Root system of seabuckthorn (Hippophaë rhamnoides L.)

Nutrient availability is one of the limiting factors for plant growth and development, and the nutrients on Earth are unevenly distributed. To overcome this, plants adapt by modifying their morphology and physiology, especially of the root system. Seabuckthorn is a small tree growing in temperate regions of Europe and Asia. It performs symbiotic N2 fixation and has high adaptability to environmental constraints. This thesis examined factors shaping the root system of seabuckthorn. It was found that seabuckthorn has the ability to produce structures with dense lateral roots called cluster roots (CRs). Different patterns of root system were found at genotype level; Pk, a wild accession of H. rhamnoides ssp. turkestanica originating from unfertile soils, produced more CRs than cultivars BHi10726 and Sunny with a breeding history in fertile soils. Reduced availability of phosphorus (P), nitrogen or iron affected root morphology by increased lateral roots and CR formation, which may explain the competitive and invasive nature of this plant on nutrient-deficient soils. Abundant compounds in CRs found by metabolite analysis using H-NMR spectroscopy were asparagine, glycine and malate, an organic acid involved in mobilization of P from soil. In Pk as compared to cv. Sunny, the concentration of root metabolites was higher. In roots under low P, RNA sequencing revealed transcripts involved in primary root metabolism, P homeostasis and metabolism consistent with a P-deficient response. To assess functions of CRs which have a determinate growth pattern, analysis of metabolites and transcripts could display changes towards a Pdeficient metabolism as well as anaerobic metabolism at later developmental stages of CR. As an enzyme central to organic acid metabolism in plants, the gene family encoding phosphoenolpyruvate carboxylase (HrPPC) was characterised. Both planttype and bacterial-type isoforms were found with tissue-specific expression patterns and with higher expression of HrPPC2 under low P. In vitro studies showed that auxin stimulated formation of lateral roots at low P levels, while high P gave high formation of shoots from roots, another trait of the seabuckthorn root system. This study provides a basis to understand functions and roles of CRs in seabuckthorn as an actinorhizal representative among the three groups of CR-forming plants.