Root strategies in response to salinity: from cell wall to root shape

This thesis is focused on the underlying mechanisms of salt signaling pathways that modulate root directional growth and root shape when exposed to salt stress. We emphasize the crucial role of cell wall modifications and cell wall localized receptors in modulating root responses and ion accumulation under salt stress. In Chapter 1 we introduce how soil salinity threatens plant growth, function and survival; emphasize the importance of maintaining Na+ and K+ ionic homeostasis during ionic stresses; introduce various phytohormones that are involved in salt signaling pathways. Moreover, the current knowledge of salt-induced root directional growth and how plant cell walls monitor the outside environment especially under salt stress are introduced in this chapter.  Chapter 2 focuses on the developmental, cellular, and molecular mechanisms that govern growth responses of root systems under salinity. In Chapter 3 we establish a new rapid salt-induced tilting assay (SITA) which also reports the specific modulation by NaCl of root growth direction upon gravitropic challenge with high time resolution. Combining SITA with a natural variation, we performed genome wide association studies (GWAS) to find the underlying genetic basis of the response. To continue, in Chapter 4 we focus on the candidate gene ExAD identified in Chapter 3. Being a member of the arabinosyltransferases enzyme family, ExAD adds the Hyp-Araf1-4 side-chain at the 4th specific position on extensins. We show the roles of ExAD in root directional growth responses and ion accumulation in response to salt treatment. By comprehensive microarray polymer profiling (CoMPP) and dot-blot, we reveal that ExAD is involved in the alteration of polysaccharide signals under salt stress. ExAD-dependent Hyp-Araf4 is increased when exposed to salt in Col-0, but is absent in the exad-1 mutant. Moreover, cell wall thickness increases in exad-1 root epidermal cells under salt stress. Therefore, we suggest a link between cell wall extensin Hyp-arabinosylation and salt responses. In Chapter 5 we focus on cell wall localized L-type LecRKs members in Arabidopsis, AtLecRK-IV.1 and AtLecRK-IV.2. We show that a double mutant of these, the lecrk4.1/4.2 mutant, shows reduced salt-induced root directional changes in both the halotropism assay and SITA. Additionally, we show that the LecRK-IV.1/LecRK-IV.2 combination regulates CWI responses that are induced by the cellulose biosynthesis inhibitor isoxaben in a THESEUS1 (THE1) dependent fashion. Interestingly, the lecrk4.1/4.2/the1-1 triple mutant showed a prolonged root salt avoidance compared to the lecrk4.1/4.2 and the1-1 mutant phenotypes. We suggest that LecRK-IV.1/LecRK-IV.2 may be involved in long-term salt stress responses via different pathways, and together contribute to salt-induced root directional growth responses in halotropism. Our data indicate a correlation between salt stress and CWI response. In Chapter 6, I further discuss the results from the previous chapters, place them in a wider context and provide future perspectives on the role of cell wall modifications in salt stress sensing and root salt responses.