Oral Paper

         Ecophysiology

Does expression of the auxin biosynthetic protein family YUCCA change during drought?

Presenting Author
Sam Pelletier
Description
As drought becomes more pronounced due to climate change, it is important to characterize the responses of plants to water stress. One area of interest includes how auxin, the hormone most associated with growth and development, is affected by drought. In particular, mature leaf size is often reduced in response to drought and auxin is known to play a key role in leaf expansion. The YUCCA proteins catalyze the conversion of indole-3-pyruvic acid to indole-3-acetic acid (IAA), the dominant form of auxin in most plants, and most of this IAA is produced in rapidly expanding leaves. However, in addition to synthesizing IAA, the YUC proteins are also associated with thiol reductase activity. This enzymatic function reduces reactive oxygen species (ROS), which can accumulate in plant tissue under stress such as drought. The YUCCA (YUC) gene family in Populus includes 12 YUCs, but they are not well described. This experiment was aimed to assess whether YUC genes in Populus leaves at different stages of development are differentially expressed in response to drought. Multi-shoot, one-year-old hybrid poplar plants (Populus tremula x alba; INRA 717-1B4) were either watered to field capacity daily (control; n = 5) or deprived of water (drought; n = 5) for nine days. Over that period, stomatal conductance and stem water potential both decreased significantly in the droughted trees relative to the control trees. The shoot apex, leaf 8, and leaf 16 (L8 and  L16, counted down from the apex) were harvested for gene expression analysis, with the apex representing rapid growth, L8 representing a leaf approximately one half the size of the average fully mature leaf, and L16 representing a fully expanded leaf. Leaf relative water content (RWC) measured on leaf disks at the time of harvest and showed that the RWC of L8 (i.e., the eighth leaf beneath the shoot apex) was significantly reduced under drought compared to controls, but the RWC of L16 was similar between treatments. qRT-PCR was used to determine expression levels of five different YUC genes that had previously been shown to be expressed in Populus leaves: YUC1, YUC2, YUC4, YUC6, and YUC12. One gene, YUC2, was significantly upregulated in drought across all leaf developmental stages, where other YUCs either remained unchanged or were reduced. In shoot apices, YUC1 was downregulated. In L8, only YUC2 was significantly affected. In L16, YUC1, YUC4, and YUC12 were all downregulated in drought. The fact that YUC2 was upregulated in L8 but not in L16, where only L8 was shown a reduction in RWC, may suggest that YUC2 was increased in response to ROS production (RWC was not measured on apices). In contrast, the fact that no YUCs were significantly reduced in expression in apices or L8 suggests that IAA biosynthesis may not be directly affected by drought. Future work should measure IAA concentrations in leaf tissue under drought to better understand these hormonal cues.