The use of lanthanum to delineate the aluminium signalling mechanisms functioning in the roots of Zea mays L.

Abstract

Controlled environment studies involving the treatment of intact roots with Al (5 mg/dm 3) and La 3+ (25 mg/dm 3) have identified some notable similarities in the early reactions by primary roots to these ions. These included root growth inhibition, changes in cap size and construction as well as a marked decline in cap secretory activity. The effects of La 3+ nevertheless appeared to be less damaging to the root than comparable Al treatments (shorter lag periods and faster growth rates). Moreover, roots subjected to “continuous” La 3+ treatments resumed growth with times ⩾ 6 days whereas roots subjected to “continuous” Al treatment did not. These similarities and differences in the response of root cells to La 3+ and Al are discussed with a view to the possible development of La 3+ as an ultrastructural tracer for Al. Roots which had previously been treated with La 3+ or Al but which received Ca 2+ during the recovery phase resumed growth during the course of the experiment (8 days). In contrast Ca 2+ deprivation of roots initially subjected to Al or La 3+ treatments delayed and in some cases prevented recovery. The effects of Ca 2+ deprivation on the stages involved in recovery are discussed in relation to identifying an external (apoplasmic) Ca 2+ requirement which influences the activity of the root cap and mediates the reversal of growth inhibition attributed to the effects of La 3+ and Al. The requirement for an exogenous Ca 2+ supply is combined with data pertaining to Al- and La 3+-induced ultrastructural changes in the apoplasm of peripheral root cap cells to identify components of a stimulus-response system functioning in the root which is responsive to La 3+ and Al. Changes in La 3+- and Al-treated roots deprived of Ca 2+ during recovery also coincided with a re-orientation of shoot growth. These effects are considered in relation to a possible external requirement, by the root, for Ca 2+ which interacts with an endogenous programme involved in modulating shoot expansion and development.