Endogenous auxin is an important regulator of in vivo organ development, but its role in in vitro organogenesis is unclear. It has been observed that the basal end of epicotyl cuttings of juvenile citrus seedlings produces fewer shoots than the apical end. Here, we report that elevated endogenous auxin levels in the basal end of citrus epicotyl cuttings are inhibitory for in vitro shoot organogenesis. Using transgenic citrus plants expressing an auxin-inducible GUS reporter gene, we have observed elevated levels of auxin at the basal end of stem cuttings that are mediated by polar auxin transport. Depleting endogenous auxin or blocking polar auxin transport enhances shoot organogenesis. An auxin transport inhibitor, N-1-naphthylphthalamic acid (NPA), can also enhance shoot organogenesis independent of its action on polar auxin transport. Finally, we demonstrate that the promotional effects of depleting endogenous auxin or blocking polar auxin transport on shoot organogenesis are cytokinin-dependent. Our study thus provides meaningful insights into possible roles of endogenous auxin and polar auxin transport, as well as auxin–cytokinin interactions, in in vitro shoot organogenesis. Meanwhile, our results may also provide practical strategies for improving in vitro shoot organogenesis for citrus and many other plant species.

Among Citrus species, lemon is one of the most susceptible to mal secco disease, a tracheomycosis caused by the mitosporic fungus Plenodomus tracheiphilus, which induces chlorosis followed by leaf drop and progressive desiccation of twigs and branches. Severe infection can cause the death of the plant. Since no effective control strategies are available to efficiently control the pathogen spread, host tolerance is the most desirable goal in the struggle against mal secco disease. To date, both traditional breeding programs and biotechnological techniques were not efficient in developing novel varieties coupling tolerance to mal secco with optimal fruit quality. Furthermore, the genetic basis of host resistance has not been fully deciphered yet, hampering the set-up of marker-assisted selection (MAS) schemes. This paper provides an overview of the biotechnological approaches adopted so far for the selection of mal secco tolerant lemon varieties and emphasizes the promising contribution of marker-trait association analysis techniques for both unraveling the genetic determinism of the resistance to mal secco and detecting molecular markers that can be readily used for MAS. Such an approach has already proved its efficiency in several crops and could represent a valuable tool to select novel lemon varieties coupling superior fruit quality traits and resistance to mal secco.