A characteristic feature of grasses and commercially important cereals is the presence of (1,3;1,4)-beta-d-glucans in their cell walls. We have used comparative genomics to link a major quantitative trait locus for (1,3;1,4)-beta-d-glucan content in barley grain to a cluster of cellulose synthase-like CslF genes in rice. After insertion of rice CslF genes into Arabidopsis, we detected (1,3;1,4)-beta-d-glucan in walls of transgenic plants using specific monoclonal antibodies and enzymatic analysis. Because wild-type Arabidopsis does not contain CslF genes or have (1,3;1,4)-beta-d-glucans in its walls, these experiments provide direct, gain-of-function evidence for the participation of rice CslF genes in (1,3;1,4)-beta-d-glucan biosynthesis.

The N-end rule pathway targets protein degradation through the identity of the amino-terminal residue of specific protein substrates. Two components of this pathway in Arabidopsis thaliana, PROTEOLYSIS6 (PRT6) and arginyl-tRNA:protein arginyltransferase (ATE), were shown to regulate seed after-ripening, seedling sugar sensitivity, seedling lipid breakdown, and abscisic acid (ABA) sensitivity of germination. Sensitivity of prt6 mutant seeds to ABA inhibition of endosperm rupture reduced with after-ripening time, suggesting that seeds display a previously undescribed window of sensitivity to ABA. Reduced root growth of prt6 alleles and the ate1 ate2 double mutant was rescued by exogenous sucrose, and the breakdown of lipid bodies and seed-derived triacylglycerol was impaired in mutant seedlings, implicating the N-end rule pathway in control of seed oil mobilization. Epistasis analysis indicated that PRT6 control of germination and establishment, as exemplified by ABA and sugar sensitivity, as well as storage oil mobilization, occurs at least in part via transcription factors ABI3 and ABI5. The N-end rule pathway of protein turnover is therefore postulated to inactivate as-yet unidentified key component(s) of ABA signaling to influence the seed-to-seedling transition.abscisic acid ͉ aminoacyl tRNA protein transferase ͉ lipid bodies ͉ targeted protein degradation

SummaryAfter-ripening (AR) is a time and environment regulated process occurring in the dry seed, which determines the germination potential of seeds. Both metabolism and perception of the phytohormone abscisic acid (ABA) are important in the initiation and maintenance of dormancy. However, molecular mechanisms that regulate the capacity for dormancy or germination through AR are unknown. To understand the relationship between ABA and AR, we analysed genome expression in Arabidopsis thaliana mutants defective in seed ABA synthesis (aba1-1) or perception (abi1-1). Even though imbibed mutant seeds showed no dormancy, they exhibited changes in global gene expression resulting from dry AR that were comparable with changes occurring in wildtype (WT) seeds. Core gene sets were identified that were positively or negatively regulated by dry seed storage. Each set included a gene encoding repression or activation of ABA function (LPP2 and ABA1, respectively), thereby suggesting a mechanism through which dry AR may modulate subsequent germination potential in WT seeds. Application of exogenous ABA to after-ripened WT seeds did not reimpose characteristics of freshly harvested seeds on imbibed seed gene expression patterns. It was shown that secondary dormancy states reinstate AR status-specific gene expression patterns. A model is presented that separates the action of ABA in seed dormancy from AR and dry storage regulated gene expression. These results have major implications for the study of genetic mechanisms altered in seeds as a result of crop domestication into agriculture, and for seed behaviour during dormancy cycling in natural ecosystems.

Phase II of germination represents a key developmental stage of plant growth during which imbibed seeds either enter stage III of germination, completing the germination process via radicle protrusion, or remain dormant. In this study, we analyzed the influence of the peroxisomal ATP-binding cassette transporter COMATOSE (CTS) on the postimbibition seed transcriptome of Arabidopsis (Arabidopsis thaliana) and also investigated interactions between gibberellin (GA) and CTS function. A novel method for analysis of transcriptome datasets allowed visualization of developmental signatures of seeds, showing that cts-1 retains the capacity to after ripen, indicating a germination block late in phase II. Expression of the key GA biosynthetic genes GA3ox1 and 2 was greatly reduced in cts seeds and genetic analysis suggested that CTS was epistatic to RGL2, a germinationrepressing DELLA protein that is degraded by GA. Comparative analysis of seed transcriptome datasets indicated that specific cohorts of genes were influenced by GA and CTS. CTS function was required for expression of the flavonoid biosynthetic pathway. Confocal imaging demonstrated the exclusive accumulation of flavonoids in the epidermis of wild-type seeds. In contrast, flavonoids were absent from cts and kat2-1 mutant seeds, but accumulated following the application of sucrose, indicating an essential role for b-oxidation in inducing flavonoid biosynthetic genes. These results demonstrate that CTS functions very late in phase II of germination and that its function is required for the expression of specific gene sets related to an important biochemical pathway associated with seedling establishment and survival.