Natural variation in the OsbZIP18 promoter contributes to branched‐chain amino acid levels in rice

Abstract

Summary Branched‐chain amino acids (BCAAs) are essential amino acids that must be obtained from the diet for humans and animals, and they play important roles in various aspects of plant growth and development. Although BCAA biosynthetic pathways in higher plants have been uncovered, knowledge of their genetic control is still limited, and no positive regulators have been identified to date. Here, we showed that variation in BCAA levels in rice is attributable to differential transcription of OsbZIP18, a basic leucine zipper (bZIP) transcription factor, due to polymorphisms in its promoter. Functional analysis revealed that OsbZIP18 positively regulates BCAA synthesis by binding directly to the ACE and C‐box cis‐elements in the promoters of the biosynthetic genes branched‐chain aminotransferase1 (OsBCAT1) and OsBCAT2. We further demonstrated that OsbZIP18 is strongly induced by nitrogen (N) deficiency and that N starvation results in enhanced BCAA levels in an OsbZIP18‐dependent manner. Overall, we identified OsbZIP18, a positive regulator of BCAA biosynthesis, which contributed to natural variation in BCAA levels and mediated BCAA accumulation through de novo synthesis by directly modulating the key biosynthetic genes OsBCAT1 and OsBCAT2.

Benefiting others and self: Production of vitamins in plants

“…That is, light induces ascorbate production by providing more substrate and enhancing production capacity. Similarly, light also induces the production of thiamine, vitamin B6, folate, and tocopherols (Jabrin et al, 2003; Havaux et al, 2009; Tanaka et al, 2010; Bocobza et al, 2013; Gest et al, 2013; Laing et al, 2015; Gramegna et al, 2019; Noordally et al, 2020).…”

“…The elucidation of the genetic determinants of the diversity of important traits through GWAS can help us to identify strong alleles and provides a theoretical basis for us to obtain biofortified crops and on agriculturally important traits such as shoot apical meristem (Leiboff et al, 2015), heading date and flowering time (Van Inghelandt et al, 2012;Yang et al, 2013Yang et al, , 2014Li et al, 2016d;Ye et al, 2018), plant height related (Dell'Acqua et al, 2015;Farfan et al, 2015;Li et al, 2016c), leaf architecture (Yang et al, 2014;Xue et al, 2016), leaf senescence (Fang et al, 2016), husk traits (Cui et al, 2016), grain shape and weight (Si et al, 2016;Duan et al, 2017;Liu et al, 2017), nitrogen use efficiency (Li et al, 2018c;Yu et al, 2020), grain protein content (Yang et al, 2019), eating and cooking quality (Wang et al 2017b), and taste and flavor (Tieman et al, 2017;Zhu et al, 2018). In addition GWAS was also used to identify the genetic determinants for diversity of plant nutrient metabolites, including thiamine, riboflavin (Li et al, 2018a), ascorbate (Ye et al, 2019), tocopherol and tocotrienol (Almeida et al, 2011;Wang et al, 2015), anthocyanin (Schulz et al, 2016;Wang et al, 2020), carotenoid (Schulz et al, 2016), and amino acid (Angelovici et al, 2013;Sun et al, 2020).…”

Combining metabolomic and transcriptomic approaches to assess and improve crop quality traits

“…GWAS for amino acid content was also performed in bread wheat, rice or soybean (Chen et al 2016;Lee et al 2019;Peng et al 2018;Qin et al 2019;Sun et al 2020). A highly diverse panel of 182 accessions of Triticum aestivum was used to map 328 significant quantitative trait nucleotides with six different multi-locus models.…”

“…Two genes annotated as tryptophan decarboxylases have also been associated with tryptophan content in maize kernel (Wen et al 2014). Very recently, a combination of GWAS and functional analysis in rice leaves of 520 accessions validated a bZIP TF, OsZIP18, as being the main genetic determinant for branched-chain amino acid (BCAA) content in rice (Sun et al 2020). As humans and animals are not able to synthesize BCAA, OsZIP18 may be a promising candidate for increasing rice nutritional value; however, OsZIP18 is mainly express in leaf tissues, for which its impact on grain BCAA content needs further validation.…”

