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ORCID IDs: 0000-0002-2121-9517 (M.C.); 0000-0002-9778-8855 (H.Y.).Seed storage reserves mainly consist of starch, triacylglycerols, and storage proteins. They not only provide energy for seed germination and seedling establishment, but also supply essential dietary nutrients for human beings and animals. So far, the regulatory networks that govern the accumulation of seed storage reserves in plants are still largely unknown. Here, we show that TRANSPARENT TESTA GLABRA1 (TTG1), which encodes a WD40 repeat transcription factor involved in many aspects of plant development, plays an important role in mediating the accumulation of seed storage reserves in Arabidopsis (Arabidopsis thaliana). The dry weight of ttg1-1 embryos significantly increases compared with that of wild-type embryos, which is accompanied by an increase in the contents of starch, total protein, and fatty acids in ttg1-1 seeds. FUSCA3 (FUS3), a master regulator of seed maturation, binds directly to the TTG1 genomic region and suppresses TTG1 expression in developing seeds. TTG1 negatively regulates the accumulation of seed storage proteins partially through transcriptional repression of 2S3, a gene encoding a 2S albumin precursor. TTG1 also indirectly suppresses the expression of genes involved in either seed development or synthesis/modification of fatty acids in developing seeds. In addition, we demonstrate that the maternal allele of the TTG1 gene suppresses the accumulation of storage proteins and fatty acids in seeds. Our results suggest that TTG1 is a direct target of FUS3 in the framework of the regulatory hierarchy controlling seed filling and regulates the accumulation of seed storage proteins and fatty acids during the seed maturation process.

Seed development is dependent on nutrients, such as a source of carbon, supplied by the parent plant. It remains largely unknown how these nutrients are distributed to zygotic and maternal tissues to coordinate storage of reserve compounds and development of protective tissues like seed coat. Here we show that phosphorylation of TRANSPARENT TESTA GLABRA1 (TTG1) is regulated by SHAGGY-like kinases 11/12 (SK11/12) and that this mediates carbon flow to fatty acid synthesis and seed coat traits in Arabidopsis seeds. SK11/12 phosphorylate TTG1 at serine 215, thus preventing TTG1 interaction with TRANSPARENT TESTA2. This compromises recruitment of TTG1 to the GLABRA2 locus and downregulates GLABRA2 expression, which enhances biosynthesis of fatty acids in the embryo, but reduces production of mucilage and flavonoid pigments in the seed coat. Therefore, site-specific phosphorylation of TTG1 by SK11/SK12 regulates carbon partitioning between zygotic and maternal sinks in seeds.

BackgroundIn wheat (Triticum aestivum L), the flag leaf has been thought of as the main source of assimilates for grain growth, whereas the peduncle has commonly been thought of as a transporting organ. The photosynthetic characteristics of the exposed peduncle have therefore been neglected. In this study, we investigated the anatomical traits of the exposed peduncle during wheat grain ontogenesis, and we compared the exposed peduncle to the flag leaf with respect to chloroplast ultrastructure, photosystem II (PSII) quantum yield, and phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activity.ResultsTransmission electron microscope observations showed well-developed chloroplasts with numerous granum stacks at grain-filling stages 1, 2 and 3 in both the flag leaf and the exposed peduncle. In the exposed peduncle, the membranes constituting the thylakoids were very distinct and plentiful, but in the flag leaf, there was a sharp breakdown at stage 4 and complete disintegration of the thylakoid membranes at stage 5. PSII quantum yield assays revealed that the photosynthetic efficiency remained constant at stages 1, 2 and 3 and then declined in both organs. However, the decline occurred more dramatically in the flag leaf than in the exposed peduncle. An enzyme assay showed that at stages 1 and 2 the PEPCase activity was lower in the exposed peduncle than in the flag leaf; but at stages 3, 4 and 5 the value was higher in the exposed peduncle, with a particularly significant difference observed at stage 5. Subjecting the exposed part of the peduncle to darkness following anthesis reduced the rate of grain growth.ConclusionOur results suggest that the exposed peduncle is a photosynthetically active organ that produces photosynthates and thereby makes a crucial contribution to grain growth, particularly during the late stages of grain-filling.

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Multiple flowering pathways in Arabidopsis (Arabidopsis thaliana) converge on the transcriptional regulation of FLOWERING LOCUS T (FT), encoding a mobile floral stimulus that moves from leaves to the shoot apex. Despite our progress in understanding FT movement, the mechanisms underlying its transport along the endoplasmic reticulum-plasmalemma pathway in phloem companion cells remain largely unclear. Here, we show that the plasma membrane-resident syntaxin-like glutamine-soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor (Q-SNARE), SYNTAXIN OF PLANTS121 (SYP121), interacts with QUIRKY (QKY), a member of the family of multiple C2 domain and transmembrane region proteins (MCTPs), to mediate FT transport in Arabidopsis. QKY and SYP121 coordinately regulate FT movement to the plasmalemma through the endosomal trafficking pathway and are required for FT export from companion cells to sieve elements, thus affecting FT transport through the phloem to the shoot apical meristem. These findings suggest that MCTP-SNARE complex-mediated endosomal trafficking is essential for the export of florigen from phloem companion cells to sieve elements to induce flowering.