Alferez Fernando M, Gerberich Kayla M, Li Jian-Liang, Zhang Yanping, Graham James H, Mou Zhonglin
Citrus Research and Education Center, University of Florida, Lake Alfred, FL, United States.
Southwest Florida Research and Education Center, University of Florida, Immokalee, FL, United States.
Front Plant Sci. 2018 Oct 9;9:1472. doi: 10.3389/fpls.2018.01472. eCollection 2018.
Nicotinamide adenine dinucleotide (NAD) is a universal electron carrier that participates in important intracellular metabolic reactions and signaling events. Interestingly, emerging evidence in animals indicates that cellular NAD can be actively or passively released into the extracellular space, where it is processed or perceived by ectoenzymes or cell-surface receptors. We have recently shown in that exogenous NAD induces defense responses, that pathogen infection leads to release of NAD into the extracellular space at concentrations sufficient for defense activation, and that depletion of extracellular NAD (eNAD) by transgenic expression of the human NAD-hydrolyzing ectoenzyme CD38 inhibits plant immunity. We therefore hypothesize that, during plant-microbe interactions, NAD is released from dead or dying cells into the extracellular space where it interacts with adjacent naïve cells' surface receptors, which in turn activate downstream immune signaling. However, it is currently unknown whether eNAD signaling is unique to or the Brassicaceae family. In this study, we treated citrus plants with exogenous NAD and tested NAD-induced transcriptional changes and disease resistance. Our results show that NAD induces profound transcriptome changes and strong resistance to citrus canker, a serious citrus disease caused by the bacterial pathogen subsp. (). Furthermore, NAD-induced resistance persists in new flushes emerging after removal of the tissues previously treated with NAD. Finally, NAD treatment primes citrus tissues, resulting in a faster and stronger induction of multiple salicylic acid pathway genes upon subsequent infection. Taken together, these results indicate that exogenous NAD is able to induce immune responses in citrus and suggest that eNAD may also be an elicitor in this woody plant species.
烟酰胺腺嘌呤二核苷酸（NAD）是一种通用的电子载体，参与重要的细胞内代谢反应和信号转导事件。有趣的是，动物研究中出现的新证据表明，细胞内的NAD可以主动或被动释放到细胞外空间，在那里它被胞外酶或细胞表面受体处理或识别。我们最近的研究表明，外源NAD能诱导防御反应，病原体感染会导致NAD以足以激活防御的浓度释放到细胞外空间，并且通过人类NAD水解胞外酶CD38的转基因表达耗尽细胞外NAD（eNAD）会抑制植物免疫。因此，我们推测在植物与微生物相互作用期间，NAD从死亡或即将死亡的细胞释放到细胞外空间，在那里它与相邻未受影响细胞的表面受体相互作用，进而激活下游免疫信号。然而，目前尚不清楚eNAD信号是否是[具体植物名称]或十字花科特有的。在本研究中，我们用外源NAD处理柑橘植物，并测试NAD诱导的转录变化和抗病性。我们的结果表明，NAD诱导了深刻的转录组变化，并对柑橘溃疡病产生了强烈抗性，柑橘溃疡病是由细菌病原体[具体病原体名称]引起的一种严重的柑橘病害。此外，在去除先前用NAD处理过的组织后新长出的嫩梢中，NAD诱导的抗性仍然存在。最后，NAD处理使柑橘组织产生了预处理效应，导致在随后感染[具体病原体名称]时，多种水杨酸途径基因的诱导更快、更强。综上所述，这些结果表明外源NAD能够在柑橘中诱导免疫反应，并表明eNAD在这种木本植物物种中也可能是一种激发子。
