Busted: Maternal modifiers of the triploid block involved in seed size control

ABSTRACT The triploid block leads to seed abortion in crosses involving tetraploid Col-0 pollen. The genetic basis underlying this phenomenon is established in the endosperm and attributed to parental genomic imprinting. This research utilised the genetic variation in Arabidopsis to identify the genomic regions harbouring the maternal modifiers of the triploid block to produce viable large seeds. Distinct chromosomal regions were identified in Bla-1 and Tsu-0 accessions. The Bla-1 maternal modifier maps to the TTG2 locus at the lower end of chromosome 2 to produce large viable seeds in response to a triploid block. Tsu-0 accession, on the other hand, recruits the TTG1 locus on the upper arm of chromosome 5 as a maternal modifier of the triploid block. TTG1 and TTG2 mutations significantly increased the proportion of large viable seeds in interploidy crosses. Both genes are involved in transcriptional regulation in the flavonoid biosynthesis pathway. However, to regulate seed size in diploids, TTG1 functions synergistically with auxin but does so independently of TTG2 . This work contributed to the genetic framework for the TTG1 and TTG2 seed size roles. HIGHLIGHTS Different Arabidopsis accessions recruit maternal modifiers to repress Col-killing in F 1 triploids. These maternal modifiers may operate in the same pathway, such as the flavonoid biosynthesis pathway or other interconnected pathways such as auxin. TTG1 and TTG2 generally increase F 1 triploid survival but in an accession-dependent manner. TTG1 differentially exhibits a strong positive additive interaction with auxin to increase diploid seed size. The TTG1 / TTG2 roles in diploid seed size control appear to have diverged somewhere in the auxin branch of the flavonoid biosynthesis pathway.