Al resistance and mechanical impedance to roots in Zea mays: Reduced Al toxicity via enhanced mucilage production

It is still debatable whether or not mucilage covering root tips can contribute to aluminium (Al) resistance. For the issue, hydroponic culture used in most studies may be responsible since such impedance-free environment to roots may make mucilage production poor. We therefore investigated mucilage production and Al sensitivity in seedlings of maize (Zea mays L.) hydroponically grown with or without sand as mechanical impedance. The presence of sand clearly enhanced mucilage production than the absence regarding either the number of root tips secreting it or its volume per tip. Supplying AlCl3 at various concentrations for 7 d, inhibited root length corresponding to Al accumulation in root tips occurred only at 10 µM without sand. With sand, however, Al accumulation was negligible until at 100 µM, and even at 1 mM approximately 70% of root tips did not accumulate Al, resulting in no significant inhibition of root length. Root tips collected from with or without sand were divided into the presence or absence of mucilage, and then exposed to Al at 1 mM for 10 min. Al accumulation became lower in tips having mucilage than the absence, supporting a protective role of mucilage against Al. However, removing mucilage from the tips could not affect it probably due to a rapid recovery of mucilage within 5 min, making the removal treatment invalid. In conclusion, there is no doubt that mechanical impedance can bring a strong Al resistance to the maize seedlings probably via enhanced number of root tips to secrete mucilage rather than the greater volume per tip.