Flowering is initiated in response to environmental and internal cues that are integrated at the shoot apical meristem (SAM). We show that SPL15 coordinates the basal floral promotion pathways required for flowering of Arabidopsis in non-inductive environments. SPL15 directly activates transcription of the floral regulators FUL and miR172b in the SAM during floral induction, whereas its paralog SPL9 is expressed later on the flanks of the SAM. The capacity of SPL15 to promote flowering is regulated by age through miR156, which targets SPL15 mRNA, and gibberellin (GA), which releases SPL15 from DELLAs. Furthermore, SPL15 and the MADS-box protein SOC1 cooperate to promote transcription of their target genes. SPL15 recruits RNAPII and MED18, a Mediator complex component, in a GA-dependent manner, while SOC1 facilitates active chromatin formation with the histone demethylase REF6. Thus, we present a molecular basis for assimilation of flowering signals and transcriptional control at the SAM during flowering.

SUMMARYThe plant growth regulator gibberellin (GA) contributes to many developmental processes, including the transition to flowering. In Arabidopsis, GA promotes this transition most strongly under environmental conditions such as short days (SDs) when other regulatory pathways that promote flowering are not active. Under SDs, GAs activate transcription of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and LEAFY (LFY) at the shoot meristem, two genes encoding transcription factors involved in flowering. Here, the tissues in which GAs act to promote flowering were tested under different environmental conditions. The enzyme GIBBERELLIN 2 OXIDASE 7 (GA2ox7), which catabolizes active GAs, was overexpressed in most tissues from the viral CaMV 35S promoter, specifically in the vascular tissue from the SUCROSE TRANSPORTER 2 (SUC2) promoter or in the shoot apical meristem from the KNAT1 promoter. We find that under inductive long days (LDs), GAs are required in the vascular tissue to increase the levels of FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF) mRNAs, which encode a systemic signal transported from the leaves to the meristem during floral induction. Similarly, impairing GA signalling in the vascular tissue reduces FT and TSF mRNA levels and delays flowering. In the meristem under inductive LDs, GAs are not required to activate SOC1, as reported under SDs, but for subsequent steps in floral induction, including transcription of genes encoding SQUAMOSA PROMOTER BINDING PROMOTER LIKE (SPL) transcription factors. Thus, GA has important roles in promoting transcription of FT, TSF and SPL genes during floral induction in response to LDs, and these functions are spatially separated between the leaves and shoot meristem.

In Arabidopsis thaliana environmental and endogenous cues promote flowering by activating expression of a small number of integrator genes. The MADS box transcription factor SHORT VEG-ETATIVE PHASE (SVP) is a critical inhibitor of flowering that directly represses transcription of these genes. However, we show by genetic analysis that the effect of SVP cannot be fully explained by repressing known floral integrator genes. To identify additional SVP functions, we analyzed genome-wide transcriptome data and show that GIBBERELLIN 20 OXIDASE 2, which encodes an enzyme required for biosynthesis of the growth regulator gibberellin (GA), is upregulated in svp mutants. GA is known to promote flowering, and we find that svp mutants contain elevated levels of GA that correlate with GA-related phenotypes such as early flowering and organ elongation. The ga20ox2 mutation suppresses the elevated GA levels and partially suppresses the growth and early flowering phenotypes of svp mutants. In wild-type plants, SVP expression in the shoot apical meristem falls when plants are exposed to photoperiods that induce flowering, and this correlates with increased expression of GA20ox2. Mutations that impair the photoperiodic flowering pathway prevent this downregulation of SVP and the strong increase in expression of GA20ox2. We conclude that SVP delays flowering by repressing GA biosynthesis as well as integrator gene expression and that, in response to inductive photoperiods, repression of SVP contributes to the rise in GA at the shoot apex, promoting rapid induction of flowering.

BACKGROUNDMultiple‐herbicide resistance in Lolium rigidum and other weed species is increasingly exerting pressure on herbicide discovery research for solutions against resistance‐prone weeds. In this study we investigate: (i) the responses of L. rigidum populations and wheat to the new herbicide cinmethylin in comparison with other pre‐emergence herbicides, (ii) the effect of seed burial depths on cinmethylin efficacy and crop selectivity, and (iii) the basis of cinmethylin selectivity in wheat.RESULTSCinmethylin at 400 g ha−1 controls herbicide‐susceptible and multiple‐resistant L. rigidum, with a reduction of >85% in plant emergence and 90% in aboveground biomass. Cinmethylin provides effective control of a large number of field populations of L. rigidum with evident resistance to trifluralin. When the wheat seed is buried ≥1 cm below the cinmethylin‐treated soil surface, the emergence of crop seedlings is not different from the untreated control. The organophosphate insecticide phorate synergizes cinmethylin toxicity in wheat, with an LD50 of 682 g ha−1 in the absence of phorate versus 109 g ha−1 in the presence of phorate (84% reduction). The synergistic effect of phorate with cinmethylin on herbicide‐susceptible L. rigidum appears smaller (a 44% reduction in the LD50 of cinmethylin).CONCLUSIONSCinmethylin is effective in controlling multiple‐resistant L. rigidum and appears safe for wheat when the seed is separated at depth from the herbicide applied to the soil surface. The basis of this metabolism‐based selectivity is likely regulated by cytochrome P450 monooxygenases. © 2020 Society of Chemical Industry