We investigated the uptake and distribution of AI in root apices of near-isogenic wheat (Triticum aesfivum L.) lines differing in AI tolerance at a single locus (Altl: aluminum tolerance). Seedlings were grown in nutrient solution that contained 100 p~ AI, and the roots were subsequently stained with hematoxylin, a compound that binds AI in vitro to form a colored complex. Root apices of Alsensitive genotypes stained after short exposures to AI (10 min and 1 h), whereas apices of AI-tolerant seedlings showed less intense staining after equivalent exposures. Differential staining preceded differences observed in either root elongation or total AI concentrations of root apices (terminal 2-3 mm of root). After 4 h of exposure to 100 p~ AI in nutrient solution, AI-sensitive genotypes accumulated more total AI in root apices than AI-tolerant genotypes, and the differences became more marked with time. Analysis of freeze-dried root apices by x-ray microanalysis showed that AI entered root apices of AI-sensitive plants and accumulated in the epidermal layer and in the cortical layer immediately below the epidermis. Long-term exposure of sensitive apices to AI (24 h) resulted in a distribution of AI coinciding with the absence of K. Quantitation of AI in the cortical layer showed that sensitive apices accumulated 5-to 10-fold more AI than tolerant apices exposed to AI solutions for equivalent times. These data are consistent with the hypothesis that Altl encodes a mechanism that excludes AI from root apices.A1 toxicity is one of the major factors that limit plant growth in many acid soils (Wright, 1989). The primary effect of A1 is to inhibit root growth in Al-sensitive genotypes with subsequent effects on nutrient and water uptake (Foy, 1983). Root elongation is affected within hours of A1 exposure (Wallace et al., 1982), and, as in many plant species, tlie primary site of A1 toxicity in wheat (Triticum aestivum L.) appears to be the root apex (Bennet and Breen, 1991). have shown that in wheat and maize, root elongation is inhibited only when apices are exposed to Al, whereas selectively exposing the remainder of the root does not inhibit elongation. Hematoxylin, a stain for Al, stains root apices of Al-sensitive wheat genotypes more intensely than root apices of Al-tolerant genotypes, but the remainder of the root shows the same degree of staining in different genotypes (Polle et al., 1978;Wallace et al., 1982), indicating that tolerance might be a property of the root apex.Differential uptake of A1 into roots could account for differences in tolerance between genotypes, but conflicting results have been reported regarding differences in A1 uptake in roots of different wheat genotypes. Some of these conflicting results appear to be due to the size of the root portion analyzed and the time of exposure to Al. Recently RincÓn and Gonzales (1992) showed that an Al-sensitive wheat cultivar accumulated more A1 in its root apices (2 mm terminus of root) than an Al-tolerant cultivar, which is consistent with the above discus...

Natural products (NPs) from microorganisms have been important sources for discovering new therapeutic and chemical entities. While their corresponding biosynthetic gene clusters (BGCs) can be easily identified by gene‐sequence‐similarity‐based bioinformatics strategies, the actual access to these NPs for structure elucidation and bioactivity testing remains difficult. Deletion of the gene encoding the RNA chaperone, Hfq, results in strains losing the production of most NPs. By exchanging the native promoter of a desired BGC against an inducible promoter in Δhfq mutants, almost exclusive production of the corresponding NP from the targeted BGC in Photorhabdus, Xenorhabdus and Pseudomonas was observed including the production of several new NPs derived from previously uncharacterized non‐ribosomal peptide synthetases (NRPS). This easyPACId approach (easy Promoter Activated Compound Identification) facilitates NP identification due to low interference from other NPs. Moreover, it allows direct bioactivity testing of supernatants containing secreted NPs, without laborious purification.

The physiological basis of plant reaction to and tolerance of aluminium (AI) is poorly understood. We review the results of investigations into A1 toxicity and root physiology to develop a theoretical basis for explaining the reaction of the root to AI, including suggested roles for Ca 2+, mucilaginous cap secretions and endogenous growth regulators in mediating a transmitted response between Al-damaged cap cells and the interacting cell populations of the cap and root.This information is used to identify possible mechanisms of A1 tolerance, notably involving signal transduction, A1 uptake pathways and root morphogenesis; and to briefly discuss how procedures selecting for AI tolerance may be improved by incorporating the concept of stimulus-response coupling.Similarities in the responses of roots to A1 and other signals (e.g. gravity, light, mechanical impedance) are used to develop the hypothesis that roots respond to environmental signals by way of a common regulatory system. New research prospects for extending our perception of A1 tolerance mechanisms are identified.

Histochemical studies of the chronological sequence involved in the uptake of AI by the primary root of Zea mays L., cv. TX 24, showed the primary sites of AI uptake to be the peripheral cells of the root cap and the mucilagenous secretions surrounding the root. AI spread rapidly through the cells of the root cap but the cap initials were the last cells to be reached. Entry of AI into the rest of the root was considerably less rapid, and was found to be limited to the outer cortical cells of the root apex. Little evidence could be found of AI reaching actively dividing cell populations of the primary root meristem during the first 20 h of this experiment. The concept of AI acting directly on cell division is consequently questioned. Decapped root experiments implied a protective function for the root cap over the quiescent centre and mitotically active cells of the root. Epidermal cells of the root apex were not an effective barrier to AI. It is postulated that AI uptake is a function of the biochemical properties particularly the presence of acid mucopolysaccharides in the cells involved. S. Afr. J. Plant Soil 1985, 2: 1 -7Histochemiese studies van die primere wortel van Zea mays L., cv. TX 24, het getoon dat aluminium hoofsaaklik deur die periferale wortelmusselle opgeneem is terwyl vergelykbare hoeveelhede van die element ook in die slymsekreet om die wortel waargeneem is. Aluminium het vinnig deur die selle van die wortelmus beweeg en die wortelmusinisiaalselle was die laaste selle wat bereik is. Opname van aluminium deur die res van die wortel was heelwat stadiger en selfs na 'n lang blootstelling was die element steeds tot die buitenste korteksselle van die wortelpunt beperk. Na 20 h kon min aanduidings van die beweging van aluminium tot by die del en de selle van die primere wortel se meristeem gevind word. Die siening dat aluminium 'n direkte invloed op seldeling uiloefen word dus bevraagleken. Uil eksperimenle waar die wortelmus verwyder is wil dil voorkom of die wortelmus 'n beskermende invloed op die stillegebied en miloties-akliewe selle van die wortelpunt uitoefen. Epidermisselle van die wortelpunl hel nie as 'n effekliewe skerm teen aluminium gedien nie. Daar word veronderslel dal aluminiumopname deur selle 'n funksie van hul biochemiese eienskappe, veral die leenwoordigheid van muko-polisakkariede, is.S.-Afr. Tydskr. Plant Grand 1985, 2: 1 -7

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