What is the definition of a flowering time gene? The answer is more complicated than you might think. Of course, flowering time genes control the transition from the vegetative to the reproductive phase. But there is increasing evidence that many of these genes have much broader functions (Auge et al., 2019). For example, FLOWERING LOCUS C (FLC), known for its role in repressing the floral transition until the plant has experienced a period of cold (vernalization), plays a role in seed germination as well (Chiang et al., 2009). And the floral integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) also regulates stomatal opening (Kimura et al., 2015). Other examples are the TEMPRANILLO genes (TEM1 and TEM2). In 2008, TEM genes were found to function redundantly as floral repressors (Castillejo and Pelaz, 2008). Later, it was found that TEMs also regulate the length of the juvenile phase (Sgamma et al., 2014) and control trichome formation (Matias‐Hernandez et al., 2016). In this issue, Michela Osnato and her colleagues found that TEMs regulate adaptive growth in response to salt stress at different developmental stages (Osnato et al., 2020).

The investigation of stress tolerance in flowering time mutants is not trivial. In Arabidopsis, vegetative growth ceases once the plant enters the reproductive phase, and after seed production the plant dies. In flowering time mutants, the length of the vegetative phase is altered, which consequently alters the length of the life cycle. As the length of the life cycle can have a direct effect on survival rates, especially under stress conditions, flowering time mutants have a different reproductive success than wild‐type plants. Indeed, the authors found that the shorter life cycle of tem1 tem2 double‐mutant plants allowed them to complete their reproductive phase even under extreme salt stress while many of the wild‐type plants died before flowering. However, further experiments showed that there was more going on. When they applied increasing NaCl concentrations, wild‐type plants showed an increased delay in flowering, and high concentrations were lethal. However, tem1 tem2 plants only showed slightly inhibited growth, and all plants flowered around the same time, regardless of the treatment (Figure 1). Thus, the mutant plants showed decreased sensitivity to salt stress at the floral transition, indicating that TEMs play a genuine role in mediating salt stress. Besides the effect on flowering time, the authors observed that salt‐induced leaf senescence was delayed in tem1 tem2 plants relative to the timing in wild‐type plants. To investigate what caused the senescence phenotype, they measured chlorophyll and carotenoid content. They found that in the double mutant the total content of these pigments declined more slowly in response to salt treatment than in wild‐type plants. As photosynthetic pigments are sensitive to oxidative damage, the authors tested if there was a difference in the levels of reactive oxygen species (ROS) between the double mutant and the wild type. Indeed, tem1 tem2 plants accumulated lower levels of hydrogen peroxide, one of the most important ROS. The smaller amounts of ROS in the mutant plants coincided with higher expression of EARLY LIGHT‐INDUCIBLE PROTEIN 2 (ELIP2), which encodes a protein that prevents excess accumulation of chlorophyll to prevent photo‐oxidative stress (Hutin et al., 2003). This suggests that tem1 tem2 plants are less sensitive to salt‐induced oxidative damage due to the accumulation of molecules with photoprotective functions. The authors also tested if jasmonic acid (JA) levels were altered, as this hormone is known to play an important role in stress responses and leaf senescence. They found that tem1 tem2 plants display lower JA levels as well as lower expression of JA biosynthetic genes. Together, these results indicate that TEMs function in salt stress‐induced leaf senescence by modulating oxidative stress and the JA content.

Osnato thinks that TEMs might have more roles besides the ones that are now identified. Transcriptome analysis showed that genes involved in stress responses were overrepresented amongst the differentially expressed genes. Hence it is possible that TEMs also play a role in other stress responses. Moreover, besides the altered JA levels that the authors observed in the tem1 tem2 plants in this study, previous research showed that TEMs regulate gibberellin content (Osnato et al., 2012). Because of their effect on hormone content, it is possible that TEMs have different functions throughout plant development that have not yet been revealed. The last author of the paper, Soraya Pelaz, said that her research team is currently trying to generate tem mutants in crops, as salt tolerance is a highly valuable trait. To do this, they have to take into account that generating such mutants might also cause a shorter life cycle, which is not always desirable in crops. Therefore, they are trying to find out if the salt tolerance of tem mutants can be separated from early flowering. If this is not possible, they will have to use other approaches to minimize the possible adverse effects of a shorter vegetative phase. For example, in rice, they have experimented with RNA interference to reduce TEM transcript levels rather than generating complete loss‐of‐function mutants. They obtained lines that only display a slightly altered flowering time, and now they will test if they still have increased salt tolerance. However, the question remains: if we try to reduce the effects on flowering time by mutating a flowering time gene, can we still call it a flowering time gene?

