No abstract

Phosphate (P) is taken up by plants through high-affinity P transporter proteins embedded in the plasma membrane of certain cell types in plant roots. Expression of the genes that encode these transporters responds to the P status of the plants, and their transcription is normally tightly controlled. However, this tight control of P uptake is lost under Zn deficiency, leading to very high accumulation of P in plants. We examined the effect of plant Zn status on the expression of the genes encoding the HVPT1 and HVPT2 high-affinity P transporters in barley (Hordeum vulgare L. cv Weeah) roots. The results show that the expression of these genes is intimately linked to the Zn status of the plants. Zn deficiency induced the expression of genes encoding these P transporters in plants grown in either P-sufficient or -deficient conditions. Moreover, the role of Zn in the regulation of these genes is specific in that it cannot be replaced by manganese (a divalent cation similar to Zn). It appears that Zn plays a specific role in the signal transduction pathway responsible for the regulation of genes encoding high-affinity P transporters in plant roots. The significance of Zn involvement in the regulation of genes involved in P uptake is discussed.Although relatively large amounts of phosphate (P) are essential to plant growth, forms that can be taken up directly by plants are only found in low concentrations (0.01-3.0 m) in most soil solutions (Barber, 1995). Low availability of P in soils limits crop yields. Under normal growing conditions the uptake of P by plants is tightly controlled. Plants normally moderate their capacity to take up P to maintain the P concentration in their tissues within physiological limits (Mimura, 1999). They therefore restrict their capacity to take up P when grown under high-P conditions but enhance this capacity when grown under low-P conditions (Clarkson and Scattergood, 1982; Jungk et al., 1990). However, under Zn-deficient conditions, high levels of P accumulate in the tissues of both dicotyledon and monocotyledon plant species (Welch et al., 1982;Cakmak and Marschner, 1986;Webb and Loneragan, 1988;Welch and Norvell, 1993) and can reach levels that are toxic to the plants if high concentrations of P are supplied (Loneragan et al., 1982; Welch et al., 1982;Cakmak and Marschner, 1986;Norvell and Welch, 1993). This suggests that Zn-deficient plants somehow lose control over the P absorption mechanism (Safaya and Gupta, 1979;Marschner and Cakmak, 1986).The kinetic characterization of the P uptake system by whole plants indicates a high-affinity transporter activity operating at the micromolar range . The high-affinity transporters are a key component of P uptake by plants at the very low concentrations of P found in many soil solutions. When the P concentration is low in the root growth medium, expression of genes encoding high-affinity P transporters is up-regulated in plant roots (Muchhal et al., 1996;Smith et al., 1997Smith et al., , 1999 Leggewie et al., 1997; Liu et al., 1998;Smi...

We report the first gene-based linkage map of Lupinus angustifolius (narrow-leafed lupin) and its comparison to the partially sequenced genome of Medicago truncatula. The map comprises 382 loci in 20 major linkage groups, two triplets, three pairs and 11 unlinked loci and is 1,846 cM in length. The map was generated from the segregation of 163 RFLP markers, 135 gene-based PCR markers, 75 AFLP and 4 AFLP-derived SCAR markers in a mapping population of 93 recombinant inbred lines, derived from a cross between domesticated and wild-type parents. This enabled the mapping of five major genes controlling key domestication traits in L. angustifolius. Using marker sequence data, the L. angustifolius genetic map was compared to the partially completed M. truncatula genome sequence. We found evidence of conserved synteny in some regions of the genome despite the wide evolutionary distance between these legume species. We also found new evidence of widespread duplication within the L. angustifolius genome.

SummaryThis paper reports the successful isolation and preliminary characterisation of a mutant of Lycopersicon esculentum Mill. with highly reduced vesicular-arbuscular (VA) mycorrhizal colonization. The mutation is recessive and has been designated rmc. Colonization by G. mosseae is characterised by poor development of external mycelium and a few abnormal appressoria. Vesicles were never formed by this fungus in association with the mutant. Gi. margarita formed large amounts of external mycelium, complex branched structures and occasional auxiliary cells. Small amounts of internal colonization also occurred. Laser scanning confocal microscopy (LSCM) gave a clear picture of the differences in development of G. intraradices and Gi. margarita in mutant and wild-type roots and confirmed that the fungus is restricted to the root surface of the mutants. The amenability of tomato for molecular genetic characterisation should enable us to map and clone the mutated gene, and thus identify one of the biochemical bases for inability to establish a normal mycorrhizal symbiosis. The mutant represents a key advance in molecular research on VA mycorrhizal symbiosis.

We investigated the chromosomal organization and developmental regulation of soybean 8-conglycinin genes. The 8-conglycinin gene family contains at least 15 members divided into two major groups encoding 2.5-kilobase and 1.7-kilobase embryo mRNAs. 8-Conglycinin genes are clustered in severa1 DNA regions and are highly homologous along their entire lengths. The two groups differ by the presence or absence of specific DNA segments. These DNA segments account for the size differences in 8-conglycinin mRNAs. The 2.5-kilobase and 1.7-kilobase 8-conglycinin mRNAs accumulate and decay at different times during embryogenesis. By contrast, genes encoding these mRNAs are transcriptionally activated and repressed at the same time periods. Our studies indicate that the 8-conglycinin family evolved by both duplication and insertion/deletion events, and that 8-conglycinin gene expression is regulated at both the transcriptional and posttranscriptional levels.