Unravelling genetic components of apple russeting

Russeting, a major concern for the apple production sector, is due to the accumulation of suberin in the inner part of the cell wall of the epidermal cell layer. Since the 60s, many studies have been performed to better understand the tight relationship occurring between the cuticle development and the suberization process. However, the genetic factors which drive the russeting process, as well as the metabolic genes which are involved in the suberization process, remain unclear. The first aim of this thesis was to identify the different genetic actors of the russeting process. For this, a transcriptomic approach comparing waxy and russeted tissues was used to investigate the different pathways which are involved in this particular trait. Among the identified genes, a large number involved in the cuticle biosynthesis had their expression repressed in the russeted skin. Conversely, genes likely involved in the suberization process were identified and displayed an enhanced expression in these tissues. A lipid surface analysis of the russeted and waxy apple skins in a semi-russeted apple confirmed these results. Interestingly, cell wall modification genes seemed to be also involved in this russeting program. In a second step, candidate transcription factor genes, which might be involved in the regulation of russeting, were investigated. Among these, MdMYB93, which is able to trigger the expression of a large part of the genes identified in our transcriptomic studies, is now considered as a master regulator of suberin biosynthesis in apple. Altogether the present thesis provides answers which help to better understand the mechanisms leading to russeting. It also opens new insights for further work aiming at unravelling the remaining unexplored processes involved in apple russeting and, more generally, in the plant suberization process.