Shojiro Tamaki, Hiroyuki Tsuji, Ayana Matsumoto, Akiko Fujita, Zenpei Shimatani, Rie Terada, Tomoaki Sakamoto, Tetsuya Kurata, and Ko Shimamoto

Significance

FLOWERING LOCUS T (FT) acts as a mobile floral activator that is synthesized in leaf and transported to shoot apex. A Rice FT-like protein, Heading date 3a (Hd3a), requires interaction with 14-3-3 proteins and transcription factor FD to induce flowering. We confirm that Hd3a and its interactors, as well as their transcriptional target, coexist in the shoot apex at the appropriate time during floral transition. RNA-sequencing analysis of shoot apices from wild-type and RNA-interference plants for FT-like genes showed that 58% of classified transposable elements are transcribed, and >200 are down-regulated in response to FT-like. Our results indicate a link between reproductive development and transposon behavior in the shoot apical meristem, supporting and extending recent evidence for such a link during gametophyte development.

Abstract

Floral induction is a crucial developmental step in higher plants. Florigen, a mobile floral activator that is synthesized in the leaf and transported to the shoot apex, was recently identified as a protein encoded by FLOWERING LOCUS T (FT) and its orthologs; the rice florigen is Heading date 3a (Hd3a) protein. The 14-3-3 proteins mediate the interaction of Hd3a with the transcription factor OsFD1 to form a ternary structure called the florigen activation complex on the promoter of OsMADS15, a rice APETALA1 ortholog. However, crucial information, including the spatiotemporal overlap among FT-like proteins and the components of florigen activation complex and downstream genes, remains unclear. Here, we confirm that Hd3a coexists, in the same regions of the rice shoot apex, with the other components of the florigen activation complex and its transcriptional targets. Unexpectedly, however, RNA-sequencing analysis of shoot apex from wild-type and RNA-interference plants depleted of florigen activity revealed that 4,379 transposable elements (TEs; 58% of all classifiable rice TEs) were expressed collectively in the vegetative and reproductive shoot apex. Furthermore, in the reproductive shoot apex, 214 TEs were silenced by florigen. Our results suggest a link between floral induction and regulation of TEs.

See: http://www.pnas.org/content/112/8/E901.abstract.html?etoc

PNAS February 24, 2015 vol. 112 no. 8 E901-E910

Fig. 1. Hd3a protein localization in the rice shoot apex during the transition from vegetative to reproductive stage. GFP fluorescence (A–D, I–L, and P) and bright-field images (E–H and M–O) of SAMs in pHd3a:Hd3a-GFP transgenic plants are shown. Development of the mature vegetative meristem (at 20–27 DAG) and different stages (R1–R4) of the inflorescence meristem (observed at 30–38 DAG and assigned to each stage by morphological characteristics) and FM (observed at >40–45 DAG) were analyzed. (A, B, E, and F) V, vegetative-stage plants at 27 DAG. At this time the plants display a meristem before the transition to reproductive development. Vas, stem vasculature. (C and G) R1, initial stage of floral transition with first bract formation. im, inflorescence meristem. (D, H, L, and P; L is an enlarged image of the area indicated by the red square in P) R2, early stage of primary branch meristem initiation. br, bract; pbm, primary branch meristem. (I and M) R3, late stage of primary panicle branch initiation. (J and N) R4, final stage of primary branch formation and initiation of secondary branch meristem (sbm). (K and O) Fl., floral organ development after R4. e.g., empty glume; fm, floret meristem; le, lemma; sl, sterile lemma. The red staining in the images indicates autofluorescence. (Scale bars, 50 µm.)