Etiolation-mediated regulation of adventitious rooting in avocado

Avocado (Persea americana) contributes $ 180 million at the farm gate and $ 420 million at retail to the Australian economy. Avocado is a woody tree species that is commercially grafted to obtain both desirable fruiting and rootstock resilience qualities. Clonal rootstocks derived from cuttings yield three times higher than sexually reproduced. However, the difficulty to generate adventitious roots on avocado stem cuttings makes propagation of rootstocks a significant challenge. The Frolich and Platt double grafting protocol is the standard practice for rootstock propagation and involves an un-interrupted etiolation step crucial for rooting. However, the molecular basis for this etiolation requirement is not yet understood. To address this, we analysed the effect of etiolation on expression of root promoting miR160 and the root inhibitor miR167 in avocado stems grafted for rootstock clonal propagation. Whilst miR167 expression was not strongly correlated to etiolation treatments, etiolation resulted in a distinct spatial regulation of miR160, with significantly stronger expression in basal tissue than in apical tissue, correlating to the site of adventitious root production. This is a trend absent in non-etiolated plants. This study suggests that etiolation facilitates a spatial gradient of miR160 abundance in grafted stems and that this, rather than general enhancement of miR160 expression, may be a key factor for adventitious rooting in avocado.