Figure 8.

Model of EGR-MASP1 Function.

EGRs and MASP1 physically associate but have opposing effects on plant growth, including effects on cell expansion. EGRs are negative growth regulators and increased EGR expression at low ψ_w restrains growth. MASP1 is a positive regulator of growth and unknown posttranscriptional mechanism(s) lead to increased MASP1 protein accumulation at low ψ_w. This increased MASP1 stabilizes MTs (as indicated by the arrows between MASP1 and MTs) and preserves the competence to grow under mild to moderate low ψ_w stress. The effects of MASP1 on growth are epistatic to those of EGRs, and MASP1 Ser-670 phosphorylation is required for its growth-promoting activity but not for MT binding [EGR attenuation of MASP1 phosphorylation is indicated by the T-bar between EGR and MASP1; MASP1 phosphorylation by unknown kinase(s) is indicated by gray dashed arrow]. MASP1 phosphorylation may affect other aspects of its function such as binding to additional MT-associated proteins or clustering at specific sites along MTs (indicated by gray dashed arrows and question mark). Functional analysis of MASP1 indicates that the C-terminal portion binds MTs, while the N-terminal portion is required for MT bundling and stabilization and may also act to sequester MASP1 away from MTs, possibly by binding to other structures in the cell cortex (binding events shown in this study are indicated by black double-sided arrows; possible association of MASP1 with other proteins or cellular structures is indicated by gray dashed double-sided arrows). By these or related mechanisms, MASP1 stabilizes and protects MT organization in a way that promotes cell expansion and continued growth. Additional EGR target proteins are likely to be involved in drought-related signaling, for example, in the regulation of proline accumulation or MASP1-independent effects on MTs.