The Stress-Induced Soybean NAC Transcription Factor GmNAC81 Plays a Positive Role in Developmentally Programmed Leaf Senescence

Abstract

The onset of leaf senescence is a highly regulated developmental change that is controlled by both genetics and the environment. Senescence is triggered by massive transcriptional reprogramming, but functional information about its underlying regulatory mechanisms is limited. In the current investigation, we performed a functional analysis of the soybean (Glycine max) osmotic stress- and endoplasmic reticulum (ER) stress-induced NAC transcription factor GmNAC81 during natural leaf senescence using overexpression studies and reverse genetics. GmNAC81-overexpressing lines displayed accelerated flowering and leaf senescence but otherwise developed normally. The precocious leaf senescence of GmNAC81-overexpressing lines was associated with greater Chl loss, faster photosynthetic decay and higher expression of hydrolytic enzyme-encoding GmNAC81 target genes, including the vacuolar processing enzyme (VPE), an executioner of vacuole-triggered programmed cell death (PCD). Conversely, virus-induced gene silencing-mediated silencing of GmNAC81 delayed leaf senescence and was associated with reductions in Chl loss, lipid peroxidation and the expression of GmNAC81 direct targets. Promoter-reporter studies revealed that the expression pattern of GmNAC81 was associated with senescence in soybean leaves. Our data indicate that GmNAC81 is a positive regulator of age-dependent senescence and may integrate osmotic stress- and ER stress-induced PCD responses with natural leaf senescence through the GmNAC81/VPE regulatory circuit.

A Regulatory Circuit Integrating Stress-Induced with Natural Leaf Senescence

“…Regarding the first hypothesis, several lines of evidence indicate that the regulatory circuit NRPs/GmNAC81:GmNAC30/VPE integrates osmotic stress-and ER stress-induced PCD response with natural leaf senescence. First, not only GmNAC30 and GmNAC81 but also the other cell death pathway components, NRP-A, NRP-B, and VPE, are induced by leaf senescence [43,59,70]. Second, the activity of VPE is also induced during the onset of leaf senescence [59].…”

“…Fourth, enhanced accumulation of BiP, which negatively regulates the NRPs/GmNAC81:GmNAC30/VPE signaling module, also promotes a delay in leaf senescence in transgenic plants [59]. Finally, GmNAC81 is a positive regulator of naturally programmed leaf senescence [70]. Although leaf senescence is genetically programmed in an age-dependent manner, it can be triggered by environmental cues and is also positively and negatively regulated by various plant hormones.…”

“…Although leaf senescence is genetically programmed in an age-dependent manner, it can be triggered by environmental cues and is also positively and negatively regulated by various plant hormones. GmNAC81 and GmNAC30 are induced by the phytohormones ABA, jasmonic acid (JA) and salicylic acid (SA), which are positive regulators of senescence, and GmNAC81-overexpressing lines display high levels of ABA, mimicking the enhanced endogenous levels of this hormone during leaf senescence [70,71]. Consistent with a role in leaf senescence, the overexpression of GmNAC81 in soybean plants accelerates leaf senescence, a phenotype associated with extensive leaf yellowing, increased chlorophyll loss, faster photosynthetic decay, and enhanced expression and activity of the GmNAC81 direct target VPE, than untransformed, wild-type plants.…”

“…Consistent with a role in leaf senescence, the overexpression of GmNAC81 in soybean plants accelerates leaf senescence, a phenotype associated with extensive leaf yellowing, increased chlorophyll loss, faster photosynthetic decay, and enhanced expression and activity of the GmNAC81 direct target VPE, than untransformed, wild-type plants. Conversely, suppressing GmNAC81 expression delays leaf senescence and decreases the expression of GmNAC81 direct target genes, including VPE [70]. Therefore, GmNAC81 is involved in developmentally programmed leaf senescence.…”

HEBE, a novel positive regulator of senescence in Solanum lycopersicum

“…NAC proteins are activators and/or repressors of gene expression and modulate plant development, plant defense and stress tolerance processes (for review see Olsen et al 13 , Nakano et al 14 , Kim et al 15 , Ohbayashi and Sugiyama 16 , Mathew and Agarwal 17 ). NAC proteins have been documented to be involved in leaf, petal and fruit senescence in Arabidopsis thaliana 3,15,[18][19][20][21][22][23][24][25][26][27][28][29][30] , Solanum lycopersicum [31][32][33] , Oryza sativa [34][35][36][37][38] , Hordeum vulgare 39,40 , Glycine max 41 , Bambusa emeiensis 42 , Trifolium pratense 43 , Helianthus annuus 44 , Gossypium hirsutum [45][46][47] , Musa x paradisiaca 48,49 , Vitis vinifera 50 and Nicotiana tabacum 51 . Among those, several NAC genes are linked to a stay-green phenotype.…”