Transcriptional Switch for Programmed Cell Death in Pith Parenchyma of Sorghum Stems

Pith parenchyma cells store water in various plant organs. These cells are especially important for producing sugar and ethanol from the sugar juice of grass stems. In many plants, the death of pith parenchyma cells reduces their stem water content. Previous studies proposed that a hypothetical gene might be responsible for the death of stem pith parenchyma cells in , a promising energy grass, although its identity and molecular function are unknown. Here, we identify the gene and note that it is located on chromosome 6 in agreement with previous predictions. Sorghum varieties with a functional allele had stems enriched with dry, dead pith parenchyma cells, whereas those with each of six independent nonfunctional alleles had stems enriched with juicy, living pith parenchyma cells. expression was spatiotemporally coupled with the appearance of dead, air-filled pith parenchyma cells in sorghum stems. Among homologs that are present in flowering plants, also is required for the death of stem pith parenchyma cells. and encode previously uncharacterized NAC transcription factors and are sufficient to ectopically induce programmed death of culture cells via the activation of autolytic enzymes. Taken together, these results indicate that and its ortholog, , are master transcriptional switches that induce programmed death of stem pith parenchyma cells. Thus, targeting the gene will provide an approach to breeding crops for sugar and ethanol production.