Iberian saxifrage side-steps the grim reaper to extend its lifespan

In the mountains of Iberia, some saxifrages sidestep death by producing lateral rosettes. These rosettes allow the plants to survive flowering, despite being monocarpic, meaning they normally die after flowering. “It appears that the evolution of clonality has allowed Pyrenean saxifrage plants to survive the harsh environmental conditions encountered when growing in their natural habitat, which pushes this plant species to its limits of life form and longevity,” say authors Alba Cotado and Sergi Munné-Bosch.

The study of Pyrenean saxifrage (Saxifraga longifolia) matters as it is adapted to what we would call extreme environments. However, these are also changing environments due to changing climates. Drought stress in particular could be a further challenge to the survival of these hardy plants. Any adaptation to drought will probably have to come at a very local level, the authors said, as populations of saxifrage live in small communities. “Pyrenean saxifrage outside the Pyrenees in the Iberian Peninsula usually only occurs in geographically highly isolated populations, and its capacity to adapt to new climate conditions will depend greatly on the plasticity of individuals because topographic barriers and long distances impede gene flow.”

The authors found that as the plants get higher in elevation, so they are more likely to form lateral rosettes. Normally S. longifolia has just the one root and one shoot, so when the rosette flowers the plant dies. But Cotado and Munné-Bosch found that at high altitudes one plant could produce multiple rosettes as lateral offshoots. The rosettes will, themselves, die after flowering, proving to be monocarpic but the plant and the co-rooted rosettes will survive – making the plant as a whole polycarpic. Effectively growing another rosette or more to the side therefore allows the genes to get a second or third shot at reproduction.

The advantage for a plant adopting this strategy is that it increases the calendar window for reproduction for the plants, say the authors. “When a rosette of a clonal individual reproduces, the other rosettes can survive this reproduction of the main rosette, because not all rosettes reproduce synchronously. Forming new rosettes helps the population to reduce reproduction-associated mortality, thus increasing longevity and the fertility period.“

Commenting on the paper, Jitka Klimešová and colleagues have an issue with using ‘clone’ to describe these extra rosettes. “[W]hen a plant produces new branches and daughter rosettes but does not initiate adventitious roots from the newly produced rosettes (as in the case of the species studied), it remains non-clonal. In this context the authors’ main message related to clonality is strictly misleading and we suggest that S. longifolia should be considered a non-clonal plant (until further research eventually finds adventitious roots in daughter rosettes).” 

Nevertheless, despite querying some of the paper’s findings, Klimešová and colleagues welcome the article. “The study by Cotado and Munné-Bosch (2020) provides valuable insights on plant stress responses at the edge of their distribution range, where contemporary and abrupt changes in temperature and precipitation prove a challenging environment for plants, likely promoting shifts in life-history strategies.”