LPA1 regulates tiller angle and leaf angle by controlling the adaxial growth of tiller node and lamina joint.
Loose Plant Architecture1,the functional ortholog of the AtIDD15/SHOOT GRAVITROPISM5 (SGR5) gene in Arabidopsis (Arabidopsis thaliana),regulates tiller angle and leaf angle by controlling the adaxial growth of tiller node and lamina joint. LPA1 was also found to affect shoot gravitropism. LPA1 encodes a predicted 438-amino acid protein.Sequence analysis indicated that LPA1 is a typical Cys-2/His-2 zinc finger protein, belonging to the plant-specific IDD protein family,where it was also known as OsIDD14 . It is interesting that OsIDD12, OsIDD13, and OsIDD14/LPA1 are more similar to one another and divergent from the other 12 rice members, like AtIDD14,AtIDD15/SHOOT GRAVITROPISM5 (SGR5), and AtIDD16 among the 16 Arabidopsis (Arabidopsis thaliana) members , suggesting the specificity of the six proteins in the IDD protein family.LPA1 defines a novel subfamily of IDD proteins with distinct domains and motifs.
Thelpa1 mutant was a naturally occurring mutant isolated from anindicavariety, Zhongxian3037. During both the vegetative and reproductive stages,lpa1 always exhibited loose plant architecture with larger tiller angle and leaf angle than those of the wild type (Fig.1, A and B).The tiller angle at heading date and found that the maximum angle was 17.3° inlpa1 but only 9.8° in the wild type (Fig. 1C). Careful observation showed that the large tiller angle of lpa1 was caused by the more symmetrical growth of the tiller node compared with the wild type (Fig. 1D).The leaf angles: each angle was larger in lpa1 than in the wild type, and this difference was more obvious in older leaves, where the maximum angle of the fourth leaf could reach up to 61.2° in lpa1 but only 27.4° in the wild type (Fig. 1E). This difference was further confirmed by the dynamic change observed in the newly developing leaf (Fig. 1F). Detailed examination revealed that the large leaf angle of lpa1 was caused by a more rapid elongation on the adaxial side of the lamina joint (Fig. 1, G and H).
In rice, the lazy1 mutant exhibits a tiller-spreading phenotype resulting from reduced shoot gravitropism . To examine whether lpa1 was also involved in the same process, we analyzed the gravity response of young seedlings. The result revealed that both light- and dark-grown mutant seedlings had a reduced gravity response and could not grow upright eventually (Fig. 2, A–C). However,lpa1roots showed a normal gravity response (Fig. 2D). These results indicated thatLPA1is only involved in shoot gravitropism in rice.
To investigate the effects of LOC_Os03g13400, an RNA interference (RNAi) vector and an overexpression (OE) vector driven by the cauliflower mosaic virus (CaMV) 35S promoter were constructed and transformed into Yandao8 (a wild-type japonicavariety with compact plant architecture). Most RNAi and OE transgenic plants showed loose and compact plant architectures with differing degrees, respectively, compared with Yandao8 (Fig. 3A), from which one typical RNAi plant and one typical OE plant were selected for detailed analysis. Following two generations of self-pollination, the RNAi and OE transgenic plants showed stable phenotypes. Detailed observation showed that both tiller angle and leaf angle increased in the RNAi plant but decreased in the OE plant (Fig. 3, B and C). Real-time PCR analysis showed that the expression level of LOC_Os03g13400 was down-regulated nearly 4-fold in the RNAi plant but up-regulated more than 20-fold in the OE plant (Fig. 3, D and E). Furthermore, the gravity response was also reduced in the RNAi seedlings (Fig. 3F). These results strongly confirmed that LOC_Os03g13400 is LPA1 and also showed that the transcription level of LPA1 is closely associated with rice plant architecture. Real-time PCR revealed thatLPA1 was highly expressed in the lamina joint and internodes, especially in young tissues. However, older tiller base also showed a high expression level equivalent to the young second internode (Fig. 4A). LPA1 was also moderately expressed in coleoptile, root, seedling, and panicle but was barely detectable in leaf blade and leaf sheath (Fig. 4A). The higher expression levels of LPA1 in the lamina joint and tiller node correspond well with the main phenotypes of the mutant.LPA1 was exceptionally abundant in leaf sheath pulvinus, about 35-fold higher than in the coleoptile, strongly suggesting an important role for LPA1 in leaf sheath pulvinus gravitropism (Fig. 4B). Additionally, the expression of LPA1 in dark-grown etiolated seedlings was 1.5-fold higher than it was in those grown in the light (Fig. 4C), indicating that light can inhibit the expression of LPA1.
There are several genes that control tiller angle in rice. LAZY1 and PROG1lead to the prostrate growth of tillers, but the major QTL,TAC1, only slightly enlarges the tiller angle in indica rice. Because of their larger effects,LAZY1andPROG1are not very suitable for rice breeding, while TAC1has been extensively utilized in rice plant architecture breeding owing to its smaller effect on tiller angle .LPA1 showed an inclined tiller angle of 17.3°,similar to TAC1. Although the equivalent effect made too many difficulties in map-based cloning of LPA1, it revealed the potential utilization of LPA1in rice plant architecture breeding. Moreover, the transgenic results further demonstrated that rice plant architecture was correlated with the endogenous expression level of LPA1.Therefore,LPA1 is a useful gene for plant architecture modification in rice breeding.
In recent years, some genes that affect rice grain yield have been identified, shedding light on some of the molecular mechanisms underlying ideal plant architecture . Gravity is an important regulator of plant architecture. In grasses, because of the short-lived coleoptiles, the leaf sheath pulvini and lamina joints are the major gravity-responding organs, which control the growth orientations of seedlings, tillers, and leaf blades . In the study, LPA1 showed a specific expression pattern, and the high expression level in the leaf sheath pulvinus and lamina joint suggests that LPA1 mainly functions in the gravitropism of these tissues,corresponding to its limited effect on coleoptile gravitropism. This analysis demonstrated that LPA1 affects rice plant architecture by regulating shoot gravitropism in later developmental stages. There are several genes that control tiller angle in rice. LAZY1 and PROG1 lead to the prostrate growth of tillers, but the major QTL,TAC1, only slightly enlarges the tiller angle in indica rice. Because of their larger effects,LAZY1 and PROG1 are not very suitable for rice breeding, while TAC1 has been extensively utilized in rice plant architecture breeding owing to its smaller effect on tiller angle . In the study,LPA1showed an inclined tiller angle of 17.3°, similar to TAC1. Although the equivalent effect made too many difficulties in map-based cloning of LPA1, it revealed the potential utilization of LPA1 in rice plant architecture breeding. Moreover, the transgenic results further demonstrated that rice plant architecture was correlated with the endogenous expression level of LPA1.Therefore, LPA1 is a useful gene for plant architecture modification in rice breeding.
Labs working on this gene
State Key Laboratory of Plant Genomics and Center for Plant Gene Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China
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