HbARF2 and HbARF16.3 function as negative regulators for the radial trunk growth of rubber tree

Abstract Auxin plays pivotal roles in the regulation of plant growth mainly via the auxin response factors (ARF), pin-formed efflux carriers (PIN) and auxin influx carriers (LAX).To identify potential responses of the expression of these genes to tapping-caused repression of radial growth, the expression of 17 ARF, 4 LAX and 4 PIN genes in tapped and untapped rubber trees (Hevea brasiliensis Muell. Arg.) and among germplasm with differential annual increment of trunk girth from six hybrid combinations was analysed by using quantitative real-time PCR (qPCR). HbARF2 and HbARF16.3 were up-regulated by tapping. The transcript levels were much higher in Reyan8–79 than that in Reken525 whereas radial trunk growth was strongly reduced in Reyan8–79 in comparison with that in Reken525 after tapping. The transcript levels negatively correlated with the annual increment in trunk girth of offsprings. Overexpression of HbARF2 or HbARF16.3 inhibited the growth of transgenic tobacco (Nicotiana tabacum L.) plants. The repressed growth caused by the HbARF2 was involved in the reduced cell sizes of pith, cell size and proliferation of the vascular bundle while the growth inhibition caused by HbARF16.3 attributed to cell proliferation within the pith of transgenic tobacco. The results suggest that HbARF2 and HbARF16.3 function as negative regulators for trunk growth and are related to the tapping-induced inhibition of trunk growth of rubber trees.