Genetic interactions reveal the antagonistic roles of FT/TSF and TFL1 in the determination of inflorescence meristem identity in Arabidopsis

During the transition to the reproductive phase, the shoot apical meristem switches from the developmental program that generates vegetative organs to instead produce flowers. In this study, we examined the genetic interactions of FLOWERING LOCUS T (FT)/TWIN SISTER OF FT (TSF) and TERMINAL FLOWER 1 (TFL1) in the determination of inflorescence meristem identity in Arabidopsis thaliana. The ft‐10 tsf‐1 mutants produced a compact inflorescence surrounded by serrated leaves (hyper‐vegetative shoot) at the early bolting stage, as did plants overexpressing TFL1. Plants overexpressing FT or TSF (or both FT and TFL1) generated a terminal flower, as did tfl1‐20 mutants. The terminal flower formed in tfl1‐20 mutants converted to a hyper‐vegetative shoot in ft‐10 tsf‐1 mutants. Grafting ft‐10 tsf‐1 or ft‐10 tsf‐1 tfl1‐20 mutant scions to 35S::FT rootstock plants produced a normal inflorescence and a terminal flower in the scion plants, respectively, although both scions showed similar early flowering. Misexpression of FT in the vasculature and in the shoot apex in wild‐type plants generated a normal inflorescence and a terminal flower, respectively. By contrast, in ft‐10 tsf‐1 mutants the vasculature‐specific misexpression of FT converted the hyper‐vegetative shoot to a normal inflorescence, and in the ft‐10 tsf‐1 tfl1‐20 mutants converted the shoot to a terminal flower. TFL1 levels did not affect the inflorescence morphology caused by FT/TSF overexpression at the early bolting stage. Taking these results together, we proposed that FT/TSF and TFL1 play antagonistic roles in the determination of inflorescence meristem identity, and that FT/TSF are more important than TFL1 in this process.     

<Emphasis Type="Italic">OsREL2</Emphasis>, a rice TOPLESS homolog functions in axillary meristem development in rice inflorescence

The morphology of the rice inflorescence, called the panicle, is determined mainly by the activities of axillary meristems including primary, secondary, and spikelet meristems. Recently, in maize, the RAMOSA1 ENHANCER LOCUS2 (REL2) gene, orthologous to the Arabidopsis shoot apical meristem fate-determining TOPLESS, was shown to be involved in the regulation of axillary meristem determinacy. In order to investigate the function of the rice REL2 homolog, we identified and characterized the rice REL2 gene (OsREL2). Compared to other rice TPL homologs, OsREL2 gene expression stayed relatively low throughout panicle development. We characterized a T-DNA insertion osrel2 mutant that showed pleiotropic phenotypic defects, such as defects in panicle heading, sterile lemma elongation, and panicle development, suggesting the OsREL2 functions in multiple developmental processes. In particular, osrel2 developed shorter axillary branches and reduced numbers of lateral organs on axillary branches in comparison to the wild-type, indicating that OsREL2 is important in axillary meristem maintenance. Interestingly, osrel2 produced more primary branches and fewer secondary branches than the wild-type. These results suggest that OsREL2 is involved in branch formation regulation, presumably by suppressing primary branch formation and promoting secondary branch formation.     

Cloning and Functional Characterization of a Vacuolar Na+/H+ Antiporter Gene from Mungbean (VrNHX1) and Its Ectopic Expression Enhanced Salt Tolerance in Arabidopsis thaliana

Plant vacuolar NHX exchangers play a significant role in adaption to salt stress by compartmentalizing excess cytosolic Na⁺ into vacuoles and maintaining cellular homeostasis and ionic equilibrium. We cloned an orthologue of the vacuolar Na⁺/H⁺ antiporter gene, VrNHX1 from mungbean (Vigna radiata), an important Asiatic grain legume. The VrNHX1 (Genbank Accession number {"type":"entrez-nucleotide","attrs":{"text":"JN656211.1","term_id":"347327345"}}JN656211.1) contains 2095 nucleotides with an open reading frame of 1629 nucleotides encoding a predicted protein of 542 amino acids with a deduced molecular mass of 59.6 kDa. The consensus amiloride binding motif (⁸⁴LFFIYLLPPI⁹³) was observed in the third putative transmembrane domain of VrNHX1. Bioinformatic and phylogenetic analysis clearly suggested that VrNHX1 had high similarity to those of orthologs belonging to Class-I clade of plant NHX exchangers in leguminous crops. VrNHX1 could be strongly induced by salt stress in mungbean as the expression in roots significantly increased in presence of 200 mM NaCl with concomitant accumulation of total [Na⁺]. Induction of VrNHX1 was also observed under cold and dehydration stress, indicating a possible cross talk between various abiotic stresses. Heterologous expression in salt sensitive yeast mutant AXT3 complemented for the loss of yeast vacuolar NHX1 under NaCl, KCl and LiCl stress indicating that VrNHX1 was the orthologue of ScNHX1. Further, AXT3 cells expressing VrNHX1 survived under low pH environment and displayed vacuolar alkalinization analyzed using pH sensitive fluorescent dye BCECF-AM. The constitutive and stress inducible expression of VrNHX1 resulted in enhanced salt tolerance in transgenic Arabidopsis thaliana lines. Our work suggested that VrNHX1 was a salt tolerance determinant in mungbean.