Transcriptome analysis of embryonic domains in Norway spruce reveals candidate genes regulating suspensor cell death

The terminal differentiation and elimination of the embryo-suspensor is the earliest manifestation of programmed cell death (PCD) during plant ontogenesis. Molecular regulation of suspensor PCD remains poorly understood. Norway spruce (Picea abies) embryos provide a powerful model for studying embryo development because of their large size, the possibility to obtain a large number of embryos at specific developmental stage through somatic embryogenesis and the availability of full genome sequencing data. Here, we have carried out global gene expression analysis of the Norway spruce embryo-suspensor versus embryonal mass (a gymnosperm analogue of embryo proper) using RNA sequencing. We have identified that suspensors have enhanced expression of NAC domain-containing transcription factors, XND1 and ANAC075, previously shown to be involved in initiating developmental PCD in Arabidiopsis. The analysis have also revealed enhanced expression of executioners of both developmental and stress-induced cell deaths, such as metacaspase 9, cysteine endopeptidase-1 (CEP1) and ribonuclease 3 (RNS3). Interestingly, a spruce homologue of bax inhibitor-1 (PaBI-1, for Picea abies BI-1), an evolutionarily conserved cell death suppressor, was likewise up-regulated in the embryo-suspensor. Since Arabidopsis BI-1 has been so far implicated only in the endoplasmic reticulum (ER)-stress induced cell death, we investigated its role in embryogenesis and suspensor PCD using RNA interference (RNAi). We have found that PaBI-1-deficient lines formed a large number of abnormal embryos with suppressed suspensor elongation and disturbed polarity. Cytochemical analysis of suspensor cells revealed that PaBI-1 deficiency suppresses vacuolar PCD and induces necrotic type of cell death previously shown to compromise embryo development. This study demonstrates that a large number of cell-death components are conserved between angiosperms and gymnosperms and establishes a new role for BI-1 in the progression of vacuolar cell death.