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Description:
When Arabidopsis RLKs were first classified into different groups based on their extracellular domains in 2001, two were defined as unknown receptor kinases (Shiu and Bleecker, 2001). Recently, a rice RLK ARK1 (Arbuscular Receptor-like Kinase 1) was found to regulate arbuscular mycorrhizal symbiosis (Roth et al., 2018). A systematic phylogenetic analysis further showed that ARK and SIMILAR PROTEIN TO ARK 1 (SPARK1) harbor a newly identified SPARK domain in their extracellular regions, which has ancient origins in streptophyte algae (Montero et al., 2021). The hidden Markov model (HMM) PF19160 was used for the SPARK classification.

References:
1. Montero H, Lee T, Pucker B, Ferreras-Garrucho G, Oldroyd G, Brockington SF, Miyao A, Paszkowski U. 2021. A mycorrhiza-associated receptor-like kinase with an ancient origin in the green lineage. Proc Natl Acad Sci U S A. 118(25):e2105281118. doi: 10.1073/pnas.2105281118. PMID: 34161289.
2. Roth R, Chiapello M, Montero H, Gehrig P, Grossmann J, O'Holleran K, Hartken D, Walters F, Yang SY, Hillmer S, Schumacher K, Bowden S, Craze M, Wallington EJ, Miyao A, Sawers R, Martinoia E, Paszkowski U. 2018. A rice Serine/Threonine receptor-like kinase regulates arbuscular mycorrhizal symbiosis at the peri-arbuscular membrane. Nat Commun. 9(1):4677. doi: 10.1038/s41467-018-06865-z. PMID: 30410018.
3. Shiu SH, Bleecker AB. 2001. Receptor-like kinases from Arabidopsis form a monophyletic gene family related to animal receptor kinases. Proc Natl Acad Sci U S A. 98(19):10763-8. doi: 10.1073/pnas.181141598. PMID: 11526204.