PhD Oral Exam - Kritika Bharti, Biology

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Stomata are microscopic valves in the plant epidermis that efficiently exchange gas while minimizing water loss. The patterning and differentiation of the stomata are regulated by multiple developmental and environmental signals. One of the primary signals involved in this regulation is the phytohormone abscisic acid (ABA), which is well known for its role in controlling stomatal movement by inducing stomatal closure under stress conditions, such as water deficit. ABA serves as a critical signal that enables plants to respond to environmental stresses, particularly drought, by minimizing water loss through the stomata. While the mechanisms by which ABA regulates the opening and closing of stomata are well-studied, our understanding of ABA’s role in controlling stomatal development and the mechanisms underlying its interaction with the stomatal development pathway are limited.

In this study, characterization of ABA-deficient and ABA-signaling mutants indicated that ABA regulation of stomatal development is mediated by upstream signaling components, including PYR/PYL/PCAR (ABA receptor), ABI1/ABI2 (ABA phosphatases), and SnRK2.2/2.3/2.6 (SnRK2 kinases) but is likely independent of the transcription factors in the core ABA signaling pathway. We examined the epidermal phenotype of ABA signaling mutants and found that abi1 abi2, which are mutants lacking two negative regulators of ABA signaling, showed a remarkable decrease in the number of stomata and non-stomatal cells on the plant epidermis. We further identified that only ABI1 and ABI2 phosphatases out of total nine ABA PP2Cs are involved in controlling stomatal development. Next, we performed a GUS assay to assess whether this was a consequence of a decrease in stomatal lineage cells and observed that the abi1 abi2 mutant had a reduced number of epidermal cells entering the stomatal lineages. Moreover, using high-order ABA and stomatal mutants, we investigated the molecular link between ABA signaling and stomatal developmental pathways. Our genetic interaction studies showed that ABIs act upstream of the stomatal receptors TOO MANY MOUTHS (TMM) and ERECTA family members, as well as mitogen-activated protein kinases (MAPKs) MPK3 and MPK6, and the transcription factor MUTE. However, ABI1/ABI2 phosphatases function downstream of stomatal ligands EPF1, EPF2, and STOMAGEN in the stomatal development pathway. Based on our genetic interaction data, we analyzed the physical interactions between ABI1/2 and ER family receptors, ER, ERL1, ERL2, and the receptor protein TMM. Our data demonstrated that ABI1 interacts with ER, ERL1, and ERL2, but not with ABI2.

Taken together, our data provide new insights into the previously undiscovered interactions between the ABA signaling pathway and the stomatal development pathway. These findings greatly expand our understanding of ABA’s role, not only in regulating stomatal movement but also in controlling stomatal patterning and development. By uncovering these connections, we indicate potential strategies that plants may use to optimize their growth and adapt their development in response to environmental stresses such as drought. This research advances our understanding of how ABA signaling modulates stomatal development, establishing the framework for future research aimed at deciphering the intricate links between ABA and stomatal developmental pathways.