The maize LAG1-O mutant suggests that reproductive cell lineages show unique gene expression patterns early in vegetative development

Excisions of a defective transposable CACTA element (Tetu1) generate new alleles of a CYCLOIDEA-like gene of Helianthus annuus

Tubular ray flower (turf) is a sunflower mutant that caught attention because it bears actinomorphic ray flowers, due to the presence of an active, although non-autonomous CACTA transposon (Tetu1) in the TCP domain of a CYCLOIDEA-like gene, HaCYC2c, a major regulator of sunflower floral symmetry. Here, we analyzed its excision rates in F3 population deriving from independent crosses of turf with common sunflower accessions. Our results suggest that the excision rate, ranging from 1.21 to 6.29%, depends on genetic background; moreover, the absence of somatic sectors in inflorescences of revertant individuals analyzed (182) and genetic analyses suggests a tight developmental control of Tetu1 excision, likely restricted to germinal cells. We individuate events of Tetu1 excision through molecular analysis that restore the wild type (WT) HaCYC2c allele, but even transposon excisions during which footprints are left. All mutations we detected occurred at the TCP basic motif and cause a change in ray flower phenotype. In particular, we selected five mutants with a one-to-four amino acid change that influence the capacity of reproductive organ development and ray flower corolla shaping (MUT-1, -2, -3, -4, -5). Revertant alleles not affecting turf phenotype (i.e. reading frame mutations) have also been identified (MUT-6). In all mutants, Real-time quantitative PCR (qPCR) experiments revealed variations of the steady state level of HaCYC2c mRNA. MUT-1 and MUT-4 showed a significant HaCYC2c down-regulation with respect to WT. A large variation within the biological replicates of MUT-2, MUT-3 and MUT-5 was detected and not significant differences in transcription levels between mutants and WT were observed. We detected low steady state level of HaCYC2c mRNA both in turf as in MUT-6. A three dimensional (3D) structure prediction tool let us predict an incorrect folding of the TCP protein already after a single amino acid deletion. This in turn is detectable as the restore of traits that are not peculiar of WT ray flowers, such as male fertility. Our analysis of an active TE sheds light on the TCP motif of the HaCYC2c gene and suggests that Tetu1 may be useful to obtain new natural mutants and for transposon tagging in different inbred lines of sunflower.

Expression of the Arabidopsis Transposable Element Tag1 Is Targeted to Developing Gametophytes

The Arabidopsis transposon Tag1 undergoes late excision during vegetative and germinal development in plants containing 35S-Tag1-GUS constructs. To determine if transcriptional regulation can account for the developmental control of Tag1 excision, the transcriptional activity of Tag1 promoter-GUS fusion constructs of various lengths was examined in transgenic plants. All constructs showed expression in the reproductive organs of developing flowers but no expression in leaves. Expression was restricted to developing gametophytes in both male and female lineages. Quantitative RT-PCR analysis confirmed that Tag1 expression predominates in the reproductive organs of flower buds. These results are consistent with late germinal excision of Tag1, but they cannot explain the vegetative excision activity of Tag1 observed with 35S-Tag1-GUS constructs. To resolve this issue, Tag1 excision was reexamined using elements with no adjacent 35S promoter sequences. Tag1 excision in this context is restricted to germinal events with no detectable vegetative excision. If a 35S enhancer sequence is placed next to Tag1, vegetative excision is restored. These results indicate that the intrinsic activity of Tag1 is restricted to germinal excision due to targeted expression of the Tag1 transposase to developing gametophytes and that this activity is altered by the presence of adjacent enhancers or promoters.