Florigen, the mobile signal that moves from an induced leaf to the shoot apex and causes flowering, has eluded identification since it was first proposed 70 years ago. Understanding the nature of the mobile flowering signal would provide a key insight into the molecular mechanism of floral induction. Recent studies suggest that the Arabidopsis FLOWERING LOCUS T (FT) gene is a candidate for encoding florigen. We show that the protein encoded by Hd3a, a rice ortholog of FT, moves from the leaf to the shoot apical meristem and induces flowering in rice. These results suggest that the Hd3a protein may be the rice florigen.

Plants sense phosphate (Pi) deficiency and initiate signaling that controls adaptive responses necessary for Pi acquisition. Herein, evidence establishes that AtSIZ1 is a plant small ubiquitin-like modifier (SUMO) E3 ligase and is a focal controller of Pi starvation-dependent responses. T-DNA insertional mutated alleles of AtSIZ1 (At5g60410) cause Arabidopsis to exhibit exaggerated prototypical Pi starvation responses, including cessation of primary root growth, extensive lateral root and root hair development, increase in root/shoot mass ratio, and greater anthocyanin accumulation, even though intracellular Pi levels in siz1 plants were similar to wild type. AtSIZ1 has SUMO E3 ligase activity in vitro, and immunoblot analysis revealed that the protein sumoylation profile is impaired in siz1 plants. AtSIZ1-GFP was localized to nuclear foci. Steadystate transcript abundances of Pi starvation-responsive genes AtPT2, AtPS2, and AtPS3 were moderate but clearly greater in siz1 seedlings than in wild type, where Pi is sufficient. Pi starvation induced the expression of these genes to the same extent in siz1 and wild-type seedlings. However, two other Pi starvation-responsive genes, AtIPS1 and AtRNS1, are induced more slowly in siz1 seedlings by Pi limitation. PHR1, a MYB transcriptional activator of AtIPS1 and AtRNS1, is an AtSIZ1 sumoylation target. These results indicate that AtSIZ1 is a SUMO E3 ligase and that sumoylation is a control mechanism that acts both negatively and positively on different Pi deficiency responses

The photoperiodic control of flowering is one of the important developmental processes of plants because it is directly related to successful reproduction. Although the molecular genetic analysis of Arabidopsis thaliana, a long-day (LD) plant, has provided models to explain the control of flowering time in this species, very little is known about its molecular mechanisms for short-day (SD) plants. Here we show how the photoperiodic control of flowering is regulated in rice, a SD plant. Overexpression of OsGI, an orthologue of the Arabidopsis GIGANTEA (GI) gene in transgenic rice, caused late flowering under both SD and LD conditions. Expression of the rice orthologue of the Arabidopsis CONSTANS (CO) gene was increased in the transgenic rice, whereas expression of the rice orthologue of FLOWERING LOCUS T (FT) was suppressed. Our results indicate that three key regulatory genes for the photoperiodic control of flowering are conserved between Arabidopsis, a LD plant, and rice, a SD plant, but regulation of the FT gene by CO was reversed, resulting in the suppression of flowering in rice under LD conditions.

RICE FLOWERING LOCUS T 1 (RFT1/FT-L3) is the closest homologue of Heading date 3a (Hd3a), which is thought to encode a mobile flowering signal and promote floral transition under short-day (SD) conditions. RFT1 is located only 11.5 kb from Hd3a on chromosome 6. Although RFT1 RNAi plants flowered normally, double RFT1-Hd3a RNAi plants did not flower up to 300 days after sowing (DAS), indicating that Hd3a and RFT1 are essential for flowering in rice. RFT1 expression was very low in wild-type plants, but there was a marked increase in RFT1 expression by 70 DAS in Hd3a RNAi plants, which flowered 90 DAS. H3K9 acetylation around the transcription initiation site of the RFT1 locus had increased by 70 DAS but not at 35 DAS. In the absence of Hd3a and RFT1 expression, transcription of OsMADS14 and OsMADS15, two rice orthologues of Arabidopsis APETALA1, was strongly reduced, suggesting that they act downstream of Hd3a and RFT1. These results indicate that Hd3a and RFT1 act as floral activators under SD conditions, and that RFT1 expression is partly regulated by chromatin modification.

