Genetic analysis of potassium use efficiency in Brassica oleracea.

BACKGROUND AND AIMS: Potassium (K) fertilizers are used in intensive and extensive agricultural systems to maximize production. However, there are both financial and environmental costs to K-fertilization. It is therefore important to optimize the efficiency with which K-fertilizers are used. Cultivating crops that acquire and/or utilize K more effectively can reduce the use of K-fertilizers. The aim of the present study was to determine the genetic factors affecting K utilization efficiency (KUtE), defined as the reciprocal of shoot K concentration (1/[K](shoot)), and K acquisition efficiency (KUpE), defined as shoot K content, in Brassica oleracea. METHODS: Genetic variation in [K](shoot) was estimated using a structured diversity foundation set (DFS) of 376 accessions and in 74 commercial genotypes grown in glasshouse and field experiments that included phosphorus (P) supply as a treatment factor. Chromosomal quantitative trait loci (QTL) associated with [K](shoot) and KUpE were identified using a genetic mapping population grown in the glasshouse and field. Putative QTL were tested using recurrent backcross substitution lines in the glasshouse. KEY RESULTS: More than two-fold variation in [K](shoot) was observed among DFS accessions grown in the glasshouse, a significant proportion of which could be attributed to genetic factors. Several QTL associated with [K](shoot) were identified, which, despite a significant correlation in [K](shoot) among genotypes grown in the glasshouse and field, differed between these two environments. A QTL associated with [K](shoot) in glasshouse-grown plants (chromosome C7 at 62.2 cM) was confirmed using substitution lines. This QTL corresponds to a segment of arabidopsis chromosome 4 containing genes encoding the K+ transporters AtKUP9, AtAKT2, AtKAT2 and AtTPK3. CONCLUSIONS: There is sufficient genetic variation in B. oleracea to breed for both KUtE and KUpE. However, as QTL associated with these traits differ between glasshouse and field environments, marker-assisted breeding programmes must consider carefully the conditions under which the crop will be grown.

Evidence of neutral transcriptome evolution in plants.

* The transcriptome of an organism is its set of gene transcripts (mRNAs) at a defined spatial and temporal locus. Because gene expression is affected markedly by environmental and developmental perturbations, it is widely assumed that transcriptome divergence among taxa represents adaptive phenotypic selection. This assumption has been challenged by neutral theories which propose that stochastic processes drive transcriptome evolution. * To test for evidence of neutral transcriptome evolution in plants, we quantified 18 494 gene transcripts in nonsenescent leaves of 14 taxa of Brassicaceae using robust cross-species transcriptomics which includes a two-step physical and in silico-based normalization procedure based on DNA similarity among taxa. * Transcriptome divergence correlates positively with evolutionary distance between taxa and with variation in gene expression among samples. Results are similar for pseudogenes and chloroplast genes evolving at different rates. Remarkably, variation in transcript abundance among root-cell samples correlates positively with transcriptome divergence among root tissues and among taxa. * Because neutral processes affect transcriptome evolution in plants, many differences in gene expression among or within taxa may be nonfunctional, reflecting ancestral plasticity and founder effects. Appropriate null models are required when comparing transcriptomes in space and time.

Interactions between selenium and sulphur nutrition in Arabidopsis thaliana.

