Unveiling Coffee Molecular Response Mechanisms to Drought: The Homeobox-Leucine Zipper I CAHB12expressed in Roots of Coffea Arabica Is Able to Confer Water Deficit and Salt Stress Tolerance to Transgenic Plants

Drought is one of the main abiotic stresses affecting plant growth, with serious negative consequences to crop yield worldwide. Among those crops, coffee is severely injured by water deficit. In the present work, 34 coffee homeobox (HB) genes were identified in the Brazilian Coffee Genome Project database, being subsequently classified through phylogenetic analyses together with Arabidopsis and rice HB genes. Three coffee HB genes (CAZHD4, CAHB1-like2 and CAHB12) showed, by in silico analysis, an expression up-regulated by osmotic stress. qPCR analyses revealed that CAHB12 is highly up-regulated in leaves and lateral roots of Coffea arabica plants, upon moderate and severe water deficit conditions. Functional characterization of transgenic Arabidopsis constitutively expressing CAHB12 resulted in increased tolerance to drought and salt stresses, during distinct developmental stages. An insight into the gene set modulated by CAHB12 ectopic expression was provided by massively parallel sequencing (RNA-Seq). Classical drought response genes were generally repressed, suggesting that other mechanisms are likely contributing to the more tolerant phenotype exhibited by over-expressing plants, such as the pathway signaled by heat shock proteins and heat shock transcriptional factors. Our data combined, suggest a possible role for CAHB12 as a positive regulator of coffee stress response, and indicate this gene as a potential candidate for biotechnological approaches.