Search

Key Message: Castor patatin-like phospholipase A IIIβ facilitates the exclusion of hydroxy fatty acids from phosphatidylcholine in developing transgenic Arabidopsis seeds. Hydroxy fatty acids (HFAs) are industrial useful, but their major natural source castor contains toxic components. Although expressing a castor OLEATE 12-HYDROXYLASE in Arabidopsis thaliana leads to the synthesis of HFAs in seeds, a high proportion of the HFAs are retained in phosphatidylcholine (PC). Thus, the liberation of HFA from PC seems to be critical for obtaining HFA-enriched seed oils. Plant phospholipase A (PLA) catalyzes the hydrolysis of PC to release fatty acyl chains that can be subsequently channeled into triacylglycerol (TAG) synthesis or other metabolic pathways. To further our knowledge regarding the function of PLAs from HFA-producing plant species, two class III patatin-like PLA cDNAs (pPLAIIIβ or pPLAIIIδ) from castor or Physaria fendleri were overexpressed in a transgenic line of A. thaliana producing C18-HFA, respectively. Only the overexpression of RcpPLAIIIβ resulted in a significant reduction in seed HFA content with concomitant changes in fatty acid composition. Reductions in HFA content occurred in both PC and TAG indicating that HFAs released from PC were not incorporated into TAG. These results suggest that RcpPLAIIIβ may catalyze the removal of HFAs from PC in the developing seeds synthesizing these unusual fatty acids.

Key Message: Ethylene biosynthesis is regulated in reproductive tissues in response to heat stress in a manner to optimize resource allocation to pollinated fruits with developing seeds. High temperatures during reproductive development are particularly detrimental to crop fruit/seed production. Ethylene plays vital roles in plant development and abiotic stress responses; however, little is known about ethylene's role in reproductive tissues during development under heat stress. We assessed ethylene biosynthesis and signaling regulation within the reproductive and associated tissues of pea during the developmental phase that sets the stage for fruit-set and seed development under normal and heat-stress conditions. The transcript abundance profiles of PsACS [encode enzymes that convert S-adenosyl-L-methionine to 1-aminocyclopropane-1-carboxylic acid (ACC)] and PsACO (encode enzymes that convert ACC to ethylene), and ethylene evolution were developmentally, environmentally, and tissue-specifically regulated in the floral/fruit/pedicel tissues of pea. Higher transcript abundance of PsACS and PsACO in the ovaries, and PsACO in the pedicels was correlated with higher ethylene evolution and ovary senescence and pedicel abscission in fruits that were not pollinated under control temperature conditions. Under heat-stress conditions, up-regulation of ethylene biosynthesis gene expression in pre-pollinated ovaries was also associated with higher ethylene evolution and lower retention of these fruits. Following successful pollination and ovule fertilization, heat-stress modified PsACS and PsACO transcript profiles in a manner that suppressed ovary ethylene evolution. The normal ethylene burst in the stigma/style and petals following pollination was also suppressed by heat-stress. Transcript abundance profiles of ethylene receptor and signaling-related genes acted as qualitative markers of tissue ethylene signaling events. These data support the hypothesis that ethylene biosynthesis is regulated in reproductive tissues in response to heat stress to modulate resource allocation dynamics.

Thlaspi arvense, a wild species from the Brassicaceae family, was shown to have a higher level of freezing tolerance than either of its close relatives, the model plant Arabidopsis thaliana or the crop Brassica napus (canola). Over 600 clones were sequenced from a subtractive cDNA library generated from cold treated T. arvense tissue, establishing that T. arvense shared significant sequence identity with both A. thaliana and B. napus (90-92%). In light of the strong sequence similarity between T. arvense and A. thaliana and to exploit the available genomics resources for Arabidopsis, the efficacy of using long 70 mer oligonucleotide whole genome Arabidopsis microarrays was tested for T. arvense. Gene expression in T. arvense leaf tissue during the very early stages of cold acclimation (or cold stress) was assayed at three time points and compared to an untreated control. This analysis highlights some of the difficulties and benefits of using cross-species microarray analysis. The data suggested that T. arvense responds in a similar fashion to cold stress as the model plant A. thaliana. However, for a number of genes quantitative differences in the level and timing of expression were identified. One of the most notable differences suggested that sulphur assimilation leading to the increased production of the methyl donor S-adenosyl-methionine was playing a role in the response of T. arvense to cold stress.

Transformation with the Arabidopsis bHLH gene 35S:GLABRA3 (GL3) produced novel B. napus plants with an extremely dense coverage of trichomes on seedling tissues (stems and young leaves). In contrast, trichomes were strongly induced in seedling stems and moderately induced in leaves of a hairy, purple phenotype transformed with a 2.2 kb allele of the maize anthocyanin regulator LEAF COLOUR (Lc), but only weakly induced by BOOSTER (B-Peru), the maize Lc 2.4 kb allele, or the Arabidopsis trichome MYB gene GLABRA1 (GL1). B. napus plants containing only the GL3 transgene had a greater proportion of trichomes on the adaxial leaf surface, whereas all other plant types had a greater proportion on the abaxial surface. Progeny of crosses between GL3+ and GL1+ plants resulted in trichome densities intermediate between a single-insertion GL3+ plant and a double-insertion GL3+ plant. None of the transformations stimulated trichomes on Brassica cotyledons or on non-seedling tissues. A small portion of bHLH gene-induced trichomes had a swollen terminal structure. The results suggest that trichome development in B. napus may be regulated differently from Arabidopsis. They also imply that insertion of GL3 into Brassica species under a tissue-specific promoter has strong potential for developing insect-resistant crop plants.

Members of the plant-specific GRAS family of putative transcription factors are involved in various aspects of plant development. SCARECROW (SCR) is a member of this protein family and plays a significant role in the radial patterning of both roots and shoots. However, little is known about the regulation of SCR expression and its mode of action in plants. Here, we report on the isolation and characterization of a Brassica napus SCARECROW-like protein, BnSCL1, isolated by selecting for proteins that interact with the Arabidopsis histone deacetylase AtHDA19 in a yeast two-hybrid screen. BnSCL1 contains domains conserved in the GRAS family of proteins, interacts with AtHDA19 through a VHIID domain, and exerts transcription activation of reporter genes . BnSCL1 is expressed predominantly in the roots, where its expression is regulated by auxin, as it also is in shoots and mature leaves. These results indicate that BnSCL1 is a member of the GRAS family, and suggest that its mode of action in plant auxin response may involve interaction with HDA19.