Rice zebra mutants are leaf variegation mutants that exhibit transverse sectors of green/yellow or green/white in developing or mature leaves. In most cases, leaf variegation is caused by defects in chloroplast biogenesis pathways, leading to an accumulation of reactive oxygen species in a transverse pattern in the leaves. Here, we examine a new type of leaf variegation mutant in rice, zebra3 (z3), which exhibits transverse dark-green/green sectors in mature leaves and lacks the typical yellow or white sectors. Map-based cloning revealed that the Z3 locus encodes a putative citrate transporter that belongs to the citrate-metal hydrogen symport (CitMHS) family. CitMHS family members have been extensively studied in bacteria and function as secondary transporters that can transport metal-citrate complexes, but whether CitMHS family transporters exist in eukaryotes remains unknown. To investigate whether Z3 acts as a citrate transporter in rice, we measured citrate levels in wild-type leaves and in the dark-green and green sectors of the leaves of z3 mutants. The results showed that citrates accumulated to high levels in the dark-green sectors of z3 mutant leaves, but not in the green sectors as compared with the wild-type leaves. These results suggest that leaf variegation in the z3 mutant is caused by an unbalanced accumulation of citrate in a transverse pattern in the leaves. Taking these results together, we propose that Z3 plays an important role in citrate transport and distribution during leaf development and is a possible candidate for a CitMHS family member in plants.

ONAC054 promotes ABA-induced leaf senescence, and its activity is regulated through alternative 3′ splice site selection and cleavage of a C-terminal transmembr

NADPH: protochlorophyllide oxidoreductase (POR) is an essential enzyme that catalyzes the photoreduction of protochlorophyllide to chlorophyllide, which is ultimately converted to chlorophyll in developing leaves. Rice has two POR isoforms, OsPORA and OsPORB. OsPORA is expressed in the dark during early leaf development; OsPORB is expressed throughout leaf development regardless of light conditions. The faded green leaf (fgl) is a loss-of-function osporB mutant that displays necrotic lesions and variegation in the leaves due to destabilized grana thylakoids, and has increased numbers of plastoglobules in the chloroplasts. To investigate whether the function of OsPORA can complement that of OsPORB, we constitutively overexpressed OsPORA in fgl mutant. In the 35S:OsPORA/fgl (termed OPAO) transgenic plants, the necrotic lesions of the mutant disappeared and the levels of photosynthetic pigments and proteins, as well as plastid structure, were recovered in developing leaves under natural long days in the paddy field and under short days in an artificially controlled growth room. Under constant light conditions, however, total chlorophyll and carotenoid levels in the developing leaves of OPAO plants were lower than those of wild type. Moreover, the OPAO plants exhibited mild defects in mature leaves beginning at the early reproductive stage in the paddy field. The physiological function of OsPORB in response to constant light or during reproductive growth cannot be completely replaced by constitutive activity of OsPORA, although the biochemical functions of OsPORA and OsPORB are redundant. Therefore, we suggest that the two OsPORs have differentiated over the course of evolution, playing distinct roles in the adaptation of rice to the environment.

MYB-type transcription factors (TFs) play important roles in plant growth and development, and in the responses to several abiotic stresses. In rice (Oryza sativa), the roles of MYB-related TFs in leaf senescence are not well documented. Here, we examined rice MYB TF gene OsMYB102 and found that an OsMYB102 T-DNA activation-tagged line (termed osmyb102-D), which constitutively expresses OsMYB102 under the control of four tandem repeats of the 35S promoter, and OsMYB102-overexpressing transgenic lines (35S:OsMYB102 and 35S:GFP-OsMYB102) maintain green leaves much longer than the wild-type under natural, dark-induced, and abscisic acid (ABA)-induced senescence conditions. Moreover, an osmyb102 knockout mutant showed an accelerated senescence phenotype under dark-induced and ABA-induced leaf senescence conditions. Microarray analysis showed that a variety of senescence-associated genes (SAGs) were down-regulated in the osmyb102-D line. Further studies demonstrated that overexpression of OsMYB102 controls the expression of SAGs, including genes associated with ABA degradation and ABA signaling (OsABF4, OsNAP, and OsCYP707A6), under dark-induced senescence conditions. OsMYB102 inhibits ABA accumulation by directly activating the transcription of OsCYP707A6, which encodes the ABA catabolic enzyme ABSCISIC ACID 8′-HYDROXYLASE. OsMYB102 also indirectly represses ABA-responsive genes, such as OsABF4 and OsNAP. Collectively, these results demonstrate that OsMYB102 plays a critical role in leaf senescence by down-regulating ABA accumulation and ABA signaling responses.

