(Oxford University Press, 2019) Ancín Rípodas, María; Fernández San Millán, Alicia; Larraya Reta, Luis María; Morales Iribas, Fermín; Veramendi Charola, Jon; Aranjuelo Michelena, Iker; Farrán Blanch, Inmaculada; IdAB. Instituto de Agrobiotecnología / Agrobioteknologiako Institutua
The activity of the protein kinase STN7, involved in phosphorylation of the light-harvesting complex II (LHCII) proteins,
has been reported as being co-operatively regulated by the redox state of the plastoquinone pool and the
ferredoxin–thioredoxin (Trx) system. The present study aims to investigate the role of plastid Trxs in STN7 regulation
and their impact on photosynthesis. For this purpose, tobacco plants overexpressing Trx f or m from the plastid
genome were characterized, demonstrating that only Trx m overexpression was associated with a complete loss
of LHCII phosphorylation that did not correlate with decreased STN7 levels. The absence of phosphorylation in Trx
m-overexpressing plants impeded migration of LHCII from PSII to PSI, with the concomitant loss of PSI–LHCII complex
formation. Consequently, the thylakoid ultrastructure was altered, showing reduced grana stacking. Moreover,
the electron transport rate was negatively affected, showing an impact on energy-demanding processes such as the
Rubisco maximum carboxylation capacity and ribulose 1,5-bisphosphate regeneration rate values, which caused a
strong depletion in net photosynthetic rates. Finally, tobacco plants overexpressing a Trx m mutant lacking the reactive
redox site showed equivalent physiological performance to the wild type, indicating that the overexpressed Trx m
deactivates STN7 in a redox-dependent way.
