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RESILIENT2: A novel thermosensory module at the interface of temperature and defense signaling

Periodic Reporting for period 1 - RESILIENCE (RESILIENT2: A novel thermosensory module at the interface of temperature and defense signaling)

Reporting period: 2015-09-01 to 2017-08-31

Plant growth and development are shaped by seasonal environmental conditions, of which temperature is a key seasonal signature. At elevated temperature growth is promoted along with accelerated developmental transitions while compromising plant defenses, leading to increased susceptibility. Climate change and the increase in average growth-season tem- peratures threaten biodiversity and food security. Despite its significance, the molecular mechanisms that link thermosensory growth and defense responses are not known. The overall objectives of this project were to understand the molecular mechanisms underlying the thermosensory suppression of immunity, using the novel resilient2 (res2) mutant that was isolated as a mutant with robust and resilient defense activation at elevated temperatures that is otherwise inhibitory. Our analyses have unearthed the intrinsic link between thermosensory response and temperature-induced defense modulation. the res2 mutant showed a remarkable suppression of classical thermosensory responses, indicating that thermosensory growth and immunity modulation are mechanistically linked. This has led to the elucidation of the key thermosensory transcription factor PIF4 as a coordinator of growth and immunity in response to temperature(Gangappa et al., 2017; Gangappa and Kumar 2017).
We investigated temperature responsiveness of res2 in context to various growth and developmental phenotypes at different temperatures. We found that res2 shows severely suppressed thermsoensory growth responses, particularly hypocotyl and rosette growth. Very interestingly, flowering phenotype of res2 mutant was comparable to wild-type both at 22°C and 27°C, suggesting that RES2 is probably not involved in controlling flowering time. We characterised key defense outputs such as the levels of Reactive oxygen species (ROS) generation upon perception of Pathogen associated molecular pattern (PAMP) such flg22; and res2 was found to to have increased levels of ROS at 22°C and 27°C. In order to map the res2 mutation, we performed bulk-seggregant RNA-seq analysis. While this has identified a number of potential candidate genes, RES2 could not be mapped in the stipulated time, possibly be due to still high number of background mutations. Efforts are underway to reduce the background mutations through repeated backcrosses. Following up on the attenuation of thermosensory growth responses in res2 in order to understand the molecular basis of the link between growth and immunity, we found that res2 suppressed PIF4-mediated temperature responses. We found that PIF4 singling is essential for the suppression of defense by elevated temperature. Moreover, PIF4 acts as a negative regulator of immunity in Arabidopsis. Modulation of PIF4 function in mutants or transgenics resulted in an altered balance between growth and defense. Importantly, reducing PIF4 singling led to resilient resistance against diseases at elevated temperatures (Gangapa et al., 2017), depicting a likely mechanism in res2. Plant growth in nature is controlled by a multiple environmental factors. We discovered that two upstream regulators, DE-ETIOLATED 1 (DET1) and CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1), are essential to maintain PIF4 function. DET1 and COP1 are required to stabilize PIF4 protein levels. This provides a molecular mechanism whereby PIF4 activity and function is coordinately controlled by DET/COP1 and HY5 in context to light and temperature signals (Gangappa and Kumar 2017). Collectively, the characterization of res2 mutant for thermosensory growth and defense phenotypes suggest that res2 mutant has defective thermosensory response, which is due to reduced PIF4-signaling. Analysis of PIF4 signaling for growth and defense responses show that PIF4 plays a key role in coordinating growth and defense responses in context to temperature. Further, we show that natural variation in PIF4 signaling could contribute to the balance between growth and defense responses in natural wild-type Arabidopsis strains. We also able to demonstrate that manipulation of PIF4 signaling could be one of the ways through which resilient resistance can be obtained. Furthermore, we also discovered the regulatory mechanism through which PIF4 function is controlled in response to different environmental signals. Future research focusing on the mapping of the res2 mutation and characterization of RES2 for molecular and biochemical functions would further reveal how exactly RES2 cross-talk with PIF4-signaling in coordinating growth and defense responses in context to different environmental signals.
As mentioned above, temperature plays a key role in the regulation of plant growth and development. Elevated temperatures promote plant growth, whereas it greatly suppresses plant defense responses to variety of pathogens. The increasing in temperature across the globe to due to global warming, has already caused a severe damage to both the crop yields and biodiversity. To combat this, first of all, we need to understand how plants integrate temperature signals and further how temperature-mediated growth and defense responses are coordinated by understanding the genetic pathways and molecular mechanism that govern these responses. Therefore, the proposed study was to understand the growth and defense responses at the interface of temperature.
Our initial characterization of res2 mutants suggest that res2 has defective thermosensory response but enhanced defense responses. Further mapping and molecular functional characterization of RES2 would add additional layer of information through which it controls growth and immunity in context to temperature. Because, res2 is defective in thermosensory growth response, we hypothesized that it could be due to defective PIF4 signaling. In fact, results from our study reveals that thermosensory pathway, which is mediated by Phytochrome interacting factor 4 (PIF4), is directly involved in suppressing defense responses at elevated temperatures. We found that PIF4 is necessary for the suppression of elevated temperature mediated suppression of defense responses. Our work might have a direct implication in designing crop plants with increased disease resistance and yield potential under warmer climatic conditions. Indeed, our results suggest that by modulating PIF4 (possibly other PIFs) we could achieve resilient resistance even at elevated temperatures. Moreover, we found that variation in PIF4 signaling could play a key role in balancing growth-defense trade-off in nature. This would further help in understanding the adaptation of plants to particular natural environments. Our future line of research in understanding how RES2 feeds into PIF4 pathway at the molecular levels would shed light on the exact mechanism through which RES2 functions. These findings provide a mechanistic framework for the coordination of growth and immunity in response to the environment, and opens up avenues for development of technologies for crop improvement towards climate resilience.
res2 mutant phenotypes
Coordination of thermosensory growth and immunity by PIF4