Ca2+ is an Important but not the Only Signal in Callose Synthesis Induced by Chitosan, Saponins and Polyene Antibiotics. In: Lugtenberg BJJ, editor. Signal Molecules in Plants and Plant-Microbe Interactions. Berlin: Springer Berlin Heidelberg; 1989. pp. 107-16. [DOI: 10.1007/978-3-642-74158-6_11]

The role of the root apoplast in aluminium-induced inhibition of root elongation and in aluminium resistance of plants: a review

BACKGROUND

Aluminium (Al) toxicity is the most important soil constraint for plant growth and development in acid soils. The mechanism of Al-induced inhibition of root elongation is still not well understood, and it is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic.

SCOPE

The present review focuses on the role of the apoplast in Al toxicity and resistance, summarizing evidence from our own experimental work and other evidence published since 1995.

CONCLUSIONS

The binding of Al in the cell wall particularly to the pectic matrix and to the apoplastic face of the plasma membrane in the most Al-sensitive root zone of the root apex thus impairing apoplastic and symplastic cell functions is a major factor leading to Al-induced inhibition of root elongation. Although symplastic lesions of Al toxicity cannot be excluded, the protection of the root apoplast appears to be a prerequisite for Al resistance in both Al-tolerant and Al-accumulating plant species. In many plant species the release of organic acid anions complexing Al, thus protecting the root apoplast from Al binding, is a most important Al resistance mechanism. However, there is increasing physiological, biochemical and, most recently also, molecular evidence showing that the modification of the binding properties of the root apoplast contributes to Al resistance. A further in-depth characterization of the Al-induced apoplastic reaction in the most Al-sensitive zone of the root apex is urgently required, particularly to understand the Al resistance of the most Al-resistant plant species.

Effect of boron on the expression of aluminium toxicity in Phaseolus vulgaris

The interaction of boron (B) and aluminium (Al) was investigated in 5-day-old seedlings of soybean cv. Maple Arrow. Al treatment inhibited root elongation and callose formation in root tips particularly after 4-h Al treatment. After 10 and 24 h, both parameters indicated increasing recovery from Al stress. B deficiency aggravated Al toxicity compared with B sufficiency. B deficiency did lead to an increase in unmethylated pectin in the first 3 mm of the root tip. This increase in potential binding sites is reflected in generally higher Al contents in root tips of B-deficient plants. A fractionated extraction of Al from the root tips showed that citrate-exchangeable and non-exchangeable Al steeply increased up to 4 h, but then decreased after 10- and 24-h Al treatment faster in B-sufficient than in B-deficient plants. This decrease of Al contents can be explained by an Al-enhanced release of citrate from the root tips after 10-h Al treatment. However, the citrate exudation rate was the same (after 10 h) or even lower (after 24 h) in B-sufficient plants and thus cannot explain the faster decrease in Al contents of the root tips compared with the B-deficient plants. We, therefore, propose that under B deficiency, Al is more strongly bound by the pectic network of the cell wall of the root tips, which delays or prevents the recovery from initial Al stress through exudation of citrate, and thus explains the greater Al sensitivity of B-deficient common bean roots.

Signal perception and intracellular signal transduction in plant pathogen defense

Disease resistance in plant/pathogen interactions requires sensitive and specific recognition mechanisms for pathogen-derived signals in plants. Cultured parsley (Petroselinum crispum) cells respond to treatment with a crude cell wall preparation derived from the phytopathogenic fungus Phytophthora sojae with transcriptional activation of the same set of defense-related genes as are activated in parsley leaves upon infection with fungal spores. A 13 amino acid core sequence (Pep-13) of a 42 kDa fungal cell wall glycoprotein was identified, which stimulates the same responses as the crude cell wall elicitor, namely macroscopic Ca2+ and H(+)-influxes, effluxes of K(+)- and Cl- ions, production of active oxygen species (oxidative burst), defense-related gene activation, and formation of antifungal phytoalexins. Using [125I]Tyr-Pep-13 as ligand in binding assays, a single-class high-affinity binding site in parsley microsomal membranes and protoplasts could be detected. Binding was specific, saturable, and reversible. By chemical crosslinking, a 91 kDa parsley plasma membrane protein was identified to be the receptor of the peptide elicitor. Isolation of this receptor protein involved in pathogen defense in plants is under way.