Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du PEPS, Québec, QC G1V 4C7, Canada.
BMC Plant Biol. 2013 Aug 12;13:116. doi: 10.1186/1471-2229-13-116.
Among the many commercial opportunities afforded by somatic embryogenesis (SE), it is the ability to clonally propagate individual plants with rare or elite traits that has some of the most significant implications. This is particularly true for many long-lived species, such as conifers, but whose long generation times pose substantive challenges, including increased recalcitrance for SE as plants age. Identification of a clonal line of somatic embryo-derived trees whose shoot primordia have remained responsive to SE induction for over a decade, provided a unique opportunity to examine the molecular aspects underpinning SE within shoot tissues of adult white spruce trees.
Microarray analysis was used to conduct transcriptome-wide expression profiling of shoot explants taken from this responsive genotype following one week of SE induction, which when compared with that of a nonresponsive genotype, led to the identification of four of the most differentially expressed genes within each genotype. Using absolute qPCR to expand the analysis to three weeks of induction revealed that differential expression of all eight candidate genes was maintained to the end of the induction treatment, albeit to differing degrees. Most striking was that both the magnitude and duration of candidate gene expression within the nonresponsive genotype was indicative of an intense physiological response. Examining their putative identities further revealed that all four encoded for proteins with similarity to angiosperm proteins known to play prominent roles in biotic defense, and that their high-level induction over an extended period is consistent with activation of a biotic defense response. In contrast, the more temperate response within the responsive genotype, including induction of a conifer-specific dehydrin, is more consistent with elicitation of an adaptive stress response.
While additional evidence is required to definitively establish an association between SE responsiveness and a specific physiological response, these results suggest that biotic defense activation may be antagonistic, likely related to the massive transcriptional and metabolic reprogramming that it elicits. A major issue for future work will be to determine how and if suppressing biotic defense activation could be used to promote a physiological state more conducive to SE induction.
在体细胞胚胎发生（SE）带来的众多商业机会中，克隆繁殖具有稀有或优良性状的个体植物的能力具有最重要的意义。对于许多长寿命物种，如针叶树，情况尤其如此，但它们的长世代时间带来了实质性的挑战，包括随着植物年龄的增长，SE 的抗逆性增加。鉴定出无性系的体细胞胚胎衍生树木，其茎原基对 SE 诱导的反应超过十年，为研究成年白云杉树木的 shoot 组织中支持 SE 的分子方面提供了独特的机会。
微阵列分析用于对来自该反应性基因型的 shoot 外植体进行全转录组表达谱分析，在 SE 诱导一周后进行，与非反应性基因型相比，导致每个基因型中最具差异表达的四个基因。使用绝对 qPCR 将分析扩展到三周的诱导，发现所有八个候选基因的差异表达在诱导处理结束时都保持不变，尽管程度不同。最引人注目的是，非反应性基因型中候选基因表达的幅度和持续时间都表明存在强烈的生理反应。进一步研究它们的假定身份表明，所有四个基因编码的蛋白都与被子植物蛋白相似，这些蛋白在生物防御中发挥着重要作用，并且它们在较长时间内高水平诱导与生物防御反应的激活一致。相比之下，反应性基因型中的更为温和的反应，包括针叶树特异性脱水素的诱导，更符合适应性应激反应的诱导。
虽然需要更多的证据来明确确定 SE 反应性与特定生理反应之间的关联，但这些结果表明生物防御的激活可能是拮抗的，这可能与它引发的大规模转录和代谢重编程有关。未来工作的一个主要问题将是确定如何以及是否可以抑制生物防御的激活，以促进更有利于 SE 诱导的生理状态。
