Dark field image of leaves. Normally, xylem vessels showing white lines form continuously. By contrast, discontinuous xylem vessels are observed in leaves overexpressing VNI2.

Several specific types of plant cells such as xylem vessels and fiber cells form thick secondary cell wall in addition to (primary) cell wall. Secondary cell wall mainly consists of polysaccharide (cellulose and xylan) and lignin, and is expected to a renewable and sustainable biomass. Dr. Taku Demura team in RIKEN, in which Dr. Yamaguchi used to be, identified NAC domain transcription factors VND6 and VND7, as key regulators of xylem vessel differentiation (Kubo et al. 2005 Genes Dev.). In addition, another NAC domain transcription factor, VNI2 was isolated as an interacting factor with VND7. VNI2 negatively regulates xylem vessel differentiation by inhibiting the VND7 function (Yamaguchi et al. 2010 Plant Cell).

VNI2 interacts with and inhibits VND7 during xylem vessel formation. Likewise, we hypothesize that VNI2 interacts with and regulates some other NAC domain transcription factors during environmental stress responses.

It has been reported that VNI2 is also involved in environmental stress responses and senescence as well as xylem vessel formation (Yang et al. Plant Cell). We have succeeded in isolation of a number of NAC transcription factors as interacting proteins with VNI2. Some of them are known to be involved in environmental stress responses. To unveil molecular function of VNI2 in response to environmental stresses, we are investigating biological role of the interaction of VNI2 with the NAC domain transcription factors.

Florescent signal of YFP fused to the VNI2 protein is hardly detected compared to that to the nuclear localized signal (NLS).

It is known that VNI2 is a fairly unstable protein. VNI2 contains a predicted PEST-motif, one of a target sequences for protein degradation, on its C-terminus. VNI2 protein lacking the C-terminal region including the PEST motif becomes more stable than full-length VNI2. In addition, ectopic expression of the C-terminally truncated VNI2 inhibits normal development of xylem vessels. We isolated ubiquitin E3 ligases as interacting proteins with VNI2. We are now studying to prove the ubiquitin E3 ligases promote the degradation of VNI2 protein.

Comparing to a control plant (Nipponbare, top), culm of a bc mutant (bc4, bottom) are easily broken.

brittle culm (bc) mutants showing reduction in mechanical strength have been isolated from various grass species. Such mutants show defects in cell wall structure of nodes and/or internodes. We are trying to identify the causal genes of some bc mutants.