Zinc-Solubilising Bacteria From Rice Rhizosphere and Their Effect on Seedling Vigour

Abstract

Zinc deficiency limits rice on many flooded and alkaline soils and shapes the zinc content of a grain that feeds a large part of the world, so bacteria that make soil zinc available to the crop have drawn steady interest. This mini review looks at what is known about zinc-solubilising bacteria from the rice rhizosphere and their effect on early seedling growth. It draws on published work on zinc chemistry in soil, on the mechanisms of solubilisation, on screening assays, and on reports from rice and other cereals, and it presents no new data. Organic acids, proton release and chelating compounds are the usual explanations, and many isolates also make auxin or siderophores, so the cause of any seedling response is hard to assign. The central difficulty is that a clear zone on a plate is a poor predictor of what an isolate does in soil or in a plant, because the insoluble zinc source, the medium, the pH and the sugar supply are all unlike a flooded paddy. Doubts about the persistence of an introduced strain and inconsistent effects across soils and cultivars add to the problem. The review concludes that rice-associated zinc solubilisers are a reasonable source of candidates, that an effect on seedling vigour is plausible but not firmly established, and it sets out the assays, controls and reporting that would make future claims comparable.