Session 2
Authors: Luan Sheng , Lin Zhen , Tessi Tomas Maria , Yang Xin , Maric Aida 
Published: July 2023

Regulation of nutrient homeostasis by a calcium signalling network

Plants are growing in a nutrient-poor environment in nature. Agricultural production is heavily relying on the application of chemical fertilizers, imposing a serious economic and environmental problem worldwide. One solution would be to breed crops that can tolerate low-nutrient soils to reduce the reliance on fertilizers. Work in Luan laboratory identified a CBL-CIPK signaling pathway that regulates the activity of a voltage-gated potassium channel involved in K-uptake in plant roots and another CBL-CIPK pathway for vacuolar K remobilization. Manipulation of CBL-CIPK network can potentially enhance the growth of plants under low-K soils, supporting sustainable agriculture and environment. The CBL-CIPK network has become a major signaling mechanism for the regulation of mineral nutrition by targeting transporters in various subcellular locations.

B2/3-RAF mediates SnRK2 activation in ABA signaling in Arabidopsis

The phytohormone abscisic acid (ABA) is crucial for plant responses to environmental challenges. The SNF1-regulated protein kinase 2s (SnRK2s) are key components in ABA-receptor coupled core signaling. It is rapidly phosphorylated and activated by ABA. Early work proved SnRK2 could activate itself by auto-phosphorylation, after ABA mediates SnRK2 release from its inhibitor complex PP2C-SnRK2. Recent studies have suggested that Raf-like protein kinases (RAFs) could phosphorylate SnRK2 and participate in ABA-triggered SnRK2 activation. Thus, how SnRK2 kinases are quickly activated during ABA signaling still needs to be clarified. Here, we used ATP analog and corresponding mutated SnRK2 to label the phosphorylation on SnRK2. The result showed that both B2/3 RAFs directly phosphorylate SnRK2.6 in the kinase activation loop. This trans-phosphorylation by RAFs stimulated SnRK2 activation, and the activated SnRK2s then intermolecularly trans-phosphorylate other SnRK2s that are not yet activated. No RAF kinases will fail the initiate of SnRK2 activation, and genetic evidence has proved that high-order Arabidopsis mutants lacking multiple B2 and B3 RAFs show ABA hyposensitivity and SnRK2 inactivation. So, our findings revealed a unique initiation and ampliﬁcation mechanism of SnRK2 activation by which RAF participate in ABA signaling.

Deciphering AGO7 interactome to understand siRNA bodies' composition and dynamics in the context of stress

AtAGO1 and AtAGO7 are the main effectors of post-transcriptional gene silencing in Arabidopsis. At AGO7 is a crucial component of the tasiRNA biogenesis pathway, which plays a crucial role in the development of leaf, flower, and root systems by targeting the auxin response factors AtARF3/4 for silencing. We conducted a proximity labelling study and found that AtFIM5, an actin-associated protein, is highly abundant in the vicinity of AtAGO7. Given that the tasiRNA biogenesis machinery localizes to the membraneless siRNA bodies that move along the actin cytoskeleton, we investigated the potential role of AtFIM5 in siRNA body dynamics. We found that AtFIM5 localization is stress-responsive and changes in AtFIM5 behaviour impact on siRNA bodies shape and movement. Additionally, we identified several putative AtAGO7 interactors that colocalize with siRNA bodies and have the potential to generate liquid-liquid phase-separated condensates. Our research aims to understand the impact of stress onto siRNA body dynamics, composition, and fluidity. We believe that unravelling the fine regulation of these condensates will shed light on long-standing questions such as how the biogenesis of secondary sRNA is favoured over RNA degradation. The integration of stress biology into tasi RNA metabolism may pave the way for innovative breeding strategies for more resilient crops.

Abscisic acid regulates stomatal production by imprinting a SnRK2 kinase–mediated phosphocode on the master regulator SPEECHLESS

Stomata, the epidermal pores for gas exchange between plants and the atmosphere, are the major sites of water loss. During water shortage, plants limit the formation of new stoma via the phytohormone abscisic acid (ABA) to conserve water. However, how ABA suppresses stomatal production is largely unknown. Here, we demonstrate that three core SnRK2 kinases of ABA signaling inhibit the initiation and proliferation of the stomatal precursors in Arabidopsis. We show that the SnRK2s function within the precursors and directly phosphorylate SPEECHLESS (SPCH), the master transcription factor for stomatal initiation. We identify specific SPCH residues targeted by the SnRK2s, which mediate the ABA/drought-induced suppression of SPCH and stomatal production. This SnRK2-specific SPCH phosphocode connects stomatal development with ABA/drought signals and enables the independent control of this key water conservation response. Our work also highlights how distinct signaling activities can be specifically encoded on a master regulator to modulate developmental plasticity.

Remember the rain: Epigenetic factors regulating flood stress memory in plants

This talk will not be available to watch on demand

Epigenetic regulation of memory has been well studied mechanism in eukaryotes. However, the ability of plants to epigenetically encode prior stress (i.e. stress memory) has only recently been reported. The current state-of-the art research supports the hypothesis that different epigenetic mechanisms play an important role in stress memory and regulation of response upon subsequent stress events. However, the capacity to epigenetically encode flooding stress memory has not been described in plants.

In order to understand the stress memory encoding mechanisms, we performed an RNA-Seq of Arabidopsis thaliana wild type and VERNALIZATION2 (VRN2) mutant plants. VRN2 is a regulator of plant chromatin signature whose mutant lacks flooding stress memory. Our preliminary results describe a transcriptional switch caused by priming stress that leads to a more targeted response upon subsequent flooding stress. This apparent encoding of the stress memory relies on several families of transcription factors (TFs). These TFs regulate the expression of a variety of tolerance genes that show flooding memory behaviour. Moreover, some of these tolerance genes are dependent on VRN2, connecting epigenetic regulation to stress memory. Collectively, genes showing memory of previous flooding events can be an important tool for improvement of future crops, ultimately responding to global food security crisis.