He onset of leaf senescence is triggered by external cues and internal factors such as phytohormones and signaling pathways involving transcription factors (TFs). Abscisic acid (ABA) strongly induces senescence and endogenous ABA levels are finely tuned by many senescence-associated TFs. Here, we report on the regulatory function of the senescence-induced TF OsWRKY5 TF in rice (Oryza sativa). OsWRKY5 expression was rapidly upregulated in senescing leaves, especially in yellowing sectors initiated by aging or dark treatment. A T-DNA insertion activation-tagged OsWRKY5-overexpressing mutant (termed oswrky5-D) promoted leaf senescence under natural and dark-induced senescence (DIS) conditions. By contrast, a T-DNA insertion oswrky5-knockdown mutant (termed oswrky5) retained leaf greenness during DIS. Reverse-transcription quantitative PCR (RT-qPCR) showed that OsWRKY5 upregulates the expression of genes controlling chlorophyll degradation and leaf senescence. Furthermore, RT-qPCR and yeast one-hybrid analysis demonstrated that OsWRKY5 indirectly upregulates the expression of senescence-associated NAM/ATAF1/2/CUC2 (NAC) genes including OsNAP and OsNAC2. Precocious leaf yellowing in the oswrky5-D mutant might be caused by elevated endogenous ABA concentrations resulting from upregulated expression of ABA biosynthesis genes OsNCED3, OsNCED4, and OsNCED5, indicating that OsWRKY is a positive regulator of ABA biosynthesis during leaf senescence. Furthermore, OsWRKY5 expression was suppressed by ABA treatment. Taken together, OsWRKY5 is a positive regulator of leaf senescence that upregulates senescence-induced NAC, ABA biosynthesis, and chlorophyll degradation genes.

Background: Gibberellic acid (GA; or gibberellin) affects the development of floral organs, especially anthers and pollen, and perturbation of development of male floral organs can cause sterility. Many studies of GA signaling have concentrated on anther development, but the effect of GA on grain production remains to be examined.Results: Using a cross of ‘Milyang23 (M23)’, which has a functional allele ofEarly flowering1(EL1), and ‘H143’, which has a nonfunctionalel1allele, we generated heterogeneous inbred family-near isogenic lines (HNILs) that are homozygous forEL1[HNIL(M23)] orel1[HNIL(H143)]. Here, we found that HNIL(H143) exhibited anther deformities and low pollen viability. The expression ofGAMYB, a major activator of GA signaling, and its downstream genesCYP703A3andKAR, mainly involved in pollen formation, increased abnormally during spikelet development; this activation of GA signaling may cause the sterility. To confirm the negative effect of theel1mutation on spikelet fertility, we examined a line carrying a T-DNA insertionel1mutant [hereafter ZH11(el1)] and its parental cultivar ‘Zhonghua11 (ZH11)’. ZH11(el1) showed nearly identical defects in anther development and pollen viability as HNIL(H143), leading to decreased seed setting rate. However, the elitejaponicacultivar Koshihikari, which has a nonfunctionalel1allele for early flowering in long days, produces fertile spikelets and normal grain yields, like other elitejaponicacultivars. This indicates that as-yet-unknown regulator(s) that can overcome the male sterile phenotype of theel1mutation must have been introduced into Koshihikari.Conclusions: Theel1mutation contributes to early flowering injaponicarice under long days but fails to limit GA signaling, thus negatively affecting spikelet fertility, which results in a loss of grain yield. Thus,EL1is essential for photoperiod sensitivity in flowering as well as spikelet fertility in grain production.