Al exposure increases proline levels by different pathways in an Al-sensitive and an Al-tolerant rye genotype.

Aluminium (Al) toxicity limits crop productivity, particularly at low soil pH. Proline (Pro) plays a role in protecting plants against various abiotic stresses. Using the relatively Al-tolerant cereal rye (Secale cereale L.), we evaluated Pro metabolism in roots and shoots of two genotypes differing in Al tolerance, var. RioDeva (sensitive) and var. Beira (tolerant). Most enzyme activities and metabolites of Pro biosynthesis were analysed. Al induced increases in Pro levels in each genotype, but the mechanisms were different and were also different between roots and shoots. The Al-tolerant genotype accumulated highest Pro levels and this stronger increase was ascribed to simultaneous activation of the ornithine (Orn)-biosynthetic pathway and decrease in Pro oxidation. The Orn pathway was particularly enhanced in roots. Nitrate reductase (NR) activity, N levels, and N/C ratios demonstrate that N-metabolism is less inhibited in the Al-tolerant line. The correlation between Pro changes and differences in Al-sensitivity between these two genotypes, supports a role for Pro in Al tolerance. Our results suggest that differential responses in Pro biosynthesis may be linked to N-availability. Understanding the role of Pro in differences between genotypes in stress responses, could be valuable in plant selection and breeding for Al resistance.