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 ehxibits 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. Overall design: For mRNA profiling, 12 pep1-1 plants were grown per biological replicate. Three biological replicates were used for each examined sampling stage. Four different developmental stages were examined. Perennial stem samples were harvested from stages I_PZ (4 weeks old), II_PZ (5 weeks old), III_PZ (7 weeks old) and IV_PZ (30 weeks old). In addition, annual stem samples were harvested from 30 weeks old plants, termed IV_AZ_si and IV_AZ_if.