Abstract
Brassinosteroids (BRs) are steroid hormones that coordinate fundamental developmental programs in plants. In this study we show that in addition to the well established roles of BRs in regulating cell elongation and cell division events, BRs also govern cell fate decisions during stomata development in Arabidopsis thaliana. In wild-type A. thaliana, stomatal distribution follows the one-cell spacing rule; that is, adjacent stomata are spaced by at least one intervening pavement cell. This rule is interrupted in BR-deficient and BR signaling-deficient A. thaliana mutants, resulting in clustered stomata. We demonstrate that BIN2 and its homologues, GSK3/Shaggy-like kinases involved in BR signaling, can phosphorylate the MAPK kinases MKK4 and MKK5, which are members of the MAPK module YODA-MKK4/5-MPK3/6 that controls stomata development and patterning. BIN2 phosphorylates a GSK3/Shaggy-like kinase recognition motif in MKK4, which reduces MKK4 activity against its substrate MPK6 in vitro. In vivo we show that MKK4 and MKK5 act downstream of BR signaling because their overexpression rescued stomata patterning defects in BR-deficient plants. A model is proposed in which GSK3-mediated phosphorylation of MKK4 and MKK5 enables for a dynamic integration of endogenous or environmental cues signaled by BRs into cell fate decisions governed by the YODA-MKK4/5-MPK3/6 module.
Highlights
Brassinosteroids (BRs) are plant steroids that signal through the inhibition of GSK3/Shaggy-like kinases such as BIN2
We demonstrate that BIN2 and its homologues, GSK3/Shaggy-like kinases involved in BR signaling, can phosphorylate the MAPK kinases MKK4 and MKK5, which are members of the MAPK module YODA-MKK4/5-MPK3/6 that controls stomata development and patterning
Brz treatment had the same effects on stomata distribution. It induced stomatal patterning defects; stomata clustering occurred at a rate of 14.0%. To investigate whether this was a phenotype induced by BR deficiency or whether it was due to defective BR signaling, we analyzed the BR signaling-deficient mutants bri1-1 [38] and bin2-1 [39], as well as plants overexpressing ASK, an A. thaliana GSK3/Shaggy-like kinase that acts redundantly with BIN2 in BR signaling [29]
Summary
Brassinosteroids (BRs) are plant steroids that signal through the inhibition of GSK3/Shaggy-like kinases such as BIN2. Results: We show here that BIN2 phosphorylates MKK4, which inhibits its activity against MPK6, in a MAPK module that controls stomata patterning. We demonstrate that BIN2 and its homologues, GSK3/Shaggy-like kinases involved in BR signaling, can phosphorylate the MAPK kinases MKK4 and MKK5, which are members of the MAPK module YODA-MKK4/5-MPK3/6 that controls stomata development and patterning. In the leaf epidermis stomatal distribution follows the onecell spacing rule; that is, adjacent stomata are spaced by at least one intervening pavement cell [16, 17] This rule is disrupted in the BR biosynthesis mutant cpd (constitutive photomorphogenesis and dwarfism) where stomatal duplications were reported [18], indicating that BRs may impact on cellular patterning in stomata development. A model in which BR signaling impacts on stomata patterning by suppressing BIN2-mediated phosphorylation of MKK4/5 at Thr-234 and activating the YDAMKK4/5-MPK3/6 module is presented and discussed
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