Abstract Detail

Physiological Section

Spicer, Rachel [1], Norcross, Samuel [2], Carraro, Nicola [3].

Reduced expression of PIN1 in Populus via RNAi causes a significant reduction in auxin transport capacity but produces no obvious structural phenotype.

Auxin transport through relatively undifferentiated tissue in the shoot apex is critical to vascular patterning in plants. In particular, polar auxin transport (PAT) – the directional, cell-to-cell transport mediated largely by the PIN proteins – plays a well-documented role in establishing leaf venation patterns. PAT may also be important in xylem development, as application of the auxin transport inhibitor NPA to woody stems has been shown to alter vessel patterning. We sought to examine the developmental role of PIN1 in the model woody plant Populus (termed PtaPIN1) by reducing expression levels of PtaPIN1 with RNAi. Several Populus RNAi lines were generated by transforming with the KANNIBAL hairpin construct developed by CSIRO, which we modified to contain an inverted repeat targeting a short region of PtaPIN1 that is highly conserved across all 16 PIN genes. The expression levels of two paralogs, PtaPIN1 and PtaPIN7, were reduced by about 60-75% depending on the genetic line and tissue studied. Even with the most severe reduction in expression there was no obvious phenotype, suggesting that redundancy in the PIN proteins allowed for similar auxin transport capacity in the RNAi plants. Interestingly however, we found that auxin transport rates determined by ³H-IAA transport assays are reduced by 55-65% in these plants. In order to exaggerate any potential developmental effects of reduced auxin transport capacity, we then established reciprocal grafts of wildtype/RNAi (alternating the genotype of the scion and rootstock) as well as control grafts (identical scion and rootstock genotype). Given the demonstrated graft-transmissibility of RNAi signals, we carefully documented the expression levels of both RNAi and wildtype (wt) stem segments within each graft type and found no evidence for graft transmission: RNAi segments continue to show PtaPIN1 expression levels that are reduced by about 65% (RNAi scion/wt rootstock) and 80% (wt scion/RNAi rootstock) relative to wt segments. In principle, having two stem segments with significantly different auxin transport rates fused together should create a region of tissue with an elevated auxin concentration. Current work in the lab is focused on a detailed study of xylem anatomy above and below the graft union, as well as auxin quantification with LC-MS/MS. If transport rates within the RNAi stem region remain reduced but auxin concentrations are unchanged, then a significant homeostatic mechanism is at work. However, if auxin concentration is significantly elevated in a localized region along the stem, an effect on xylem development is expected.

1 - Connecticut College, Botany Department, 270 Mohegan Avenue, New London, CT, 06320, USA
2 - Connecticut College, Biology Department, 270 Mohegan Avenue, New London, CT, 06320, USA
3 - Purdue University, Botany and Plant Pathology, 915 West State St, West Lafayette, IN, 47907, USA

Keywords:
auxin
xylem
RNAi
Populus.

Presentation Type: Oral Paper:Papers for Sections
Session: 42
Location: Pines South/Boise Centre
Date: Wednesday, July 30th, 2014
Time: 3:45 PM
Number: 42009
Abstract ID:758
Candidate for Awards:None