开花时间基因的定义是什么？答案比您想象的要复杂。当然，开花时间基因控制着从营养期到生殖期的过渡。但是越来越多的证据表明，这些基因中有许多具有更广泛的功能（Auge等人，2019）。例如，以其在植物经历寒冷（春化）之前抑制花色过渡的作用而闻名的FLOWERING LOCUS C（FLC）在种子发芽中也起作用（Chiang等，2009）。以及花积分抑制因子1（SOC1的过表达。）还调节气孔的开放（Kimura et al。，2015）。其他例子是TEMPRANILLO基因（TEM1和TEM2）。在2008年，发现TEM基因作为花卉阻遏物有多余的功能（Castillejo和Pelaz，2008年）。后来发现，TEM也调节了幼年期的长度（Sgamma等，2014）并控制了毛状体的形成（Matias-Hernandez等，2016）。在本期中，Michela Osnato和她的同事发现了TEM它们在不同的发育阶段响应盐胁迫来调节适应性生长（Osnato等，2020）。

在开花时间突变体中对胁迫耐受性的研究并非无关紧要。在拟南芥中，一旦植物进入生殖期，营养生长就会停止，种子生产后植物就会死亡。在开花时间突变体中，营养期的长度发生了变化，从而改变了生命周期的长度。由于生命周期的长短直接影响存活率，尤其是在胁迫条件下，开花时间突变体的繁殖成功率与野生型植物不同。实际上，作者发现tem1 tem2的生命周期较短即使在极端的盐胁迫下，双突变植物也可以使它们完成生殖阶段，而许多野生型植物在开花前就死亡了。但是，进一步的实验表明还有更多的事情要做。当他们增加NaCl浓度时，野生型植物的开花延迟增加，高浓度的植物具有致命性。但是，无论采用何种处理方法，tem1 tem2植物仅显示出轻微的生长抑制作用，并且所有植物都在同一时间开花。因此，突变植物在花期过渡期对盐胁迫的敏感性降低，表明TEM在介导盐胁迫中起着真正的作用。除了对开花时间的影响外，作者还观察到盐诱导的叶片衰老被推迟。tem1 tem2植物相对于野生型植物中的时间。为了研究引起衰老表型的原因，他们测量了叶绿素和类胡萝卜素的含量。他们发现，在双突变体中，这些盐的总含量对盐处理的响应比野生型植物的下降更为缓慢。由于光合色素对氧化损伤敏感，因此作者测试了双突变体和野生型之间活性氧（ROS）水平是否存在差异。实际上，tem1 tem2植物积累的双氧水含量较低，这是最重要的ROS之一。突变植物中较少的ROS含量与早期光诱导蛋白2（ELIP2），其编码的蛋白质可防止叶绿素过度积累，从而防止光氧化应激（Hutin等人，2003）。这表明tem1 tem2植物由于具有光保护功能的分子的积累而对盐诱导的氧化损伤不那么敏感。作者还测试了茉莉酸（JA）的水平是否改变，因为已知该激素在胁迫反应和叶片衰老中起重要作用。他们发现tem1 tem2植物显示出较低的JA水平以及JA生物合成基因的较低表达。总之，这些结果表明TEM通过调节氧化胁迫和JA含量在盐胁迫诱导的叶片衰老中起作用。

Osnato认为TEM除了现在已经确定的角色以外，还可能扮演更多的角色。转录组分析表明，在差异表达的基因中，参与应激反应的基因被过度表达。因此，TEM在其他应力响应中也可能起作用。此外，除了作者在本研究中观察到的tem1 tem2植物中JA水平的改变外，先前的研究还表明TEM可以调节赤霉素含量（Osnato等人，2012年）。）。由于它们对激素含量的影响，因此TEM在整个植物发育过程中可能具有不同的功能，但尚未揭示出来。该论文的最后作者索拉亚·佩拉兹（Soraya Pelaz）说，她的研究小组目前正在尝试在农作物中产生tem突变体，因为耐盐性是非常有价值的特性。为此，他们必须考虑到生成此类突变体也可能导致较短的生命周期，这在作物中并不总是希望的。因此，他们正在尝试找出tem的耐盐性突变体可以与早期开花分离。如果无法做到这一点，他们将不得不使用其他方法来尽量缩短较短的营养期可能带来的不利影响。例如，在水稻中，他们已经对RNA干扰进行了实验，以降低TEM转录本水平，而不是生成完全丧失功能的突变体。他们获得了仅显示开花时间略有变化的品系，现在他们将测试它们是否仍具有更高的耐盐性。但是，问题仍然存在：如果我们试图通过突变开花时间基因来减少对开花时间的影响，我们还能称其为开花时间基因吗？