Pectin of middle lamellae consists predominantly of homogalacturonan. Known enzymes in homogalacturonan biosynthesis are the galacuronosyl transferase QUASIMODO 1 (QUA1), the pectin methyltransferase QUASIMODO 2 (QUA2) and GALACTURONOSYLTRANSFERASE 9 (GAUT9), which is a homolog of QUA1. The qua mutants have a dwarf phenotype and reduced cell adhesion due to defective middle lamellae, while the gaut9 mutant has no visible growth defect. The ELMO proteins were discovered previously in a mutant screen for diminished cell adhesion, and mutants have enhanced Ruthenium Red staining of hypocotyls and curling cells at the surface, resembling Elmo’s fur. This phenotype is similar to the cell adhesion phenotype of qua mutants, however, ELMOs have no predicted enzymatic function. While it has been demonstrated that ELMO1 can interact with QUA1, QUA2 and GAUT9 in a heterologous system, the in planta function of ELMOs remains unclear.
The researchers found that Na+ AND K+-SENSITIVE 1 (NKS1), which was previously described as ELMO4, is co-expressed with cellulose synthase, QUA1 and GAUT9. It localizes to the medial Golgi, and mutants have a reduced hypocotyl elongation rate, cell adhesion and galacturonic acid content. Interaction of NKS1/ELMO4 with QUA1, QUA2, ELMO1, ELMO2, GAUT9 and QUA2-LIKE2 (QUL2) was verified in planta, suggesting that it may form an interaction platform for a pectin synthesis complex. Intriguingly, while the trans-Golgi network is not affected in nks1/elmo4 mutants, the Golgi shows curved stacks with fewer cisternae and slower stack movement. This suggests that functional pectin synthesis in the Golgi is not just required for cell wall and tissue morphology, but also for Golgi morphology itself, which could potentially be a side effect of defective secretion.
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