Different Molecular Mechanisms Account for Drought Tolerance in Coffea canephora var. Conilon

The effects of water deficit on photochemical parameters and expression of several candidate genes were investigated in drought-tolerant clone 73 of Coffea canephora submitted to slowly imposed water limitation. Under irrigation, this clone showed low values of stomatal conductance (g(s)) and of CO2 assimilation rates (A) suggesting that it had a great efficiency in controlling stomatal closure and transpiration. After water withdrawal, this clone reached a -3.0 MPa after 15 days without irrigation and showed a slow decrease in the pre-dawn leaf water potential. Under drought, the suppression of A was accompanied by maintenance of photochemical quenching (q(P)) and internal to ambient CO2 concentration (Ci/Ca) ratios as well as by a decrease of non-photochemical quenching (q(N)). This is confirmed by the transport rate/CO2 assimilation (ETR/A) rates that suggested the participation of an alternative electron sink protecting the photosynthetic apparatus against photoinhibition. At the transcriptomic level, high up-regulation of genes encoding for a dehydrin (CcDH3), an ascorbate peroxidase (CcAPX1), a prephenate-dehydrogenase like protein (CcPDH1) and a non-symbiotic haemoglobin (CcNSH1) was also observed upon drought suggesting a strong induction of antioxidant and osmoprotection systems in this clone. High expression levels of gene-encoding ABA receptors (CcPYL3 and CcPYL7) under water limitation were also observed suggesting the involvement of the ABA signaling pathway in response to drought. All these results where compared to those previously obtained for drought-tolerant clones 14 and 120. Our results demonstrated the existence of different mechanisms amongst the drought-tolerant coffee clones regarding water deficit.