GmNAC81 Inversely Modulates Leaf Senescence and Drought Tolerance

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.

Senescence-Associated Glycine max (Gm)NAC Genes: Integration of Natural and Stress-Induced Leaf Senescence

“…Different TF families participate in the ongoing senescence, mainly the NAC [ 5 , 25 , 28 , 38 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 ], MYB [ 13 , 16 ], AP2/ERF [ 49 , 50 ], and WRKY [ 17 , 51 , 52 , 53 ] families. In Arabidopsis, several regulatory gene networks have already been characterized and the crosstalk regulation between phytohormones and TFs is extensively understood [ 54 ].…”

“…SNAC-A (ATAF) is the most represented subfamily in senescence; 90% of the genes are differentially expressed in leaf senescence (10 of 11 members). Within this family, GmNAC018 , GmNAC030 , GmNAC039, and GmNAC043 are SAGs displaying stress-responsiveness [ 5 , 41 ] ( Table 1 ). GmNAC018 and GmNAC039 are putative paralogs and display similar induction patterns, with expressive upregulation by simulated drought, ER, and biotic stresses [ 41 ].…”

“…Within this family, GmNAC018 , GmNAC030 , GmNAC039, and GmNAC043 are SAGs displaying stress-responsiveness [ 5 , 41 ] ( Table 1 ). GmNAC018 and GmNAC039 are putative paralogs and display similar induction patterns, with expressive upregulation by simulated drought, ER, and biotic stresses [ 41 ]. Similarly, GmNAC030 belongs to a partially overlapped regulatory circuit integrating drought, ER, and biotic stresses with natural senescence in planta , and thus is considered a GmNAC-SAG .…”

“…Not surprisingly, ATAF1 targets genes involved in ABA biosynthesis and transport, NCED3 and ABCG40 , respectively, which are essential for ABA signaling [ 59 ]. Despite the high structural conservation of SNAC-A (ATAF)-SAGs, a soybean ortholog of ATAF1, GmNAC030, forms a heterodimer with GmNAC081 to repress the expression of ABCG40 and regulators of ABA signaling [ 41 ]. These results indicate that the underlying mechanism of NAC-SAG-mediated senescence may differ across species.…”

“…GmNAC08 1 has also been functionally characterized as a downstream component of the DCD/NRP-mediated cell death response [ 19 , 56 ] and as a positive regulator of leaf senescence [ 5 , 28 , 41 , 43 ]. Transcript expression analysis and GmNAC081 promoter-GUS assay indicated that GmNAC081 transcriptional control is correlated with leaf senescence events [ 41 , 43 ].…”