Abstract
NAC proteins are plant-specific transcription factors that play essential roles in regulating development and responses to abiotic and biotic stresses. We show that over-expression of the cotton GhNAC2 under the CaMV35S promoter increases root growth in both Arabidopsis and cotton under unstressed conditions. Transgenic Arabidopsis plants also show improved root growth in presence of mannitol and NaCl while transgenic cotton expressing GhNAC2 show reduced leaf abscission and wilting upon water stress compared to control plants. Transgenic Arabidopsis plants also have larger leaves, higher seed number and size under well watered conditions, reduced transpiration and higher relative leaf water content. Micro-array analysis of transgenic plants over-expressing GhNAC2 reveals activation of the ABA/JA pathways and a suppression of the ethylene pathway at several levels to reduce expression of ERF6/ERF1/WRKY33/ MPK3/MKK9/ACS6 and their targets. This probably suppresses the ethylene-mediated inhibition of organ expansion, leading to larger leaves, better root growth and higher yields under unstressed conditions. Suppression of the ethylene pathway and activation of the ABA/JA pathways also primes the plant for improved stress tolerance by reduction in transpiration, greater stomatal control and suppression of growth retarding factors.
Highlights
Plants being sessile in nature have to continuously respond to abiotic and biotic stresses that restrict their growth and yield
It contained a NARD like sequence from amino acids 104–132 (Fig. S1). This motif functions as a repressor when fused to VP16 in plant protoplast assays and has been reported in other repressor type NAC members such as GmNAC20, GmNAC11, AtNAC2, NST1, ATAF1, RD26 and SNAC7,10,20
About 73 NAC domain proteins belonging to four subfamilies namely, ATAF, AtNAC3, NAM and NAP were identified in three different studies in G. hirsutum[19,21,22] while about 145 NAC genes were identified in G. raimondii[23]
Summary
Plants being sessile in nature have to continuously respond to abiotic and biotic stresses that restrict their growth and yield. NAC genes, first identified in petunia and Arabidopsis as genes that control shoot apical meristem (SAM) development[5] were subsequently shown to be involved in other developmental processes such as formation of lateral roots[6,7], secondary wall formation[8], senescence[9], petal expansion[10], fruit ripening/pigmentation[11] etc. Besides development, these genes are associated with nutrient remobilization[12] and resistance to biotic and abiotic stresses[3,13]. We show that expression of GhNAC2, isolated from the relatively drought tolerant G.herbaceum, leads to much improved root growth under well watered conditions both in transgenic Arabidopsis and cotton and primes the plant for improved drought tolerance through modulation of the ABA/JA and ethylene pathways
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