Abstract
Glycine max NAC81 (GmNAC81) is a downstream effector of the DCD/NRP-mediated cell death signaling, which interacts with GmNAC30 to fully induce the caspase 1-like vacuolar processing enzyme (VPE) expression, the executioner of the cell death program. GmNAC81 has been previously shown to positively modulate leaf senescence via the NRP/GmNAC81/VPE signaling module. Here, we examined the transcriptome induced by GmNAC81 overexpression and leaf senescence and showed that GmNAC81 further modulates leaf senescence by regulating an extensive repertoire of functionally characterized senescence-associated genes (SAGs). Because the NRP/GmNAC81/VPE signaling circuit also relays stress-induced cell death signals, we examined the effect of GmNAC81 overexpression in drought responses. Enhanced GmNAC81 expression in the transgenic lines increased sensitivity to water deprivation. Under progressive drought, the GmNAC81-overexpressing lines displayed severe leaf wilting, a larger and faster decline in leaf Ψw, relative water content (RWC), photosynthesis rate, stomatal conductance, and transpiration rate, in addition to higher Ci/Ca and lower Fm/Fv ratios compared to the BR16 control line. Collectively, these results indicate that the photosynthetic activity and apparatus were more affected by drought in the transgenic lines. Consistent with hypersensitivity to drought, chlorophyll loss, and lipid peroxidation were higher in the GmNAC81-overexpressing lines than in BR16 under dehydration. In addition to inducing VPE expression, GmNAC81 overexpression uncovered the regulation of typical drought-responsive genes. In particular, key regulators and effectors of ABA signaling were suppressed by GmNAC81 overexpression. These results suggest that GmNAC81 may negatively control drought tolerance not only via VPE activation but also via suppression of ABA signaling.
Highlights
The NAC superfamily of plant-specific transcriptional factors (TF) is largely distributed in the plant kingdom and constitutes one of the largest families of plant TFs (Shao et al, 2015)
At 90 days after germination (DAG), developmental stage R7, leaf yellowing was more intense in transgenic lines than in wild type (Figure 1A), which was associated with the higher accumulation of hydrogen peroxide in overexpressing lines, as judged by the intensity of DAB staining in senescent leaves (Supplementary Figure S1A)
Glycine max NAC81 (GmNAC81) is a downstream component of the multiple stressand natural leaf senescence-induced developmental cell death (DCD)/NRP-mediated cell death signaling
Summary
The NAC (an acronym for NAM, ATAF, and CUC) superfamily of plant-specific transcriptional factors (TF) is largely distributed in the plant kingdom and constitutes one of the largest families of plant TFs (Shao et al, 2015). The primary structural organization of NAC TFs includes a 150-aminoacid conserved N-terminal DNA binding domain, designated the NAC domain, and a more variable C-terminal domain involved in transcriptional regulation (Nakashima et al, 2011). The NAC domain is divided into five subdomains (A-E). The C and D subdomains are highly conserved and are responsible for binding DNA. The most divergent B and E subdomains may be responsible for the functional diversity of NAC genes, whereas the A subdomain may be involved in the formation of functional homo- or heterodimers or oligomers (Puranik et al, 2012). The D subdomain of some NACs may harbor a highly hydrophobic, negative regulatory domain (NRD), which inhibits NAC and TF partners (Puranik et al, 2012)
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