SummaryThe Arabidopsis thaliana vacuolar Na + /H + antiporter AtNHX1 is a salt tolerance determinant. Predicted amino acid sequence similarity, protein topology and the presence of functional domains conserved in AtNHX1 and prototypical mammalian NHE Na + /H + exchangers led to the identi®cation of ®ve additional AtNHX genes (AtNHX2±6). The AtNHX1 and AtNHX2 mRNAs are the most prevalent transcripts among this family of genes in seedling shoots and roots. A lower-abundance AtNHX5 mRNA is present in both shoots and roots, whereas AtNHX3 transcript is expressed predominantly in roots. AtNHX4 and AtNHX6 mRNAs were detected only by RT±PCR. AtNHX1, 2 or 5 suppress, with differential ef®cacy, the Na + /Li + -sensitive phenotype of a yeast mutant that is de®cient in the endosomal/vacuolar Na + /H + antiporter ScNHX1. Ion accumulation data indicate that these AtNHX proteins function to facilitate Na + ion compartmentalization and maintain intracellular K + status. Seedling steady-state mRNA levels of AtNHX1 and AtNHX2 increase similarly after treatment with NaCl, an equi-osmolar concentration of sorbitol, or ABA, whereas AtNHX5 transcript abundance increases only in response to salt treatment. Hyper-osmotic up-regulation of AtNHX1, 2 or 5 expression is not dependent on the SOS pathway that controls ion homeostasis. However, steady-state AtNHX1, 2 and 5 transcript abundance is greater in sos1, sos2 and sos3 plants growing in medium that is not supplemented with sorbitol or NaCl, providing evidence that transcription of these genes is negatively affected by the SOS pathway in the absence of stress. AtNHX1 and AtNHX2 transcripts accumulate in response to ABA but not to NaCl in the aba2-1, mutant indicating that the osmotic responsiveness of these genes is ABA-dependent. An as yet unde®ned stress signal pathway that is ABA-and SOS-independent apparently controls transcriptional up-regulation of AtNHX5 expression by hyper-saline shock. Similar to AtNHX1, AtNHX2 is localized to the tonoplast of plant cells. Together, these results implicate AtNHX2 and 5, together with AtNHX1, as salt tolerance determinants, and indicate that AtNHX2 has a major function in vacuolar compartmentalization of Na + .

Successful sexual reproduction in flowering plants depends on the accurate timing of flowering, which transits from vegetative stages to reproductive stages. Floral transition is regulated by both endogenous and environmental signals. Photoperiodic flowering is one of the most important factors in controlling floral transition among these various signals and is regulated both by day length and by the endogenous circadian rhythm (Thomas and Vince, 1977). Plants fall into one of three photoperiod-sensing classes: long-day plants (LDP), which promote flowering by sensing long-day (LD) photoperiods, short-day plants (SDP), which promote flowering by sensing short-day (SD) photoperiods, and day-natural plants, which are not regulated by photoperiod. The signaling cascades of photoperiodic flowering have been extensively studied in Arabidopsis thaliana (LDP) (Baurle and Dean, 2006;Imaizumi and Kay, 2006) and rice (SDP) (Izawa, 2007; Tuji et al., 2008). A number of signaling cascade genes have been identified and characterized. In Arabidopsis, GIGANTEA (GI) integrates cellular signals from light sensory transduction and the circadian clock, and activates CONSTANS (CO), which encodes a zinc-finger transcriptional activator (Park et al., 1999;Samach et al., 2000). CO induces FLOWERING LOCUS T (FT), which encodes a mobile flowering signal under LD conditions (Corbesier et al., 2007;Jaeger and Wigge, 2007;Lin et al., 2007;Mathieu et al., 2007). The GI-CO-FT pathway is conserved in rice (OsGI-Hd1-Hd3a) (Yano et al., 2000;Kojima et al., 2002;Hayama et al., 2002). Expression of Hd3a, the rice ortholog of FT, is also induced by Ehd1, a B-type response regulator that functions independently of Hd1 under SD conditions (Doi et al., 2004). OsMADS51, which is regulated by OsGI, functions upstream of Ehd1 (Kim et al., 2007). It was recently reported that RID1/Ehd2/OsId1 is a positive regulator of both SD and LD flowering in rice (Wu et al., 2008;Matsubara et al., 2008;Park et al., 2008). By contrast, under LD conditions, Hd1 suppresses the expression of Hd3a and causes delayed flowering (Hayama et al., 2003). Ghd7 encodes a transcription factor with a CCT motif, which acts as an LD-specific repressor of flowering (Xue et al., 2008). Thus, these studies revealed that rice flowering is regulated both by a 'SD activation pathway' and a 'LD suppression pathway' as an SDP. However, cultivated rice is grown extensively throughout Asia, and at the northern extremes of rice cultivation, including Japan and northern provinces of China and Korea, natural day length during rice cultivation is nearly LD (13-14.5 hours light) (Izawa, 2007), making LD flowering agronomically important in these regions. However, the genetic pathways governing LD flowering in rice are not well understood.FT/Hd3a, which is a common floral inducer in Arabidopsis thaliana (LDP) and rice (SDP), encodes florigen, the mobile flowering signal (Tamaki et al., 2007;Corbesier et al., 2007;Jaeger and Wigge, 2007;Lin et al., 2007;Mathieu et al., 2007), although the regulation ...