Selenium (Se) is an essential plant micronutrient, but is toxic at high tissue concentrations. It is chemically similar to sulphur (S), an essential plant macronutrient. The interactions between Se and S nutrition were investigated in the model plant Arabidopsis thaliana (L.) Heynh. Arabidopsis plants were grown on agar containing a complete mineral complement and various concentrations of selenate and sulphate. The Se/S concentration ratio in the shoot ([Se](shoot)/[S](shoot)) showed a complex dependence on the ratio of selenate to sulphate concentration in the agar ([Se](agar)/[S](agar)). Increasing [S](agar) increased shoot fresh weight (FW) and [S](shoot), but decreased [Se](shoot). Increasing [Se](agar) increased both [Se](shoot) and [S](shoot), but reduced shoot FW. The reduction in shoot FW in the presence of Se was linearly related to the shoot Se/S concentration ratio. These data suggest (i) that Se and S enter Arabidopsis through multiple transport pathways with contrasting sulphate/selenate selectivities, whose activities vary between plants of contrasting nutritional status, (ii) that rhizosphere sulphate inhibits selenate uptake, (iii) that rhizosphere selenate promotes sulphate uptake, possibly by preventing the reduction in the abundance and/or activity of sulphate transporters by sulphate and/or its metabolites, and (iv) that Se toxicity occurs because Se and S compete for a biochemical process, such as assimilation into amino acids of essential proteins.

Influx and accumulation of Cs(+) by the akt1 mutant of Arabidopsis thaliana (L.) Heynh. lacking a dominant K(+) transport system.

An extensive literature reports that Cs(+), an environmental contaminant, enters plant cells through K(+) transport systems. Several recently identified plant K(+) transport systems are permeable to Cs(+). Permeation models indicate that most Cs(+) uptake into plant roots under typical soil ionic conditions will be mediated by voltage-insensitive cation (VIC) channels in the plasma membrane and not by the inward rectifying K(+) (KIR) channels implicated in plant K nutrition. Cation fluxes through KIR channels are blocked by Cs(+). This paper tests directly the hypothesis that the dominant KIR channel in plant roots (AKT1) does not contribute significantly to Cs(+) uptake by comparing Cs(+) uptake into wild-type and the akt1 knockout mutant of Arabidopsis thaliana (L.) Heynh. Wild-type and akt1 plants were grown to comparable size and K(+) content on agar containing 10 mM K(+). Both Cs(+) influx to roots of intact plants and Cs(+) accumulation in roots and shoots were identical in wild-type and akt1 plants. These data indicate that AKT1 is unlikely to contribute significantly to Cs(+) uptake by wild-type Arabidopsis from 'single-salt' solutions. The influx of Cs(+) to roots of intact wild-type and akt1 plants was inhibited by 1 mM Ba(2+), Ca(2+) and La(3+), but not by 10 microM Br-cAMP. This pharmacology resembles that of VIC channels and is consistent with the hypothesis that VIC channels mediate most Cs(+) influx under 'single-salt' conditions.

Cell marking in Arabidopsis thaliana and its application to patch-clamp studies.

Ion transport processes at the plasma membrane of plant cells are frequently studied by applying membrane-patch voltage-clamp (patch-clamp) electrophysiological techniques to isolated protoplasts. As plants are composed of many tissues and cell types, and each tissue and cell type may be specialized to a particular function and possess a unique complement of transport proteins, it is important to certify the anatomical origin of the protoplasts used for patch-clamp studies. This paper describes a general molecular genetic approach to marking specific cell types for subsequent patch-clamp studies and presents a specific example: a comparison of the K+ currents in protoplasts from cortical and stelar cells of Arabidopsis roots. Transgenic Arabidopsis were generated in which the expression of green fluorescent protein (GFP) from Aequoria victoria was driven by the CaMV 35S promoter (line mGFP3). In roots of the transgenic mGFP3 line, visible fluorescence was restricted to the stele. Protoplasts were generated from roots of the mGFP3 line and K+ currents in non-fluorescent (cortical/epidermal) and fluorescent (stelar) protoplasts were assayed using patch-clamp techniques. It was found that both the frequency of observing inward rectifying K+ channel (IRC) activity and the relative occurrence of IRC compared to outward rectifying K+ channels were significantly lower in protoplasts from cortical/epidermal cells compared to cells of the stele. The presence of GFP did not affect the occurrence or biophysical properties of K+ channels. It is concluded that the generation of transgenic Arabidopsis expressing GFP in a cell-specific fashion is a convenient and reliable way to mark protoplasts derived from contrasting cell types for subsequent patch-clamp studies.