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Somatic embryogenesis receptor-like kinase 1 (SERK1) is a membrane receptor that might serve as common co-regulator of plant cell differentiation processes by forming heterodimers with specific receptor-like kinases. The Coffea canephora SERK1 homolog (CcSERK1) was cloned in this work, and its early function in the transcription of embryogenesis master genes and of genes encoding proteins involved in auxin metabolism was investigated by externally manipulating its expression in embryogenic leaf explants, before the appearance of embryogenic structures. Overexpression of CcSERK1 early during embryogenesis caused an increase in the number of somatic embryos when the 55-day process was completed. Suppression of CcSERK1 expression by RNA interference almost abolished somatic embryogenesis. Real time-PCR experiments revealed that the transcription of the CcAGL15, CcWUS, CcBBM, CcPKL, CcYUC1, CcPIN1 and CcPIN4 homologs was modified in direct proportion to the expression of CcSERK1 and that only CcLEC1 was inversely affected by the expression levels of CcSERK1. The expression of the CcYUC4 homolog was induced to more than 80-fold under CcSERK1 overexpression conditions, but it was also induced when CcSERK1 expression was silenced. The level of CcTIR1 was not affected by CcSERK1 overexpression but was almost abolished during CcSERK1 silencing. These results suggest that CcSERK1 co-regulates the induction of somatic embryogenesis in Coffea canephora by early activation of YUC-dependent auxin biosynthesis, auxin transport mediated by PIN1 and PIN4, and probably auxin perception by the TIR1 receptor, leading to the induction of early-stage homeotic genes (CcAGL15, CcWUS, CcPKL and CcBBM) and repression of late-stage homeotic genes (CcLec1)., (© 2018 Scandinavian Plant Physiology Society.)

Addition of a toxic concentration of aluminum (Al) to cell suspension cultures of Coffea arabica L. induced the rapid and transient activation of a protein kinase that phosphorylates myelin basic protein (MBP), as revealed by in-gel kinase assays. This enzyme with an apparent molecular mass of 58 kDa was activated shortly after cells were exposed to 50 microM AlCl(3), a concentration previously shown to produce toxicity in plant cells in vitro. The activity of this kinase dropped to basal levels after 20 min of Al addition; this activity is specific for MBP as it could not be detected when casein or histone H1 were used as substrates. Analysis of the same cell extracts with antibodies that specifically recognize bis-phosphorylated (active) mitogen-activated protein kinases (MAP kinases), revealed the presence of a phosphoprotein with an apparent molecular mass of 58 kDa, which showed the same response to Al as the protein kinase revealed by the in-gel kinase assays. Furthermore, immunoprecipitation with an antibody directed against mammalian MAP kinases depleted both the enzymatic activity and the phosphoprotein from the cell extracts, suggesting that the 58 kDa kinase and the 58 kDa phosphoprotein from C. arabica cells are the same protein, and that it can be actually a member of the MAP kinase family of protein kinases. Since its activity is enhanced dramatically after addition of AlCl(3) to the medium, we can speculate that Al toxicity in plants could be perceived through the MAP kinase signal transduction pathway.