Two Arabidopsis thaliana extragenic mutations that suppress NaCl hypersensitivity of the sos3-1 mutant were identified in a screen of a T-DNA insertion population in the genetic background of Col-0 gl1 sos3-1. Analysis of the genome sequence in the region flanking the T-DNA left border indicated that sos3-1 hkt1-1 and sos3-1 hkt1-2 plants have allelic mutations in AtHKT1. AtHKT1 mRNA is more abundant in roots than shoots of wild-type plants but is not detected in plants of either mutant, indicating that this gene is inactivated by the mutations. hkt1-1 and hkt1-2 mutations can suppress to an equivalent extent the Na ؉ sensitivity of sos3-1 seedlings and reduce the intracellular accumulation of this cytotoxic ion. Moreover, sos3-1 hkt1-1 and sos3-1 hkt1-2 seedlings are able to maintain [K ؉ ]int in medium supplemented with NaCl and exhibit a substantially higher intracellular ratio of K ؉ ͞Na ؉ than the sos3-1 mutant. Furthermore, the hkt1 mutations abrogate the growth inhibition of the sos3-1 mutant that is caused by K ؉ deficiency on culture medium with low Ca 2؉ (0.15 mM) and <200 M K ؉ . Interestingly, the capacity of hkt1 mutations to suppress the Na ؉ hypersensitivity of the sos3-1 mutant is reduced substantially when seedlings are grown in medium with low Ca 2؉ (0.15 mM). These results indicate that AtHKT1 is a salt tolerance determinant that controls Na ؉ entry and high affinity K ؉ uptake. The hkt1 mutations have revealed the existence of another Na ؉ influx system(s) whose activity is reduced by high [Ca 2؉ ]ext. H igh [NaCl] ext disturbs intracellular ion homeostasis of plants, which leads to membrane dysfunction, attenuation of metabolic activity, and secondary effects that cause growth inhibition and lead ultimately to cell death (1). Both glycophytes and halophytes use a similar strategy that involves regulation of net Na ϩ flux across the plasma membrane and vacuolar compartmentalization of the internalized cation to mediate intracellular Na ϩ homeostasis. This strategy requires the coordinated function of numerous ion transport determinants and effectively partitions the toxic ion away from critical cytosolic and organellar machinery. Under conditions of high [Na ϩ ] ext , the functioning of these determinants also facilitates the use of Na ϩ as an osmolyte to mediate osmotic adjustment that is necessary for cell expansion (1-3). Because vacuolar expansion is the primary mechanism of plant cell enlargement, this strategy is likely to be an essential adaptation to saline environments.Recently, putative plasma membrane and tonoplast localized Na ϩ ͞H ϩ transporters were identified in plants that are presumed to mediate energized transport of Na ϩ outward from the cytosol to the apoplast or into the vacuole (4-7). These transporters are apparently the molecular effectors of Na ϩ ͞H ϩ antiporter activities associated with plasma membrane and tonoplast vesicles that were described more than a decade ago (1,3,8,9). The plasma membrane Na ϩ ͞H ϩ

Rice is a facultative short-day plant, and molecular genetic studies have identified the major genes involved in short-day flowering. However, the molecular mechanisms promoting the diversity of flowering time in cultivated rice are not known. We used a core collection of 64 rice cultivars that represent the genetic diversity of 332 accessions from around the world and studied the expression levels and polymorphisms of 6 genes in the short-day flowering pathway. The RNA levels of Heading date 3a (Hd3a), encoding a floral activator, are highly correlated with flowering time, and there is a high degree of polymorphism in the Heading date 1 (Hd1) protein, which is a major regulator of Hd3a expression. Functional and nonfunctional alleles of Hd1 are associated with early and late flowering, respectively, suggesting that Hd1 is a major determinant of variation in flowering time of cultivated rice. We also found that the type of Hd3a promoter and the level of Ehd1 expression contribute to the diversity in flowering time and Hd3a expression level. We evaluated the contributions of these 3 factors by a statistical analysis using a simple linear model, and the results supported our experimental observations.) has evolved during the last 8,000 to 10,000 years of domestication and breeding (1, 2). A major reason for the spread of rice cultivation to a wide range of geographical regions, and for the increases in yield, is the diversification of flowering time (1). In general, rice is known as a short-day plant that induces transition from the vegetative phase to the reproductive phase when it senses a decrease in day length. The molecular genetic pathway for short-day flowering in cultivated rice (Fig. 1A) is relatively well characterized (3-5). Signals from light and circadian clocks are received by OsGI, the rice orthologue of Arabidopsis GIGANTEA, and it regulates expression of Heading date 1 (Hd1) and OsMADS51 (6-8). Hd1 and its Arabidopsis orthologue CONSTANS encode zinc-finger type transcriptional activators with the CO, CO-like, and TOC1 (CCT) domains (9). Hd1 regulates Heading date 3a (Hd3a) expression (7, 9, 10). Hd3a is a rice orthologue of Arabidopsis FLOWERING LOCUS T (FT), and these genes recently were shown to encode a mobile flowering signal (11-16). RICE FLOWERING LOCUS T1 (RFT1) belongs to the rice FT-like gene family and functions as a floral activator, acting redundantly with Hd3a (17, 18). OsMADS51 encodes a type I MADSbox gene and functions upstream of Early heading date 1 (Ehd1) (8). Ehd1 encodes a B-type response regulator and acts as an activator of Hd3a independently from Hd1 (19). No clear orthologues of Ehd1 or OsMADS51 are found in the Arabidopsis genome. Although the genetic pathway for short-day flowering in rice is relatively well understood, the molecular mechanisms generating the diversity of flowering time in cultivated rice are not known. In this study, we analyzed the expression and nucleotide sequences of genes involved in short-day flowering in rice. Our study revealed that allelic vari...