GERMIN3 regulates tuber initiation and axillary bud activation by facilitating plasmodesmatal gating

GERMIN3 has previously been identified as a target of the tuberigen activation complex, suggesting a function in potato tuberisation, but its role is presently unknown. In the present study, we analysed morphological, agronomic and molecular phenotypes of GERMIN3 transgenic lines in Solanum tuberosum ssp. andigena and in the tuberosum cultivar Desiree. GERMIN3 over-expressing lines of S. tuberosum ssp. andigena exhibited increased tuber yields and enhanced tuber numbers. Post-harvest tuber sprouting exhibited greater bud activation with increased numbers of sprouts. Axillary buds were also activated in aerial tissues of mature plants, resulting in increased stem branching. Similar results were observed in the commercial cultivar Desiree. Over-expression of GERMIN3 had no impact on the expression of SP6A, a positive regulator of tuberisation, or TFL1B, a negative regulator. The GERMIN3 protein localised to the endoplasmic reticulum, and transient expression in N. benthamiana leaves resulted in plasmodesmatal gating, allowing intercellular transport of GFP-tagged sporamin independent of GERMIN3 oxalate oxidase activity. We propose that GERMIN3 affects tuberisation and other developmental processes by facilitating meristem activation. This identifies GERMIN3 as a novel protein associated with control of plasmodesmatal transport and supports the importance of plasmodesmatal gating in the regulation of key potato developmental processes.