Hey,

There are two approaches here.

1, org.At.tair.db

You can use the annotation DB packages from Bioconductor, specifically org.At.tair.db.

Copying my own answer from here: A: Biomart query returns NA when searching for entrez_id, while manual search works

library(org.At.tair.db)

genes <- c("AT2G14610","AT4G23700","AT3G26830",
  "AT3G15950","AT3G54830","AT5G24105")

keytypes(org.At.tair.db)

mapIds(org.At.tair.db, keys = genes,
  column = c('SYMBOL'), keytype = 'TAIR')
 AT2G14610  AT4G23700  AT3G26830  AT3G15950  AT3G54830  AT5G24105 
 "AtCAPE9"  "ATCHX17" "CYP71B15"     "NAI2"         NA    "AGP41"

select(org.At.tair.db, keys = genes,
  column = c('ENTREZID', 'SYMBOL', 'REFSEQ'), keytype = 'TAIR')

       TAIR ENTREZID   SYMBOL       REFSEQ
1  AT2G14610   815949  AtCAPE9    NM_127025
2  AT2G14610   815949  AtCAPE9    NP_179068
3  AT2G14610   815949    ATPR1    NM_127025
4  AT2G14610   815949    ATPR1    NP_179068
5  AT2G14610   815949       PR    NM_127025
6  AT2G14610   815949       PR    NP_179068
7  AT2G14610   815949      PR1    NM_127025
8  AT2G14610   815949      PR1    NP_179068
9  AT4G23700   828470  ATCHX17 NM_001341626
10 AT4G23700   828470  ATCHX17    NM_118501
11 AT4G23700   828470  ATCHX17 NP_001328705
12 AT4G23700   828470  ATCHX17    NP_194101
13 AT4G23700   828470    CHX17 NM_001341626
14 AT4G23700   828470    CHX17    NM_118501
15 AT4G23700   828470    CHX17 NP_001328705
16 AT4G23700   828470    CHX17    NP_194101
17 AT3G26830   822298 CYP71B15    NM_113595
18 AT3G26830   822298 CYP71B15    NP_189318
19 AT3G26830   822298     PAD3    NM_113595
20 AT3G26830   822298     PAD3    NP_189318
21 AT3G15950   820839     NAI2 NM_001035631
22 AT3G15950   820839     NAI2 NM_001338191
23 AT3G15950   820839     NAI2 NM_001338192
24 AT3G15950   820839     NAI2 NM_001338193
25 AT3G15950   820839     NAI2    NM_112465
26 AT3G15950   820839     NAI2 NP_001030708
27 AT3G15950   820839     NAI2 NP_001326807

2, biomaRt

require(biomaRt)
tair_mart <- useMart(biomart = 'plants_mart',
  host = 'plants.ensembl.org', dataset = 'athaliana_eg_gene')

head(listAttributes(tair_mart), 15)

annot <- getBM(
  values = genes,
  mart = tair_mart,
  attributes = c('ensembl_gene_id', 'entrezgene_id',
    'description', 'external_gene_name'),
  filters = 'ensembl_gene_id')

  ensembl_gene_id entrezgene_id
1       AT2G14610        815949
2       AT3G15950        820839
3       AT3G26830        822298
4       AT3G54830            NA
5       AT4G23700        828470
6       AT5G24105       2745995
                                                                                          description
1                             Pathogenesis-related protein 1 [Source:UniProtKB/Swiss-Prot;Acc:P33154]
2                                          TSA1-like protein [Source:UniProtKB/Swiss-Prot;Acc:Q9LSB4]
3 Bifunctional dihydrocamalexate synthase/camalexin synthase [Source:UniProtKB/Swiss-Prot;Acc:Q9LW27]
4                                                                                                    
5                                  Cation/H(+) antiporter 17 [Source:UniProtKB/Swiss-Prot;Acc:Q9SUQ7]
6                                 Arabinogalactan protein 41 [Source:UniProtKB/Swiss-Prot;Acc:Q8L9T8]
  external_gene_name
1                PR1
2               NAI2
3           CYP71B15
4                   
5              CHX17
6              AGP41

If you want a complete table from biomaRt, just use:

annotComplete <- getBM(
  mart = tair_mart,
  attributes = c('ensembl_gene_id', 'entrezgene_id',
    'description', 'external_gene_name'))

dim(annotComplete)
[1] 33528     4

Kevin

Using EntrezDirect:

$ esearch -db gene -query "Arabidopsis thaliana [ORGN]" | efetch -format tabular | head -5
tax_id  Org_name    GeneID  CurrentID   Status  Symbol  Aliases description other_designations  map_location    chromosome  genomic_nucleotide_accession.version    start_position_on_the_genomic_accession end_position_on_the_genomic_accession   orientation exon_count  OMIM
3702    Arabidopsis thaliana    816394  0   live    PHYB    AT2G18790, HY3, MSF3.17, MSF3_17, OOP1, OUT OF PHASE 1, PHYTOCHROME B, phytochrome B    phytochrome B   phytochrome B       2   NC_003071.7 8139756 8144461 plus    3
3702    Arabidopsis thaliana    830878  0   live    FLC AT5G10140, AGAMOUS-like 25, AGL25, FLF, FLOWERING LOCUS C, FLOWERING LOCUS F, MADS BOX PROTEIN FLOWERING LOCUS F, REDUCED STEM BRANCHING 6, RSB6, T31P16.130, T31P16_130    K-box region and MADS-box transcription factor family protein   K-box region and MADS-box transcription factor family protein       5   NC_003076.8 3173382 3179448 minus   8
3702    Arabidopsis thaliana    817857  0   live    COP1    AT2G32950, ARABIDOPSIS THALIANA CONSTITUTIVE PHOTOMORPHOGENIC 1, ATCOP1, CONSTITUTIVE PHOTOMORPHOGENIC 1, DEETIOLATED MUTANT 340, DET340, EMB168, EMBRYO DEFECTIVE 168, FUS1, FUSCA 1, T21L14.11, T21L14_11 Transducin/WD40 repeat-like superfamily protein Transducin/WD40 repeat-like superfamily protein     2   NC_003071.7 13977881    13983609    plus    13
3702    Arabidopsis thaliana    842859  0   live    FT  AT1G65480, F5I14.3, F5I14_3, FLOWERING LOCUS T, REDUCED STEM BRANCHING 8, RSB8  PEBP (phosphatidylethanolamine-binding protein) family protein  PEBP (phosphatidylethanolamine-binding protein) family protein      1   NC_003070.9 24331373    24333999    plus    4

OR

$ esearch -db gene -query "Arabidopsis thaliana [ORGN]" | esummary | xtract -pattern DocumentSummary -element Name,OtherAliases | head -5 | awk -F "\t|," '{OFS="\t"}{print $2,$1}'
AT2G18790   PHYB
AT5G10140   FLC
AT2G32950   COP1
AT1G65480   FT
AT1G09570   PHYA

Remove head command to get them all.