The RAF‐like mitogen‐activated protein kinase kinase kinases RAF22 and RAF28 are required for the regulation of embryogenesis in Arabidopsis

Abstract

Summary In Arabidopsis, embryonic development follows a stereotypical pattern of cell division. Although many factors have been reported to participate in establishment of the proper embryonic pattern, the molecular mechanisms underlying pattern formation are unclear. In this study we showed that RAF22 and RAF28, two RAF‐like mitogen‐activated protein kinase kinase kinases (MAPKKKs) in Arabidopsis, are involved in the regulation of embryogenesis. The double knockout mutant of RAF22 and RAF28 was embryo lethal. A large proportion of the raf22−/−raf28+/− mutant embryos exhibited various defects, including disordered proembryo cell divisions, disruption of the bilaterally symmetrical structure, abnormally formative divisions of hypophysis and exaggerated suspensor growth. Whereas the kinase active form of RAF22 could complement these embryonic aberrant phenotypes, the kinase inactive form could not. The restrictive expression of the basal cell fate marker WOX8 in the abnormally dividing suspensor cells and the apical cell linage marker WOX2 in the abnormal proembryos indicated that apical and basal cell fates were unchanged in the abnormal embryos. The polar distribution of the auxin maxima and the PIN1 and PIN7 auxin transporters was markedly altered in the abnormal embryos. Our results suggest that RAF22 and RAF28 are important components of embryogenesis and that auxin polar transport may be involved in this regulation.

Arabidopsis group C Raf-like protein kinases negatively regulate abscisic acid signaling and are direct substrates of SnRK2

“…S11 C and D). It is known that some group C Raf-like kinases function in various tissues, for example, HT1 or BLUE LIGHT-DEPENDENT H + -ATPASE PHOSPHORYLATION (BHP) in guard cells and Raf28 in embryogenesis (29,33,34). Thus, Raf36 and Raf22 may also have different functions in different tissues and developmental stages.…”

Auxin and Target of Rapamycin Spatiotemporally Regulate Root Organogenesis

“…Auxin synthesis in the integuments of the ovule is upregulated and transported to the apical domain of proembryo, leading to an increase in auxin levels and ultimately mediating early embryonic development [ 55 ]. During embryonic development, the root meristem initiates at the globular stage, where the auxin transporters PIN1/7 transport auxin to the suspensor cells, while the transported auxin accumulates in the uppermost cells of the suspensor [ 56 ]. However, auxin accumulation is insufficient to complete the establishment of hypophysis, and additional auxin-dependent signals are required.…”

The Raf‐like kinase Raf36 negatively regulates plant resistance against the oomycete pathogen Phytophthora parasitica by targeting MKK2

“…Plant MAPKKKs consist of three families: the MEKK family, the Raf-like family, and the ZIK family (Ichimura et al, 2002;Jonak et al, 2002). The MEKK kinases usually function upstream and activate the MAPKK-MAPK cascades (Thulasi Devendrakumar et al, 2018), but the Raf-like kinases interact with different kinds of substrates and participate in diverse life activities (Fàbregas et al, 2020;Hayashi et al, 2020;Wang et al, 2018). Raf-like kinases also play roles in plant-pathogen interactions.…”