ABSTRACT
Meristems of seed plants continuously produce new cells for incorporation into maturing tissues. A tightly controlled balance between cell proliferation in the center and cell differentiation at the periphery of the shoot meristem maintains its integrity. Here, we describe the role of three GRAS genes, named LOST MERISTEMS genes, in shoot apical meristem maintenance and axillary meristem formation. Under short photoperiods, the lom1 lom2 and lom1 lom2 lom3 mutants have arrested meristems characterized by an over-proliferation of meristematic cells and loss of polar organization. They also show early arrest of axillary meristem development and formation of ectopic meristematic cell clusters within the stem. LOM1 and LOM2 transcripts accumulate in the peripheral and basal zones of the SAM and in vascular strands. We show that LOM1 and LOM2 promote cell differentiation at the periphery of shoot meristems and help to maintain their polar organization.
Subject(s)
Arabidopsis Proteins/metabolism , Arabidopsis/physiology , Cell Differentiation , Meristem/growth & development , MicroRNAs/metabolism , Arabidopsis/cytology , Arabidopsis/genetics , Arabidopsis Proteins/genetics , Cell Division , Gene Expression Profiling , Gene Expression Regulation, Plant , Meristem/cytology , Mutation , Photoperiod , Plant Leaves/growth & developmentABSTRACT
An Arabidopsis cDNA coding for a previously uncharacterized isoform of inorganic pyrophosphatase was isolated. It was used to complement an E. coli mutant, demonstrating that it coded for an active enzyme. MgCl(2) was necessary for the protein's activity, whilst NaF inhibited it. The K(m) for pyrophosphate and the pH optimum of the protein was determined. The gene coding for this protein was expressed in all tissues, and its expression in rosette leaves was induced by incubation on metabolizable sugars. In vitro import experiments demonstrated that the protein could be imported into chloroplasts and localized to the stromal compartment.
Subject(s)
Arabidopsis Proteins/biosynthesis , Arabidopsis Proteins/chemistry , Arabidopsis/enzymology , Chloroplasts/enzymology , Inorganic Pyrophosphatase/biosynthesis , Inorganic Pyrophosphatase/chemistry , Amino Acid Sequence , Arabidopsis Proteins/genetics , Chloroplasts/metabolism , DNA, Complementary/metabolism , Escherichia coli/metabolism , Genetic Complementation Test , Hydrogen-Ion Concentration , Inorganic Pyrophosphatase/genetics , Inorganic Pyrophosphatase/metabolism , Kinetics , Magnesium Chloride/pharmacology , Molecular Sequence Data , Pisum sativum/enzymology , Phylogeny , Plastids/metabolism , Protein Isoforms , Protein Transport , RNA/chemistry , RNA, Messenger/metabolism , Sequence Homology, Amino Acid , Sodium Fluoride/pharmacology , Tissue DistributionABSTRACT
A functional screen in Escherichia coli was established to identify potato genes coding for proteins involved in transitory starch degradation. One clone isolated had a sequence very similar to a recently described chloroplast-targeted beta-amylase of Arabidopsis. Expression of the gene in E. coli showed that the protein product was a functional beta-amylase that could degrade both starch granules and solubilized amylopectin, while import experiments demonstrated that the beta-amylase was imported and processed into pea chloroplasts. To study the function of the protein in transitory starch degradation, transgenic potato plants were generated where its activity was reduced using antisense techniques. Analysis of plants reduced in the presence of this beta-amylase isoform showed that their leaves had a starch-excess phenotype, indicating a defect in starch degradation. In addition, it was shown that the antisense plants degraded only 8-30% of their total starch, in comparison with 50% in the wild type, over the dark period. This is the first time that a physiological role for a beta-amylase in plants has been demonstrated.
