Researchers from National Institute of Plant Genome Research (NIPGR) have uncovered how plant cells attacked by insects communicate distress to other cells of the plant. In the first 5 – 10 minutes after the attack, a spike in calcium levels and the plant immune protein Plant Elicitor Peptide Receptor (PEPR) amplifies the damage signal, as plants brace for insect attack. The findings of this study can pave the way for protecting crops from pests.
Unlike animals, plants cannot flee from herbivores and insects that munch on their leaves. Instead, they rely on chemical defences when under attack. How do plants quickly sense the attack to turn on their defences? When attacked, damaged plant cells spill out a defence hormone called ‘Jasmonic Acid Isoleucine (JA-Ile)’. Nearby plant cells have COI1 (coronatine- insensitive 1) receptors that sense JA-Ile and activate defence mechanisms in the plant. However, this canonical pathway takes about 20 minutes to act after an initial attack.
Jyothilakshmi Vadassery, Scientist, National Institute of Plant Genome Research (NIPGR), New Delhi, studies insect-defence pathways and calcium Signalling in plants. A new study from her lab published in the Plant, Cell and Environment journal details how plant cells attacked by insects communicates distress to other cells of the plant. Vadassery says,
We wanted to see if there is any pathway other than the slow canonical pathway which can sense JA-Ile very rapidly.
Researchers treated plant sections to JA-Ile to mimic plant herbivory. They used a fluorescence reporter to measure cellular calcium levels, which usually increase during immune response. The findings revealed a sharp rise in calcium levels in leaf cells but not in root cells following JA-Ile exposure. This influx of calcium ions appears to prime leaves for defence responses, reflecting that plant leaves are more vulnerable to insect threats compared to underground roots.
To understand what causes the calcium spike, they examined genes that are turned on by JA-Ile. They found that a defence protein called Plant Elicitor Peptide Receptor (PEPR) is produced more when JA-Ile is present. PEPR is located on the plant cell surface and detects damage-associated proteins. A protein called MYC2 (basic helix-loop-helix (bHLH) transcription factor) binds near the PEPR1 and PEPR2 genes in the DNA, amplifying PEPR production when cells are exposed to JA-Ile.
Debabrata Laha, Assistant Professor at the Indian Institute of Science (IISc), Bangalore, who was not associated with the study, says, “It is interesting that JA-Ile induced calcium spike is independent of the COI1 receptor of the canonical pathway. Previously it was thought that the JA-Ile and the calcium signalling pathway are linear but this study suggests that it is a complex pathway and there is more to discover.”
On the one hand, JA-Ile causes a calcium spike through PEPR, which activates MYC2 (basic helix-loop-helix (bHLH) transcription factor) and turns on the canonical JA-Ile pathway. On the other hand, JA-Ile also turns on the canonical pathway by chopping the PROREP protein on one end, to form AtPep, a plant damage signal that activates PEPR. These two pathways together help in the rapid sensing of JA-Ile, finally switching on the canonical pathway.
In this study, researchers have identified a new signalling pathway that acts faster than the previously known canonical JA-Ile defence signalling pathway, with potential applications in crop protection. However, the mechanism by which undamaged plant cells uptake JA-Ile from nearby damaged cells is still unknown. The authors plan to identify the specific receptor protein that binds JA-Ile and initiates signalling cascades within plant cells when they detect the spilled JA-Ile. Laha says,
Global warming related change in temperature inhibits JAIle signalling pathway and plants are more susceptible to pests as a result. Studies along this direction can help increase crop fitness against pests.