Hormonal responses associated with acclimation to freezing stress in Lolium perenne

Complex phytohormonal analyses revealed hormone changes underlying the freezing response of perennial ryegrass (Lolium perenne L.). Comparisons among sensitive (code of accession 1/61), highly tolerant (5/101) and semi-tolerant (6/104; sensitive without acclimation, but tolerant after acclimation) genotypes allowed identification of changes associated with freezing tolerance. The main response to freezing stress (-7 degrees C for four days) was a significant elevation of abscisic acid (ABA) in all tissues of all genotypes, supported by elevation of jasmonic acid (JA) in leaves, and of salicylic acid (SA) in crowns and roots. Jasmonate-isoleucine increase was observed in roots of the tolerant genotype. The auxin (indole-3-acetic acid) levels showed relatively minor changes, while cytokinins exhibited up-regulation in crowns of sensitive genotypes. Tolerant genotypes exhibited decreased levels of highly active cytokinin trans-zeatin compensated by elevated low-active cis-zeatin, involved in stress responses. Cold acclimation (3 degrees C for seven days) applied prior frost treatment was associated with stimulation of plant defences, especially expression of ice recrystallisation inhibition protein LpIRI3. Acclimation diminished the impact of subsequent freezing treatment as indicated by lower up-regulation of stress hormones ABA, JA and SA in all tested tissues (with the exception of SA in crowns and roots of sensitive genotype) and upregulation of active cytokinins in tolerant and semi-tolerant genotypes, especially in crowns and roots.

Automated summary

Complex phytohormonal analyses revealed changes in hormones such as ABA, JA, and SA in perennial ryegrass in response to freezing stress. Tolerant genotypes showed lower levels of active cytokinin trans-zeatin compensated by elevated low-active cis-zeatin. Cold acclimation stimulated plant defenses and reduced the impact of subsequent freezing treatment by lowering the up-regulation of stress hormones ABA, JA, and SA.