| Summary: | The <i>cue1</i> nitric oxide (NO) overproducer mutants are impaired in a plastid phosphoenolpyruvate/phosphate translocator, mainly expressed in Arabidopsis thaliana roots. <i>cue1</i> mutants present an increased content of arginine, a precursor of NO in oxidative synthesis processes. However, the pathways of plant NO biosynthesis and signaling have not yet been fully characterized, and the role of CUE1 in these processes is not clear. Here, in an attempt to advance our knowledge regarding NO homeostasis, we performed a deep characterization of the NO production of four different <i>cue1</i> alleles (<i>cue1-1</i>, <i>cue1-5</i>, <i>cue1-6</i> and <i>nox1</i>) during seed germination, primary root elongation, and salt stress resistance. Furthermore, we analyzed the production of NO in different carbon sources to improve our understanding of the interplay between carbon metabolism and NO homeostasis. After in vivo NO imaging and spectrofluorometric quantification of the endogenous NO levels of <i>cue1</i> mutants, we demonstrate that CUE1 does not directly contribute to the rapid NO synthesis during seed imbibition. Although <i>cue1</i> mutants do not overproduce NO during germination and early plant development, they are able to accumulate NO after the seedling is completely established. Thus, CUE1 regulates NO homeostasis during post-germinative growth to modulate root development in response to carbon metabolism, as different sugars modify root elongation and meristem organization in <i>cue1</i> mutants. Therefore, <i>cue1</i> mutants are a useful tool to study the physiological effects of NO in post-germinative growth.
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