Universite Catholique de Louvain, Laboratoire de Cytogenetique, Place Croix du Sud, 4-5 (bte. 13), B-1348 Louvain-la-Neuve, Belgium, 1) Research assistant at the U.C.L., 2) Corresponding author.
Somaclonal variation and in vitro selection were used to improve
aluminium tolerance of two sensitive rice genotypes (Oryza sativa L. var.
I Kong Pao (IKP) and Aiwu). After preliminary tests, checking the in vitro
ability of these two genotypes and adjusting the selective culture media,
five selection procedures were tested and compared. All these procedures
permitted selection at the callus level (during the phase of callogenesis),
and two of them permitted also selection at the level of regenerated
plants. Amongst these five selection methods, one involved two in vitro
cycles. The seeds obtained from the regenerated plants were used to
induce the second in vitro cycle.
These five procedures allowed the selection of different callus and plant lines showing an improved aluminium tolerance in comparison to the parental plants. In order to verify the stability of the selected character, we tested the first progeny plantlets for 80 days, in a modified Yoshida (1976) nutrient solution with unchelated iron, low pH (3.85), low phosphorus and calcium concentrations, and 1500 micro-atg l-1 of Al. This aluminium concentration is strongly toxic to the sensitive plants of the two parental genotypes. Some of the tested plantlets showed a clear improvement in their aluminium tolerance. The study of their progeny is in progress, in order to fit the character and to understand the inheritance of tolerance to aluminium.
Simultaneously, we undertook the mineral characterization of some tolerant calli and plants. This study corroborated the hypothesis according to which the tolerance mechanisms selected in vitro are distinct from those found in the tolerant line IR 6115-6-1, especially at the callus level. In contrast to the IR 6115-6-1 calli, the selected IKP and Aiwu calli absorb less aluminium rather than tolerate large quantities of this element in their cells. When the plants were cultured in the presence of a high concentration of aluminium, both the selected IKP and Aiwu lines, as well as the IR 6115-6-1 tolerant line, accumulated less aluminium in their leaves than the unselected plants. Moreover, the [Al]\leaf\/ [Al]\root\ ratio was always lower than 1 in tolerant plants, while it was usually greater than 1 in the sensitive ones.
At the leaf level, there is a very strong parallelism between the quantities of iron and aluminium absorbed by plants cultured in the presence of aluminium. We postulate that there is a common mechanism for the absorption of these two minerals. In rice, the mechanism of iron mobilization and uptake is of the strategy II type, involving phytosiderophores (Romheld and Marschner 1986). Since aluminium can replace iron on certain bacterial siderophores (Gascoyne et al. 1991), we suggest that, in the presence of aluminium in the nutrient solution, tolerant plants in contrast to the sensitive plants, would develop a phyto- siderophore-FeIII uptake mechanism, functionning only when plants are Fe- deficient.
References
Gascoyne, D.J., J.A. Connor and A.T. Bull, 1991. Capacity of siderophore-producing alkalophilic bacteria to accumulate iron, gallium and aluminium. Appl. Microbiol. Biotechnol. 36: 136-141.
Romheld, V. and H. Marschner, 1986. Evidence for a specific uptake system for iron phyto-siderophores in roots of grasses. Plant Physiol. 80: 175-180.
Yoshida, S., 1976. Routine procedure for growing rice plants in culture solution. In Laboratory manual for physiological studies of rice, S. Yoshida, D.A. Forno, J.H. Cook and K.A. Gomez (eds.), p. 61-66, I.R.R.I., Manilla, Philippines.