins. Mutants carrying the smg7-6 allele are semi-fertile due to reduced pollen production. We found that instead of forming tetrads, smg7-6 pollen mother cells undergo multiple rounds of chromosome condensation and spindle assembly at the end of meiosis, resembling aberrant attempts to undergo additional meiotic divisions. A suppressor screen uncovered a mutation in centromeric histone H3 (CENH3) that increased fertility and promoted meiotic exit in smg7-6 plants. The mutation led to inefficient splicing of the CENH3 mRNA and a substantial decrease of CENH3, resulting in smaller centromeres. The reduced level of CENH3 delayed formation of the mitotic spindle but did not have an apparent effect on plant growth and development. We suggest that impaired spindle re-assembly at the end of meiosis limits aberrant divisions in smg7-6 plants and promotes formation of tetrads and viable pollen. Furthermore, the reduced level of CENH3 did not induce haploid plants in crosses with wild plants, indicating that differences in centromere size is not the key determinant of centromere-mediate genome elimination.

High temperature increases centromere-mediated genome elimination frequency in Arabidopsis deficient in cenH3 or its assembly factor KNL2

“…Nevertheless, the reduced level of cenH3 induced haploid plants only with a very low frequency (0.2%) when crossed with wild-type plants. Thus the smaller centromere size was not efficient to trigger haploidization in Arabidopsis (Capitao et al ., 2021). Considering the effect of heat stress on the knl2 mutant, we tested the haploid induction rate with cenh3-4 mutants cultivated under heat stress.…”

“…Moreover, the same heat stress treatment applied to the cenh3-4 mutant (Capitao et al ., 2021) resulted in an increase in HIR from 0.2% under standard conditions to 4.1%. Interestingly, our data also show that the more efficient HIR is achieved when heat stress is prolonged to the postfertilization period.…”

“…RNAi-mediated knockdown of cenH3 showed a reduction in its mRNA level (27-43%) and also resulted in a dwarf plant phenotype and meiotic defects in Arabidopsis (Lermontova et al ., 2011). Recently, a mutation in cenH3 named cenh3-4 has been discovered from the genetic suppressor screen, which increased fertility and promoted meiotic exit in smg7-6 plants (Capitao et al ., 2021). The cenh3-4 is a point mutation (G →A) in the splicing donor site of the 3rd exon of cenH3 showed a reduced amount of cenH3 at centromeres and thus, forming small centromeres.…”