Current progress and prospect of crop quality research

“…在鲜食甜玉米杂交选育中得到广泛应用 [3] 。 颗粒结合型淀粉合成酶编码基因 Waxy/Wx/ GBSSI 是谷类胚乳直链淀粉合成的主效基因 [4] 。在水稻中 Wx 是蒸煮和食味品质的主要决定因子， 同时还影响胶稠度和垩白等品质指标 [4,5] 。Wx 基因有数量众多的自然变异，如水稻 Wx mw ， Wx lv 、Wx a 、Wx in 、Wx b 、Wx op/hp 、Wx mq 、Wx mp 、wx 等，为设计直链淀粉含量在 0-30%之间的 不同品种提供了丰富的选择 [6,7] 。ALK/SSIIa/SSII-3 参与支链淀粉的长链合成，该基因是造成 水稻品种间糊化温度差异的主要基因 [8] 。将 Wx 的优异等位变异 Wx b 单独导入或与 SSIIa 的 优异等位变异同时导入超级杂交稻或其亲本中 [9][10][11] ，或者协调 Wx、SSIIa 及其同工酶 SSII-2 的表达 [12] ，都能改良稻米食味品质。粘糯的鲜食糯玉米也是由 Wx 基因突变导致的 [13] 。在水 稻中，负责支链淀粉长链合成的 FLO5/SSIIIa 不仅直接影响，同时还与 Wx 或 SSI 协作影响 垩白、直链淀粉含量、糊化温度和抗性淀粉含量等 [14][15][16] 。淀粉分支酶 BEI、BEIIb 以及脱支 酶 ISA1 均参与了稻米糊化温度调控 [17,18] 。玉米 Ae1/BEIIb 编码基因突变可使直链淀粉含量 从 25%提高到 50%-70%，是目前已知的高直链淀粉玉米选育的一个主要靶点 [19] 。 除以上淀粉合成关键酶类， 还鉴定了多个淀粉代谢相关基因通过调控淀粉合成关键酶类 的表达或活性，或者通过其他淀粉代谢相关途径影响淀粉合成及胚乳淀粉粒形成，从而影响 垩白、直链淀粉含量、糊化特性等品质性状。例如 BT1、FLO2、FLO13、PDIL1-1 以及 AP2/EREBP 类转录因子 RSR1 参与淀粉合成关键酶类的表达或活性调控 [20][21][22][23][24] 。DU1 和 DU3 在 Wx b 的 mRNA 前体剪切中发挥功能 [25,26] 。FLO6 直接与 ISA1 互作 [27] ；FLO7 参与淀粉质 体发育 [28] 。FLO4、FLO8 和 PHO1 参与淀粉代谢的机制目前仍不清楚 [29][30][31] [32] 。转录因子 RISBZ1 和 RPBF 协同激活种子储藏蛋白基因的表达，提高蛋白含量 [33] ；而单独降低 RISBZ1 或 RPBF 表达能 降低赖氨酸降解酶 LKR/SDH 的水平，从而增加稻米蛋白所缺乏的赖氨酸的含量 [34] 。AAP6 编码氨基酸转运蛋白，正调控所有四类种子储藏蛋白，提高稻米的营养品质 [35] 。转录因子 bZIP18 正调控支链氨基酸(亮氨酸、异亮氨酸、缬氨酸)合成途径关键酶支链氨基转移酶 编码基因 BCAT1 和 BCAT2 的表达，是稻米支链氨基酸含量的正调控因子 [36] Gli-D2 决定了小麦谷蛋白含量以及麦谷蛋白与麦醇溶蛋白的配比，是影响面筋、面团、面 包、 面条烘培和食味品质最主要的调控因子 [37] 。 在优良小麦品种背景中导入醇溶蛋白 Gli-D2 基因的优异变异 Gli-D2-null，可显著改善小麦面团和面包烘培品质 [38] 。小麦胚乳特异性转 [43] 。通过将大豆 O-乙酰丝氨酸巯基酶 OASS 过量表达，不仅提高 了总蛋白含量，而且显著提高了半胱氨酸和甲硫氨酸的含量…”