Manganese toxicity and callose deposition in leaves are attenuated in mycorrhizal soybean

Abstract

Callose (1,3-β-glucan) deposition has been proposed as a sensitive indicator of plant susceptibility to Mn toxicity. In addition, it has been repeatedly observed that mycorrhization causes alleviation of Mn toxicity symptoms. The objective of this paper was thus to evaluate the mechanisms responsible for Mn sensitivity under Mn stress in mycorrhizal and non-mycorrhizal soybean plants, especially with regard to callose deposition in leaves. Non-inoculated or inoculated plants with G. etunicatum were submitted to increasing levels of Mn (0, 5, 10, 20 and 40 mg kg−1) and harvested at 80 days. Mycorrhizal plants showed better growth and less Mn toxicity symptoms. Non-mycorrhizal plants showed more relative growth decrease than mycorrhizal ones due to the two highest Mn additions comparing to the treatment without Mn addition. Callose deposition decreased in mycorrhizal plants only in the second youngest leaves, which were formed when the symbiosis was effective in attenuating Mn stress. In the second oldest leaves, formed before the complete functioning of the symbiosis, callose deposition increased in both mycorrhizal and non-mycorrhizal plants upon Mn addition. The improved nutritional status (mainly P) of mycorrhizal plants may be related to the attenuation of Mn toxicity in soybean, possibly by rendering Mn physiologically inactive (formation of P/Mn complexes), since Mn concentrations in mycorrhizal and non-mycorrhizal plants were the same. Similarly, Si concentration was increased in mycorrhizal plants and could also be involved in decreasing Mn toxicity.