A two-staged model of Na+ exclusion in rice explained by 3D modeling of HKT transporters and alternative splicing.

The HKT family of Na(+) and Na(+)/K(+) transporters is implicated in plant salinity tolerance. Amongst these transporters, the cereal HKT1;4 and HKT1;5 are responsible for Na(+) exclusion from photosynthetic tissues, a key mechanism for plant salinity tolerance. It has been suggested that Na(+) is retrieved from the xylem transpiration stream either in the root or the leaf sheath, protecting the leaf blades from excessive Na(+) accumulation. However, direct evidence for this scenario is scarce. Comparative modeling and evaluation of rice (Oryza sativa) HKT-transporters based on the recent crystal structure of the bacterial TrkH K(+) transporter allowed to reconcile transcriptomic and physiological data. For OsHKT1;5, both transcript abundance and protein structural features within the selectivity filter could control shoot Na(+) accumulation in a range of rice varieties. For OsHKT1;4, alternative splicing of transcript and the anatomical complexity of the sheath needed to be taken into account. Thus, Na(+) accumulation in a specific leaf blade seems to be regulated by abundance of a correctly spliced OsHKT1;4 transcript in a corresponding sheath. Overall, allelic variation of leaf blade Na(+) accumulation can be explained by a complex interplay of gene transcription, alternative splicing and protein structure.

Root-specific transcript profiling of contrasting rice genotypes in response to salinity stress.

Elevated salinity imposes osmotic and ion toxicity stresses on living cells and requires a multitude of responses in order to enable plant survival. Building on earlier work profiling transcript levels in rice (Oryza sativa) shoots of FL478, a salt-tolerant indica recombinant inbred line, and IR29, a salt-sensitive cultivar, transcript levels were compared in roots of these two accessions as well as in the roots of two additional salt-tolerant indica genotypes, the landrace Pokkali and the recombinant inbred line IR63731. The aim of this study was to compare transcripts in the sensitive and the tolerant lines in order to identify genes likely to be involved in plant salinity tolerance, rather than in responses to salinity per se. Transcript profiles of several gene families with known links to salinity tolerance are described (e.g. HKTs, NHXs). The putative function of a set of genes identified through their salt responsiveness, transcript levels, and/or chromosomal location (i.e. underneath QTLs for salinity tolerance) is also discussed. Finally, the parental origin of the Saltol region in FL478 is further investigated. Overall, the dataset presented appears to be robust and it seems likely that this system could provide a reliable strategy for the discovery of novel genes involved in salinity tolerance.

Enhancing gene targeting efficiency in higher plants: rice is on the move.

Meeting the challenge of routine gene targeting (GT) in higher plants is of crucial interest to researchers and plant breeders who are currently in need of a powerful tool to specifically modify a given locus in a genome. Higher plants have long been considered the last lineage resistant to targeting technology. However, a recent report described an efficient method of T-DNA-mediated targeted disruption of a non-selectable locus in rice [Terada et al., Nat Biotechnol 20: 1030-1034 (2002)]. Though this study was an obvious breakthrough, further improvement of GT frequencies may derive from a better understanding of the natural mechanisms that control homologous recombination (HR) processes. In this review, we will focus on what is known about HR and the factors which may hamper the development of routine GT by HR in higher plants. We will also present the current strategies envisaged to overcome these limitations, such as expression of recombination proteins and refinements in the design of the transformation vector.