Overexpression of the plastidal pseudo-protease AtFtsHi3 confers drought tolerance without penalizing growth

Drought is one of the most severe environmental stresses affecting plant biomass production and quality, however, the molecular mechanisms of drought response in plants remain unclear. Here, we demonstrate that overexpression of the Arabidopsis gene FTSHi3 under the influence of its endogenous, or the 35S constitutive promoter results in drought-tolerant phenotypes without penalising plant growth. FTSHi3 encodes a pseudo-protease located in the chloroplast envelope and knock-down mutants (ftshi3-1) have previously been found to be drought tolerant, but highly reduced in growth. Changes in FtsHi3 transcript abundance therefore seems to induce drought tolerance in Arabidopsis thaliana. Overexpression of FTSHi3 (pFTSHi3-OE) impacts leaf stomatal density, lowers stomatal conductance and increases water use efficiency. To explore the underlying mechanisms behind this, we compared the proteomes of ftshi3-1 and pFTSHi3-OE to wild type plants grown under drought and watered conditions. Under drought conditions, the drought related processes osmotic stress, water transport and response to abscisic acid were enriched, indicating that pFtsHi3-OE and ftshi3-1 mutants are more active in their response to drought than the wild-type. The proteins HSP90, HSP93 and TIC110 were more abundant in the knock-down mutant, which suggests that FtsHi3 might play a downstream role in chloroplast pre-protein import. Increased abundance of FtsH7/9 and FtsH11, FtsH12 and FtsHi4 in ftshi3-1 combined with the fact that FtsH proteases function as homo- or heteromeric complexes suggests that these proteases may be possible interacting partners. To explore this, we constructed mathematical models that show FtsHi3 likely interacts with at least two other (pseudo-) proteases.