The papaya CpAUX1/LAX and CpPIN genes: structure, phylogeny and expression analysis related to root formation on in vitro plantlets

Carica papaya L. plantlets, normally exhibit low rooting capacity when cultured in vitro. It has been suggested in other species that auxin concentration at root tissues, is the result of a reflux system driven by auxin influx transporters (AIT; AUX1/LAX) and auxin efflux transporters (AET; PIN), that regulates the mechanism of initiation and development of lateral roots. Therefore, in the present paper, we studied the structure, phylogeny and the expression patterns of the whole family of AIT and AET in C. papaya, and their possible relation with the limited capacity to generate adventitious roots of in vitro cultured papaya plantlets. We found 4 AUX1/LAX genes (CpAUX1, CpLAX1, CpLAX2, CpLAX3) and 6 PIN genes (CpPIN1, CpPIN2, CpPIN3, CpPIN4, CpPIN5, CpPIN6) within the genome of C. papaya. The expression patterns and levels of those genes were studied in stem-base and root tissues from C. papaya cv. Maradol plants under four different treatments: (1) in vitro plantlets without IBA (that did not generate roots), (2) in vitro plantlets treated with 2 mg L−1 IBA (that did generate roots), (3) de-rooted seedlings treated with the same concentration of IBA (that also generated adventitious roots), and (4) intact seedlings used as controls. Histological studies made on the stem base and root tissues from all treatments showed that the IBA-induced roots were histologically equivalent, to those naturally formed in intact seedlings. In vitro plantlets non-treated with IBA had low expression of all auxin transporters genes in stem-base tissues and they were unable to produce roots. On the contrary, in vitro plantlets treated with IBA experienced a marked increase in the expression of most auxin transporters genes, in particular of CpLAX3 and CpPIN2, and they were capable to produce roots. Those roots generated in the IBA-treated in vitro plantlets, showed expression levels and patterns of auxin transporter genes, equivalent to those shown in both, the IBA-treated de-rooted seedlings, and in the naturally formed roots from intact seedlings.