Page 246 - Plant Canada 2024 Proceeding

PLANT CANADA 2024

enveloped by a plant-derived membrane that maintains N-fixation under oxygenated conditions. The
nitrogenase enzyme is deactivated by oxygen, so the nodule interior is compartmentalized and protected
by antioxidant systems that scavenge free reactive oxygen species formed during bacterial respiration.
Copper (Cu) is a common environmental pollutant, and can reduce nodulation and N-fixation in various
legume species. Excess copper causes oxidative stress in plant tissues and destroys membranes
essential to cellular function via lipid peroxidation, but it is not known if Cu-induced reductions in rhizobial
activity are due to a reduced ability of a stressed plant to support the symbiont, or to oxidative stress
within the nodule itself.

To determine the physiological mechanisms by which Cu inhibits nodulation and N-fixation, the model
legume Lotus japonicus was inoculated with Mesorhizobium loti and grown in a sand-hydroponic system
spiked with 0, 300 or 450 μM CuSO4. Cu uptake in roots, nodules, and shoots was confirmed by
inductively coupled plasma-mass spectrometry. The concentration of Cu in the roots was up to 4-fold
higher than in the nodules, and 11-fold higher than the shoots. A 30-40% decrease in both the mass and
number of nodules was observed at both 300 and 450 μM CuSO4 compared to control. An acetylene
reduction assay showed a 50% decrease in nitrogenase activity at 450 μM CuSO4 compared to control. A
thiobarbituric reactive substances assay was used to determine lipid peroxidation in the roots and
nodules, and showed 20-30% higher concentrations of malondialdehyde in roots compared to nodules,
but no significant difference among Cu treatments.
These findings indicate that Cu does impact the health of the plant and its ability to form nodules. The
nodules were both smaller and less numerous in Cu-treated plants, reducing total nitrogen fixation. The
formation of malondialdehyde does not indicate that oxidative stress is occurring unduly in the nodules
themselves. This, combined with the finding that nodules were relatively low in Cu compared to adjacent
roots, suggests that Cu does not directly affect rhizobial activity within the nodule via oxidative stress, but
rather that reduced N-fixation under Cu-stress is due solely to the plants having fewer and less developed
nodules.

*[P86] RECOMBINANT INBRED LINES OF PLANTS ADAPTED TO EXTREME ENVIRONMENTS CAN
HELP IDENTIFY THE GENETIC BASIS OF LOW-PHOSPHATE TOLERANCE IN CROPS. Laura Li ,
1
1 1
2
2
Yong Li , Barbara Moffatt , and Elizabeth Weretilnyk . Department of Biology, McMaster University, 1280
Main St W, Hamilton, Ontario, Canada, L8S 4L8; and Department of Biology, Waterloo University, 200
2
University Ave W, Waterloo, Ontario, Canada N2L 3G1
Correspondence to: lil127@mcmaster.ca

Phosphorus is an essential nutrient for plants with many critical roles including energy transfer (ATP and
photosynthesis) and as cellular components ranging from cell membranes to nucleic acids. Not
surprisingly, agriculture relies on phosphate containing fertilizers to maximize crop yields and food
security. However, decreasing global deposits of accessible rock phosphate for fertilizer production and
excessive algal production from phosphate runoff makes it imperative to reduce our reliance on fertilizer
use. Eutrema salsugineum is an extremophile crucifer and halophyte that is related to Arabidopsis
thaliana and canola. An ecotype native to the semi-arid, subarctic Yukon, Canada, is tolerant to many
abiotic stressors, including drought, freezing temperatures and low-phosphate conditions. In contrast, an
ecotype from Shandong, China, an area that is more temperate, is sensitive to low-phosphate conditions.
We are using recombinant inbred lines (RILs) produced by a cross between the Yukon and Shandong
plants to identify traits associated with low-phosphate tolerance. For screening, we compared plants
grown on a medium (agar or soil) supplemented with phosphate or a medium where phosphate addition
was either low or absent. We identified lines that resemble their parents, either by displaying tolerance or
sensitivity to low phosphate conditions. We measured several traits to rank low phosphate tolerance
among the RILs including seedling root architecture, root and shoot biomass, rosette leaf area, and the
expression of low-phosphate responsive genes. Among non-destructive measurements used on plants,
chlorophyll fluorescence tests of non-photochemical quenching correlated well with low phosphate
tolerance while quantum yield (Fv/Fm) values did not. Ongoing work is directed to identifying the
physiological and genetic basis contributing to low phosphate tolerance in RILs displaying this tolerance
to low phosphate. Identifying gene(s) associated with low phosphate tolerance in plants like Yukon E.
salsugineum can help us generate crops that are more phosphate efficient to reduce our reliance on
phosphate fertilizers.

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