Cell expansion is a central process in plant morphogenesis, and the elongation of roots and root hairs is essential for uptake of minerals and water from the soil. Ca2+ influx from the extracellular store is required for (and sets the rates of) cell elongation in roots. Arabidopsis thaliana rhd2 mutants are defective in Ca2+ uptake and consequently cell expansion is compromised--rhd2 mutants have short root hairs and stunted roots. To determine the regulation of Ca2+ acquisition in growing root cells we show here that RHD2 is an NADPH oxidase, a protein that transfers electrons from NADPH to an electron acceptor leading to the formation of reactive oxygen species (ROS). We show that ROS accumulate in growing wild-type (WT) root hairs but their levels are markedly decreased in rhd2 mutants. Blocking the activity of the NADPH oxidase with diphenylene iodonium (DPI) inhibits ROS formation and phenocopies Rhd2-. Treatment of rhd2 roots with ROS partly suppresses the mutant phenotype and stimulates the activity of plasma membrane hyperpolarization-activated Ca2+ channels, the predominant root Ca2+ acquisition system. This indicates that NADPH oxidases control development by making ROS that regulate plant cell expansion through the activation of Ca2+ channels.
Monoclonal antibodies recognizing un-esterified (JIM5) and methyl-esterified (JIM7) epitopes of pectin have been used to locate these epitopes by indirect immunofluorescence and immunogold electron microscopy in the root apex of carrot (Daucus carota L.). Both antibodies labelled the walls of cells in all tissues of the developing root apex. Immunogold labelling observed at the level of the electron microscope indicated differential location of the pectin epitopes within the cell walls. The un-esterified epitope was located to the inner surface of the primary cell walls adjacent to the plasma membrane, in the middle lamella and abundantly to the outer surface at intercellular spaces. In contrast, the epitope containing methyl-esterified pectin was located evenly throughout the cell wall. In root apices of certain other species the JIM5 and JIM7 epitopes were found to be restricted to distinct tissues of the developing roots. In the root apex of oat (Avena sativa L.), JIM5 was most abundantly reactive with cell walls at the region of intercellular spaces of the cortical cells. JIM7 was reactive with cells of the cortex and the stele. Neither epitope occurred in walls of the epidermal or root-cap cells. These pattern of expression were observed to derive from the very earliest stages of the development of these tissues in the oat root meristem and were maintained in the mature root. In the coleoptile and leaf tissues of oat seedlings, JIM5 labelled all cells abundantly whereas JIM7 was unreactive. Other members of the Gramineae and also the Chenopodiaceae are shown to express similar restricted spatial patterns of distribution of these pectin epitopes in root apices.
Root hairs and rhizoids are cells with rooting functions in land plants. We describe two basic helix-loop-helix transcription factors that control root hair development in the sporophyte (2n) of the angiosperm Arabidopsis thaliana and rhizoid development in the gametophytes (n) of the bryophyte Physcomitrella patens. The phylogeny of land plants supports the hypothesis that early land plants were bryophyte-like and possessed a dominant gametophyte and later the sporophyte rose to dominance. If this hypothesis is correct, our data suggest that the increase in morphological complexity of the sporophyte body in the Paleozoic resulted at least in part from the recruitment of regulatory genes from gametophyte to sporophyte.
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