Hormonal regulation of moss protonema development and the possible origin of plant hormonal responses in bryophytes

Abstract

The protonema of mosses is a far simpler paradigm to understand the mechanism of hormonal action and tolerance to abiotic stresses in plants. Its developmental biology, responses to hormones and the similarity of signaling mechanisms with higher plants are reviewed. There is strong evidence for second messenger role of calcium ions in the action of cytokinin. Multiple calcium-dependent protein kinases (CDPKs) are present in the protonema. The Funaria hygrometrica CDPK gene (FhCDPK) shows the characteristic catalytic and autoinhibitory domains, the four EF hands and the highest homology to CDPKs from higher plants but far lower to liverwort or other moss CDPK genes. A 38 kDa myelin basic protein kinase (MBP kinase) is activated within minutes by abscisic acid (ABA) and salinity. As ABA also confers tolerance against desiccation and freezing and the wheat ABA-inducible promoter is fully functional in mosses, the ABA signaling mechanism seems to be highly conserved. In plants, the CDPKs are involved not only in hormonal signaling but also in the acclimation response against abiotic stresses. The manipulation of signal transduction components such as transcription factors, CDPKs and calcineurin have emerged as viable strategies to genetically engineer the stress tolerant plants. There is increasing evidence to support the origin of plant hormonal responses at the level of bryophytes.