Andrea J. Massiah

Florigenic and Antiflorigenic Signaling in Plants

The evidence that FLOWERING LOCUS T (FT) protein, and its paralog TWIN SISTER OF FT, act as the long-distance floral stimulus, or at least that they are part of it in diverse plant species, has attracted much attention in recent years. Studies to understand the physiological and molecular apparatuses that integrate spatial and temporal signals to regulate developmental transitions in plants have occupied countless scientists and have resulted in an unmanageably large amount of research data. Analysis of these data has helped to identify multiple systemic florigenic and antiflorigenic regulators. This study gives an overview of the recent research on gene products, phytohormones and other metabolites that have been demonstrated to have florigenic or antiflorigenic functions in plants.

Wheat ribosome-inactivating proteins : seed and leaf forms with different specificities and cofactor requirements

Distinct forms of ribosome-inactivating proteins were purified from wheat (Triticum aestivum L.) germ and leaves and termed tritin-S and tritin-L, respectively. These differ in size and charge and are antigenically unrelated. They are both RNA N-glycosidases which act on 26S rRNA in native yeast (Saccharomyces cerevisiae) ribosomes by the removal of A3024 located in a universally conserved sequence in domain VII which has previously been identified as the site of action of ricin A-chain. Tritin-S and tritin-L differ in both their ribosome substrate specificities and cofactor requirements. Tritin-S shows only barely detectable activity on ribosomes from the endosperm, its tissue of synthesis, whereas tritin-L is highly active on leaf ribosomes. Additionally, tritin-S is inactive on wheat germ, tobacco leaf and Escherichia coli ribosomes but active on rabbit reticulocyte and yeast ribosomes. Tritin-L is active on ribosomes from all of the above sources. Tritin-S, unlike tritin-L shows a marked requirement for ATP in its action.

Conservation of Arabidopsis thaliana Photoperiodic Flowering Time Genes in Onion ( Allium cepa L.)

The genetics underlying onion development are poorly understood. Here the characterization of onion homologs of Arabidopsis photoperiodic flowering pathway genes is reported with the end goal of accelerating onion breeding programs by understanding the genetic basis of adaptation to different latitudes. The expression of onion GI, FKF1 and ZTL homologs under short day (SD) and long day (LD) conditions was examined using quantitative reverse transcription-PCR (qRT-PCR). The expression of AcGI and AcFKF1 was examined in onion varieties which exhibit different daylength responses. Phylogenetic trees were constructed to confirm the identity of the homologs. AcGI and AcFKF1 showed diurnal expression patterns similar to their Arabidopsis counterparts, while AcZTL was found to be constitutively expressed. AcGI showed similar expression patterns in varieties which exhibit different daylength responses, whereas AcFKF1 showed differences. It is proposed that these differences could contribute to the different daylength responses in these varieties. Phylogenetic analyses showed that all the genes isolated are very closely related to their proposed homologs. The results presented here show that key genes controlling photoperiodic flowering in Arabidopsis are conserved in onion, and a role for these genes in the photoperiodic control of bulb initiation is predicted. This theory is supported by expression and phylogenetic data.

Starch metabolism and antiflorigenic signals modulate the juvenile-to-adult phase transition in Arabidopsis

The physiology and genetics underlying juvenility is poorly understood. Here, we exploit Arabidopsis as a system to understand the mechanisms that regulate floral incompetence during juvenility. Using an experimental assay that allows the length of juvenility to be estimated and mutants impaired in different pathways, we show that multiple inputs influence juvenility. Juvenile phase lengths of wild type (WT) accessions Col-0, Ler-0 and Ws-4 are shown to differ, with Col-0 having the shortest and Ws-4 the longest length. Plants defective in sugar signalling [gin1-1, gin2-1, gin6 (abi4)] and floral repressor mutants [hst1, tfl1, tfl2 (lhp1)] showed shortened juvenile phase lengths compared to their respective WTs. Mutants defective in starch anabolism (adg1-1, pgm1) and catabolism (sex1, sex4, bam3) showed prolonged juvenile phase lengths compared to Col-0. Examination of diurnal metabolite changes in adg1-1 and sex1 mutants indicates that their altered juvenile phase length may be due to lack of starch turnover, which influences carbohydrate availability. In this article, we propose a model in which a variety of signals including floral activators and repressors modulate the juvenile-to-adult phase transition. The role of carbohydrates may be in their capacity as nutrients, osmotic regulators, signalling molecules and/ or through their interaction with phytohormonal networks.

TEMPRANILLO is a regulator of juvenility in plants

Many plants are incapable of flowering in inductive daylengths during the early juvenile vegetative phase (JVP). Arabidopsis mutants with reduced expression of TEMPRANILLO (TEM), a repressor of FLOWERING LOCUS T (FT) had a shorter JVP than wild-type plants. Reciprocal changes in mRNA expression of TEM and FT were observed in both Arabidopsis and antirrhinum, which correlated with the length of the JVP. FT expression was induced just prior to the end of the JVP and levels of TEM1 mRNA declined rapidly at the time when FT mRNA levels were shown to increase. TEM orthologs were isolated from antirrhinum (AmTEM) and olive (OeTEM) and were expressed most highly during their juvenile phase. AmTEM functionally complemented AtTEM1 in the tem1 mutant and over-expression of AmTEM prolonged the JVP through repression of FT and CONSTANS (CO). We propose that TEM may have a general role in regulating JVP in herbaceous and woody species.

FLC expression is down-regulated by cold treatment in Diplotaxis tenuifolia (wild rocket), but flowering time is unaffected

Wild rocket (Diplotaxis tenuifolia) has become a very popular salad leaf due to its peppery taste. It is part of the Brassicaceae family and thus has a high level of homology at the DNA level to other Brassica species including Arabidopsis thaliana. The vernalization and photoperiodic requirements of wild rocket have not been reported to date. Photoperiodic experiments described here demonstrate that rocket is a facultative long day plant. To investigate the vernalization requirement, both seed and young plants were given vernalization treatments at 4 °C for different lengths of time. A rocket homologue of FLOWERING LOCUS C (DtFLC) was isolated and shown to functionally complement the Arabidopsis FRI+flc3 null mutant. Whilst the expression of DtFLC was significantly reduced after just one week of cold treatment, cold treatments of two to eight weeks had no significant effect on bolting time of wild rocket indicating that rocket does not have a vernalization requirement. These findings illustrate that important fundamental differences can exist between model and crop plant species, such as in this case where down-regulation of DtFLC expression does not enable earlier flowering in wild rocket as it does in Arabidopsis and many other Brassica species.