<it>RBCS1 </it>expression in coffee: <it>Coffea </it>orthologs, <it>Coffea arabica </it>homeologs, and expression variability between genotypes and under drought stress

<p>Abstract</p> <p>Background</p> <p>In higher plants, the inhibition of photosynthetic capacity under drought is attributable to stomatal and non-stomatal (i.e., photochemical and biochemical) effects. In particular, a disruption of photosynthetic metabolism and Rubisco regulation can be observed. Several studies reported reduced expression of the <it>RBCS </it>genes, which encode the Rubisco small subunit, under water stress.</p> <p>Results</p> <p>Expression of the <it>RBCS1 </it>gene was analysed in the allopolyploid context of <it>C. arabica</it>, which originates from a natural cross between the <it>C. canephora </it>and <it>C. eugenioides </it>species. Our study revealed the existence of two homeologous <it>RBCS1 </it>genes in <it>C. arabica</it>: one carried by the <it>C. canephora </it>sub-genome (called <it>CaCc</it>) and the other carried by the <it>C. eugenioides </it>sub-genome (called <it>CaCe</it>). Using specific primer pairs for each homeolog, expression studies revealed that <it>CaCe </it>was expressed in <it>C. eugenioides </it>and <it>C. arabica </it>but was undetectable in <it>C. canephora</it>. On the other hand, <it>CaCc </it>was expressed in <it>C. canephora </it>but almost completely silenced in non-introgressed ("pure") genotypes of <it>C. arabica</it>. However, enhanced <it>CaCc </it>expression was observed in most <it>C. arabica </it>cultivars with introgressed <it>C. canephora </it>genome. In addition, total <it>RBCS1 </it>expression was higher for <it>C. arabica </it>cultivars that had recently introgressed <it>C. canephora </it>genome than for "pure" cultivars. For both species, water stress led to an important decrease in the abundance of <it>RBCS1 </it>transcripts. This was observed for plants grown in either greenhouse or field conditions under severe or moderate drought. However, this reduction of <it>RBCS1 </it>gene expression was not accompanied by a decrease in the corresponding protein in the leaves of <it>C. canephora </it>subjected to water withdrawal. In that case, the amount of RBCS1 was even higher under drought than under unstressed (irrigated) conditions, which suggests great stability of RBCS1 under adverse water conditions. On the other hand, for <it>C. arabica</it>, high nocturnal expression of <it>RBCS1 </it>could also explain the accumulation of the RBCS1 protein under water stress. Altogether, the results presented here suggest that the content of RBCS was not responsible for the loss of photosynthetic capacity that is commonly observed in water-stressed coffee plants.</p> <p>Conclusion</p> <p>We showed that the <it>CaCe </it>homeolog was expressed in <it>C. eugenioides </it>and non-introgressed ("pure") genotypes of <it>C. arabica </it>but that it was undetectable in <it>C. canephora</it>. On the other hand, the <it>CaCc </it>homeolog was expressed in <it>C. canephora </it>but highly repressed in <it>C. arabica</it>. Expression of the <it>CaCc </it>homeolog was enhanced in <it>C. arabica </it>cultivars that experienced recent introgression with <it>C. canephora</it>. For both <it>C. canephora </it>and <it>C. arabica </it>species, total <it>RBCS1 </it>gene expression was highly reduced with WS. Unexpectedly, the accumulation of RBCS1 protein was observed in the leaves of <it>C. canephora </it>under WS, possibly coming from nocturnal <it>RBCS1 </it>expression. These results suggest that the increase in the amount of RBCS1 protein could contribute to the antioxidative function of photorespiration in water-stressed coffee plants.</p>