OsFTL12, a member of FT-like family,modulates the heading date and plant architecture by florigen repression complex in rice

FLOWERING LOCUS T (FT), a florigen in Arabidopsis, plays critical roles in floral transition. Among 13 FT-like members in rice, OsFTL2 (Hd3a) and OsFTL3 (RFT1), two rice homologues of FT, have been well characterized to act as florigens to induce flowering under short-day (SD) and long-day (LD) conditions, respectively, but the functions of other rice FT-like members remain largely unclear. Here, we show that OsFTL12 plays an antagonistic function against Hd3a and RFT1 to modulate the heading date and plant architecture in rice. Unlike Hd3a and RFT1, OsFTL12 is not regulated by daylength and highly expressed in both SD and LD conditions, and delays the heading date under either SD or LD conditions. We further demonstrate that OsFTL12 interacts with GF14b and OsFD1, two key components of the florigen activation complex (FAC), to form the florigen repression complex (FRC) by competing with Hd3a for binding GF14b. Notably, OsFTL12-FRC can bind to the promoters of the floral identity genes OsMADS14 and OsMADS15 and suppress their expression. The osmads14 osmads15 double mutants could not develop panicles and showed erect leaves. Taken together, our results reveal that different FT-like members can fine-tune heading date and plant architecture by regulating the balance of FAC and FRC in rice.     

GTSF‐1 is required for formation of a functional RNA‐dependent RNA Polymerase complex in Caenorhabditis elegans

Argonaute proteins and their associated small RNAs (sRNAs) are evolutionarily conserved regulators of gene expression. Gametocyte‐specific factor 1 (Gtsf1) proteins, characterized by two tandem CHHC zinc fingers and an unstructured C‐terminal tail, are conserved in animals and have been shown to interact with Piwi clade Argonautes, thereby assisting their activity. We identified the Caenorhabditis elegans Gtsf1 homolog, named it gtsf‐1 and characterized it in the context of the sRNA pathways of C. elegans. We report that GTSF‐1 is not required for Piwi‐mediated gene silencing. Instead, gtsf‐1 mutants show a striking depletion of 26G‐RNAs, a class of endogenous sRNAs, fully phenocopying rrf‐3 mutants. We show, both in vivo and in vitro, that GTSF‐1 interacts with RRF‐3 via its CHHC zinc fingers. Furthermore, we demonstrate that GTSF‐1 is required for the assembly of a larger RRF‐3 and DCR‐1‐containing complex (ERIC), thereby allowing for 26G‐RNA generation. We propose that GTSF‐1 homologs may act to drive the assembly of larger complexes that act in sRNA production and/or in imposing sRNA‐mediated silencing activities.     

Enhanced resistance to soybean cyst nematode Heterodera glycines in transgenic soybean by silencing putative CLE receptors

CLE peptides are small extracellular proteins important in regulating plant meristematic activity through the CLE‐receptor kinase‐WOX signalling module. Stem cell pools in the SAM (shoot apical meristem), RAM (root apical meristem) and vascular cambium are controlled by CLE signalling pathways. Interestingly, plant‐parasitic cyst nematodes secrete CLE‐like effector proteins, which act as ligand mimics of plant CLE peptides and are required for successful parasitism. Recently, we demonstrated that Arabidopsis CLE receptors CLAVATA1 (CLV1), the CLAVATA2 (CLV2)/CORYNE (CRN) heterodimer receptor complex and RECEPTOR‐LIKE PROTEIN KINASE 2 (RPK2), which transmit the CLV3 signal in the SAM, are required for perception of beet cyst nematode Heterodera schachtii CLEs. Reduction in nematode infection was observed in clv1, clv2, crn, rpk2 and combined double and triple mutants. In an effort to develop nematode resistance in an agriculturally important crop, orthologues of Arabidopsis receptors including CLV1, CLV2, CRN and RPK2 were identified from soybean, a host for the soybean cyst nematode Heterodera glycines. For each of the receptors, there are at least two paralogues in the soybean genome. Localization studies showed that most receptors are expressed in the root, but vary in their level of expression and spatial expression patterns. Expression in nematode‐induced feeding cells was also confirmed. In vitro direct binding of the soybean receptors with the HgCLE peptide was analysed. Knock‐down of the receptors in soybean hairy roots showed enhanced resistance to SCN. Our findings suggest that targeted disruption of nematode CLE signalling may be a potential means to engineer nematode resistance in crop plants.     

