Quantitative resistance to blackleg can work to reduce disease severity even when the major resistance gene is no longer effective. The mechanism for QR, at least in the one variety tested, is possibly through programmed cell death (PCD) and reactive oxygen species (ROS) to cut off the growth of L. maculans.
Most canola varieties grown in Western Canada carry the specific blackleg resistance (R) genes Rlm1 and/or Rlm3, but recent field monitoring data indicate that these R genes are no longer effective. Despite this, severe blackleg damage is still uncommon on these resistant cultivars, suggesting additional resistance mechanisms, known as quantitative resistance (QR), may be present.
QR has the potential to provide a more durable, if less complete, protection of canola against blackleg. However, the effectiveness of QR may also vary widely in the field, and it has long been suspected that elevated temperatures may negatively affect the expression of QR. This study set out to understand more about QR.
Methods and results
Researchers took three blackleg-resistant canola cultivars carrying Rlm1 and Rlm3 and inoculated them with virulent isolates of Leptosphaeria maculans that should overcome these two major resistance genes. The variety Westar was used as a susceptible control.
All inoculated cotyledons showed infection symptoms, but the severity was lower for the R-rated cultivars, relative to Westar. These results indicate that quantitative resistance (QR) plays a role for these R-rated canola cultivars.
To figure out the mechanisms underlying QR, researchers studied the variety 74-44 BL, which has QR against a range of L. maculans isolates without the direct involvement of any major R genes. In response to L.maculans infection, 74-44 BL showed high expression for genes involved in programmed cell death (PCD), reactive oxygen species (ROS) generation and/or intracellular endomembrane transport. Inoculated 74-44 BL cotyledons also produced hydrogen peroxide, a trigger of PCD, in a larger area than was colonized by hyphae. These mechanisms are quite different from those of the major gene Rlm1. The result suggests that QR for 74-44 BL is achieved through increased PCD and ROS to limit the growth of L. maculans inside the plant.
To test the potential temperature effect, researchers used three common canola cultivars (74-44 BL, PV 530 G and 45H29) known to show QR. Plants were treated with seven-hour daily exposure to 32°C for one week during early plant flowering under controlled-environment conditions. The impact of elevated temperature on the susceptibility of these cultivars was compared to performance of the same cultivars under a moderate 22°C day-time high. Westar was used as a control. The elevated temperature often increased blackleg severity on Westar, occasionally on PV 530 G, but generally not on 74-44 BL or 45H29.
Key findings and conclusions
1) It was demonstrated that QR is of value in alleviating blackleg impact on canola without the direct involvement of major R genes. This is achieved by limiting the spread of fungal hyphae in infected cotyledons further into stems (reduced blackleg incidence) and/or the infection in stem tissues after the pathogen enters it (reduced disease severity).
2) These resistance mechanisms are different from those of single R genes showed by Rlm1 that induce localized reactions in response to the infection by L. maculans carrying AvrLm1, which halts the infection immediately by upregulation of many genes involved in the jasmonic acid (JA) and salicylic acid (SA) pathways. This is the first time that molecular mechanism with a specific blackleg resistance gene (Rlm1) is identified.
3) Using 74-44 BL, the mechanism underlying QR against blackleg was explored and different genes (as opposed to those involved in Rlm1) were found to express differentially, with the highest gene expression associated with those involved in programmed cell death, reactive oxygen species generation and intracellular endomembrane transport. Once confirmed, this will be first report on the molecular mechanisms of QR against plant diseases.
4) The study on the impact of elevated temperature on QR expression indicate that common canola cultivars with a QR background can perform effectively under high-temperature conditions during heat waves and this finding shows that QR traits can be stable under a wide range of field temperatures.
These findings suggest that the QR traits are highly useful for blackleg management in Western Canada, even with warmer temperatures encountered during rosette to early flowering stages.