Stem cell lineage in body layer specialization and vascular patterning of rice root and leaf

Since the first appearance of vascular plants during evolution, the plant body has become specialized for adaption to land conditions. Much of our knowledge of plant body specialization and the origins of tissues from stem cells have been obtained from studies on the dicot Arabidopsis thaliana. However, less is known about plant body specialization in monocots, another important branch of angiosperms. In this study, we analyzed stem cell lineage and differentiation during development of the root and leaf of the monocot model plant rice (Oryza sativa). Our results showed that three body layers of rice are established from stem cells accompanied by progressively reduced pluripotency. Layer 1 (L1) is a single-cell layer of epidermis; L2 is the cortex/endodermis in the root and the mesophyll in the leaf; and L3 is the site of vascular initiation. At least two common steps in vascular development are shared between rice root and leaf. The preprocambium divides to form the procambium and root pericycle or leaf outer sheath. The procambium further differentiates into the xylem, phloem and circumambient cells. We found that the outer sheath of leaf vascular bundles originates not only from the preprocambium of L3, but also from the mesophyll precursor cells of L2. In addition, WUSCHEL-RELATED HOMEOBOX (WOX) genes are expressed in not only the stem cell niche but also metaxylem precursor in rice. This pattern differs from that of homologs in Arabidopsis, suggesting that WOX functions have been recruited in different stem cells in dicots and monocots.