Mutations in the N‐terminal kinase‐like domain of the repressor of photomorphogenesis SPA1 severely impair SPA1 function but not light responsiveness in Arabidopsis

The COP1/SPA complex is an E3 ubiquitin ligase that acts as a key repressor of photomorphogenesis in dark‐grown plants. While both COP1 and the four SPA proteins contain coiled‐coil and WD‐repeat domains, SPA proteins differ from COP1 in carrying an N‐terminal kinase‐like domain that is not present in COP1. Here, we have analyzed the effects of deletions and missense mutations in the N‐terminus of SPA1 when expressed in a spa quadruple mutant background devoid of any other SPA proteins. Deletion of the large N‐terminus of SPA1 severely impaired SPA1 activity in transgenic plants with respect to seedling etiolation, leaf expansion and flowering time. This ΔN SPA1 protein showed a strongly reduced affinity for COP1 in vitro and in vivo, indicating that the N‐terminus contributes to COP1/SPA complex formation. Deletion of only the highly conserved 95 amino acids of the kinase‐like domain did not severely affect SPA1 function nor interactions with COP1 or cryptochromes. In contrast, missense mutations in this part of the kinase‐like domain severely abrogated SPA1 function, suggesting an overriding negative effect of these mutations on SPA1 activity. We therefore hypothesize that the sequence of the kinase‐like domain has been conserved during evolution because it carries structural information important for the activity of SPA1 in darkness. The N‐terminus of SPA1 was not essential for light responsiveness of seedlings, suggesting that photoreceptors can inhibit the COP1/SPA complex in the absence of the SPA1 N‐terminal domain. Together, these results uncover an important, but complex role of the SPA1 N‐terminus in the suppression of photomorphogenesis.     

Arabidopsis thaliana CENTRORADIALIS homologue (ATC) acts systemically to inhibit floral initiation in Arabidopsis

Floral initiation is orchestrated by systemic floral activators and inhibitors. This remote‐control system may integrate environmental cues to modulate floral initiation. Recently, FLOWERING LOCUS T (FT) was found to be a florigen. However, the identity of systemic floral inhibitor or anti‐florigen remains to be elucidated. Here we show that Arabidopsis thaliana CENTRORADIALIS homologue (ATC), an Arabidopsis FT homologue, may act in a non‐cell autonomous manner to inhibit floral initiation. Analysis of the ATC null mutant revealed that ATC is a short‐day‐induced floral inhibitor. Cell type‐specific expression showed that companion cells and apex that express ATC are sufficient to inhibit floral initiation. Histochemical analysis showed that the promoter activity of ATC was mainly found in vasculature but under the detection limit in apex, a finding that suggests that ATC may move from the vasculature to the apex to influence flowering. Consistent with this notion, Arabidopsis seedling grafting experiments demonstrated that ATC moved over a long distance and that floral inhibition by ATC is graft transmissible. ATC probably antagonizes FT activity, because both ATC and FT interact with FD and affect the same downstream meristem identity genes APETALA1, in an opposite manner. Thus, photoperiodic variations may trigger functionally opposite FT homologues to systemically influence floral initiation.     

CONSTANS delays Arabidopsis flowering under short days

Long days (LD) promote flowering of Arabidopsis thaliana compared with short days (SD) by activating the photoperiodic pathway. Here we show that growth under very‐SD (3 h) or darkness (on sucrose) also accelerates flowering on a biological scale, indicating that SD actively repress flowering compared with very‐SD. CONSTANS (CO) repressed flowering under SD, and the early flowering of co under SD required FLOWERING LOCUS T (FT). FT was expressed at a basal level in the leaves under SD, but these levels were not enhanced in co. This indicates that the action of CO in A. thaliana is not the mirror image of the action of its homologue in rice. In the apex, CO enhanced the expression of TERMINAL FLOWER 1 (TFL1) around the time when FT expression is important to promote flowering. Under SD, the tfl1 mutation was epistatic to co and in turn ft was epistatic to tfl1. These observations are consistent with the long‐standing but not demonstrated model where CO can inhibit FT induction of flowering by affecting TFL1 expression.     

Trichome regulator SlMIXTA‐like directly manipulates primary metabolism in tomato fruit

Trichomes are storage compartments for specialized metabolites in many plant species. In trichome, plant primary metabolism is significantly changed, providing substrates for downstream secondary metabolism. However, little is known of how plants coordinate trichome formation and primary metabolism regulation. In this report, tomato (Solanum lycopersicum) trichome regulator SlMIXTA‐like is indicated as a metabolic regulation gene by mGWAS analysis. Overexpression of SlMIXTA‐like in tomato fruit enhances trichome formation. In addition, SlMIXTA‐like can directly bind to the promoter region of gene encoding 3‐deoxy‐7‐phosphoheptulonate synthase (SlDAHPS) to activate its expression. Induction of SlDAHPS expression enhances shikimate pathway activities and provides substrates for downstream secondary metabolism. Our data provide direct evidence that trichome regulator can directly manipulate primary metabolism, in which way plants can coordinate metabolic regulation and the formation of storage compartments for specialized metabolites. The newly identified SlMIXTA‐like can be used for future metabolic engineering.