The plant hormone auxin is involved in virtually every aspect of plant growth and development. Because of its vital role in plant physiology, it is transported through controlled mechanisms. The PIN family of efflux carrier proteins are highly involved in this transport, shuttling molecules of auxin from cell to cell. This PIN-driven transport of auxin guides the differentiation of plant vasculature. In this study, I used a clonally-propagated line of the woody plant species Populus tremula x alba that had been genetically altered to induce an RNAi response against genes coding for the PIN proteins. These plants were shown to have reduced auxin transport rates, however, there was no difference when compared to wild-type in the external phenotype. First, reduction of PIN gene expression was confirmed in this Populus line through qRT-PCR, having been reduced by nearly 80% in shoot apices and over 90% in developing xylem. But despite this reduction of auxin transport, there was still no observable phenotype. It was then considered that this phenotype may be revealed if auxin transport rates were altered in localized areas of a single plant. Thus, this line of Populus was then used in reciprocal grafting experiments, in which RNAi line plants and wild-type plants were used alternatively as either scions or rootstocks. This resulted in whole plants with localized areas of reduced auxin transport. qRT-PCR was conducted on the scion and rootstocks of these plants to confirm that the RNAi signal is not graft transmissible, PIN expression in the RNAi plant segments having been reduced by over 65%, regardless of position (scion or rootstock). The vascular anatomy was then analyzed above and below the graft site. Again, regardless of the position of the RNAi line plant segment, digital image analysis revealed negligible difference in vessel size and distribution within developing xylem. These results may speak to the efficiency of homeostatic auxin mechanisms, which may have regulated the alterations in auxin transport such that the phenotype remained consistent with wild-type.
Norcross, Samuel, "The Effect of PIN Reduction in Populus: The Search for the Missing Phenotype" (2014). Biology Honors Papers. 16.
The views expressed in this paper are solely those of the author.