Spatial differences in stoichiometry of EGR phosphatase and Microtubule-Associated Stress Protein 1 control root meristem activity during drought stress

Abstract

During moderate severity drought and low water potential (Ψw) stress, poorly understood signaling mechanisms restrict both meristem cell division and subsequent cell expansion. We found that the Clade E Growth-Regulating 2 (EGR2) protein phosphatase and Microtubule Associated Stress Protein 1 (MASP1) differed in their stoichiometry of expression across the root meristem and had opposing effects on root meristem activity at low Ψw. Ectopic MASP1 or EGR expression increased or decreased, respectively, root meristem size and root elongation during low Ψw stress. This, along with the ability of phosphomimic MASP1 to overcome EGR suppression of root meristem size and observation that ectopic EGR expression had no effect on unstressed plants, indicated that during low Ψw EGR activation and attenuation of MASP1 phosphorylation in their overlapping zone of expression determines root meristem size and activity. Ectopic EGR expression also decreased root cell size at low Ψw. Conversely, both the egr1-1egr2-1 and egr1-1egr2-1masp1-1 mutants had similarly increased root cell size; but, only egr1-1egr2-1 had increased cell division. These observations demonstrated that EGRs affect meristem activity via MASP1 but affect cell expansion via other mechanisms. Interestingly, EGR2 was highly expressed in the root cortex, a cell type important for growth regulation and environmental response.

Size and activity of the root meristem: A key for drought resistance and a key model of drought‐related signaling

“…In our previous study (Longkumer et al, 2022), we measured fully expanded cell size and the number of dividing cells in the root meristem, along with the root elongation rates mentioned above, for wild‐type, egr1‐1egr2‐1 , masp1‐1 , and egr1‐1egr2‐1masp1‐1 mutants as well as transgenic lines ectopically expressing EGR1 or MASP1 ( 35S:EGR1 , 35S:MASP1 ). Average cell size and meristem cell number are replotted in Figure 2B.…”

“…Instead, the ability to maintain a larger population of dividing cells in the root tip (i.e., increased meristem size) allowed enhanced growth maintenance without a substantial change in T c . The most likely cause of this is that in 35S:MASP1 plants, the zone of MASP1 expression extends out further from the quiescent center (Longkumer et al, 2022), and this may allow cells at the distal end of the meristem to complete an additional cell division cycle(s) before transitioning to cell expansion.…”

“…EGRs are negative regulators of root growth during low ψ w . Ectopic expression of EGR1 ( 35S:EGR1 ) had no detectable effect on plant growth under standard conditions but strongly downregulated root elongation after plants were transferred to moderate severity low ψ w (Figure 2A; Longkumer et al, 2022). The decreased root elongation of 35S:EGR1 compared to wild‐type was most dramatic in the initial 48 h after transfer to low ψ w .…”

“…In the sections below, we further explore these and other key questions of how root growth and the root meristem respond to low ψ w indicative of moderate severity drought stress. To illustrate these questions, we will draw upon our recent study of the clade E Growth‐Regulating (EGR) protein phosphatases and Microtubule Associated Stress Protein 1 (MASP1) (Bhaskara et al, 2017; Longkumer et al, 2022). EGRs are negative regulators of root growth during low ψ w .…”

“…(B) Length of fully expanded root epidermal cells and number of epidermal cells in the root meristem measured 4 or 5 days, respectively, after transfer to control or low ψ w stress (−0.7 MPa) treatments. Graphs show the means of data originally reported in Longkumer et al (2022). (C) Average cell flux, cell production rate, and cell cycle times ( T c ) for the whole root meristem were calculated for the epidermal cell file using root elongation rates (A), final cell length (B), and meristem cell number (B).…”