Cytokinin-promoted secondary growth and nutrient storage in the perennial stem zone of Arabis alpina

Perennial plants maintain their life span through several growth seasons. Arabis alpina serves as model Brassicaceae species to study perennial traits. A. alpina lateral stems have a proximal vegetative zone with a dormant bud zone, and a distal senescing seed-producing inflorescence zone. We addressed the questions of how this zonation is distinguished at the anatomical level, whether it is related to nutrient storage, and which signals affect the zonation. We found that the vegetative zone exhibits secondary growth, which we termed the perennial growth zone (PZ). High-molecular weight carbon compounds accumulate there in cambium and cambium derivatives. Neither vernalization nor flowering were requirements for secondary growth and sequestration of storage compounds. The inflorescence zone with only primary growth, termed annual growth zone (AZ), or roots exhibited different storage characteristics. Following cytokinin application, cambium activity was enhanced and secondary phloem parenchyma was formed in the PZ and also in the AZ. In transcriptome analysis cytokinin-related genes represented enriched gene ontology terms and were expressed at higher level in PZ than AZ. Thus, A. alpina uses primarily the vegetative PZ for nutrient storage, coupled to cytokinin-promoted secondary growth. This finding lays a foundation for future studies addressing signals for perennial growth.HighlightArabis alpina stems have a perennial zone with secondary growth, where cambium and derivatives store high-molecular weight compounds independent of vernalization. Cytokinins are signals for the perennial secondary growth zone.

Données sur le contexte végétatif lié à la floraison chez le cerisier (Prunus avium)

A unique feature of Prunus avium L. is the distribution of growth between two types of morphologically and functionally distinct branches: auxiblasts and brachyblasts. The first ones are perennial and bear flowers at the basis of annual growth units. They are determinant for the architecture of the tree. The second type, born as lateral ramifications on the previous ones, have a much shorter life and support the flowers. A morphometric study was conducted on the growth units of the different types of ramifications in their primary phase of growth, on young and aged trees. It was showed that floral zones are characterized by a predominance of the foliar components over the stem components of the ramification. On the opposite, the vegetative zone of the auxiblasts is characterized by a predominance of the stem component. To designate these two phenomena, the terms "foliarization" and "axialization" were coined. With gradual aging of the tree and parallel evolution of the auxiblasts, toward progressively shorter as well as less and less ramified structures, an increasing preponderance of the leaves over the stem was observed or, to use the new terminology, a foliarization of the ramifications. In the meantimes, these become invaded by flowers. This type of morphometric study allows to describe simultaneously the progressive aging of the vegetative system along with the evolution of the distribution of flowering on different types of ramifications. Key words: architecture, morphometry, Prunus avium, aging, flowering. [Journal translation]