SummaryConditions to promote dark morphogenesis and¯ower-ing in Arabidopsis have previously been limited to liquid cultures and to a few laboratory ecotypes. We have obtained development and¯owering of Arabidopsis plants under complete darkness by growing them on vertical Petri dishes containing solid agar medium with sucrose. Under these conditions, all the ecotypes tested were able to develop, giving rise to etiolated plants that¯owered after producing a certain number of leaves. Dark-grown plants showed similarities with phytochrome-de®cient mutants and were different from de-etiolated or constitutive photomorphogenesis mutants such as det and cop. Late-and early-¯owering ecotypes, showing large differences in¯owering time and leaf number under long days,¯owered with a similar number of leaves when grown in the dark. Rapid dark¯owering of late-¯owering ecotypes was not an effect of darkness but the result of the interaction between dark and sucrose availability at the aerial part of the plant, since sucrose also had an effect when plants were grown in the light. Gibberellin-de®cient and insensitive mutants were delayed in the initiation of owers in the dark, indicating a role for these hormones in¯owering promotion in the dark. The late-¯owering phenotype of mutants at different loci of the autonomous and long-day-dependent¯owering induction pathways was rescued in dark growth conditions. However, the late-¯owering phenotype of ft and fwa mutants was not rescued by sucrose either in the dark or in the light, suggesting a different role for these genes in¯owering induction.
Genes involved in the mechanisms of plant responses to salt stress may be used as biotechnological tools for the genetic improvement of salt tolerance in crop plants. This would help alleviate the increasing problem of salinization of lands cultivated under irrigation in arid and semi-arid regions. We have isolated a novel halotolerance gene from Arabidopsis thaliana , A. thaliana Li-tolerant lipase 1 ( AtLTL1 ), on the basis of the phenotype of tolerance to LiCl conferred by its expression in yeast. AtLTL1 encodes a putative lipase of the GDSL-motif family, which includes bacterial and a very large number of plant proteins. In Arabidopsis , AtLTL1 expression is rapidly induced by LiCl or NaCl, but not by other abiotic stresses. Overexpression of AtLTL1 increases salt tolerance in transgenic Arabidopsis plants, compared to non-transformed controls, allowing germination of seeds in the presence of toxic concentrations of LiCl and NaCl, and stimulating vegetative growth, flowering and seed set in the presence of NaCl. These results clearly point to a role of AtLTL1 in the mechanisms of salt tolerance. In addition, we show that AtLTL1 expression is also activated, although only transiently, by salicylic acid (SA), suggesting that the lipase could also be involved in defence reactions against pathogens.
A new chloroplastic Cu/Zn-superoxide dismutase (SOD) isoenzyme was identified in Arabidopsis thaliana ecotype Cvi. Genetic analyses indicated that the new isoenzyme was encoded by a Cvi-specific allele of Csd2 that was named Csd2-2. Paraquat treatments of A. thaliana ecotypes Ler and Cvi resulted in higher levels of chloroplastic Cu/Zn-SOD activity in Cvi, suggesting that the Cvi isoenzyme has a higher stability and/or turnover rate than the Ler variant under photo-oxidative conditions. In addition, Cvi showed a higher tolerance to paraquat treatments. Hybrid plant populations expressing Csd2-2 also exhibited an increased tolerance, suggesting that the Cvi isoenzyme is one of the factors that contribute to a better fitness in photo-oxidative stress conditions.
SummaryThe development of wild-type Arabidopsis thaliana (L.) Heyhn and two lete-flowering fve mutants has been analysed under different environmental conditions. In wildtype plants, short-day photoperiods delay the floral transition as a consequence of lengthening all the developmental phases of the plant. Moreover, short days also alter the inflorescence structure by reducing the internode elongation and delaying the establishment of the floral developmental programme in the lateral meristems of the inflorescence and co-florescences. Mutations at the FVE locus cause a delay in flowering time, and a change in the inflorescence structure, similar to the effect of short photoperiods on wild-type plants. However, the effect of the fve mutations is additive to the effect of short days, and all the aspects of the Fve phenotype are corrected by vernalization. These results seem to indicate that FVE is not simply involved in timing the transition from vegetative to reproductive growth, but that it could play a role during all stages of plant development.
SummarySearching for novel targets of salt toxicity in eukaryotic cells, we have screened an Arabidopsis thaliana cDNA library to isolate genes conferring increased tolerance to salt stress when expressed in the yeast Saccharomyces cerevisiae. Here we show that expression of the`alternating arginine-rich' (or RS) domains of two different SR-like, putative splicing proteins from Arabidopsis allows yeast cells to tolerate higher lithium and sodium concentrations. Protection against salt stress appears to require the in vivo phosphorylation of these plant polypeptides, since the yeast SR protein kinase Sky1p, which was able to phosphorylate in vitro at least one of them, also proved to be essential for the observed salt tolerance phenotype. In addition, a clone encoding the U1A protein, a previously characterised Arabidopsis splicing factor, was also isolated in the screening. No signi®cant decrease in the intracellular concentration of lithium was observed in yeast cells incubated in the presence of LiCl upon expression of any of the Arabidopsis proteins, suggesting that their effects are not mediated by the stimulation of ion transport. In support of the general signi®cance of these data, we also show that the expression of the RS domain of one of the SR-like proteins in transgenic Arabidopsis plants increases their tolerance to LiCl and NaCl. These results point to an important role of pre-mRNA splicing and SR-like proteins in the salt tolerance of eukaryotic cells, offering a novel route to improve this important trait in crop plants.
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