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The functionality of SUS5 and SUS6 remain unclear.The research shows that the transcripts of the two Rice Sus genes classiﬁed in the New Group, SUS5 and SUS6,exhi- bited a similar expression pattern to each other, being detected in most of the tissues examined, except for leaf sheaths, but at much lower levels compared to ex- pression levels of the other members of the rice Sus gene family. These results are similar to those observed for the members of the NG in Arabidopsis and L.japo- nicus. It has been reported that the transcript levels of Arabidopsis SUS5 and 6 do not respond to either oxygen deﬁciency or cold stress. In contrast to this, in the current work the mRNA levels of SUS5 and 6 were found to be suppressed in response to submergence, which is the ﬁrst report of Sus genes of the NG class respond- ing to an environmental stress. However, the function of the NG class Sus proteins are still unclear. The existence of conserved NG Sus members in a range of higher plant species suggests that the members may have speciﬁc function(s).
1.Tissue-speciﬁc expression of Sus genes Fig. 2A illustrates the total abundance of the mRNAs for the six SUS genes as normalized to the constitutive polyubiquitin(RUBIQ1) gene transcript level. The mRNA levels for the SUS genes were highest in elongating sink organs such as roots,developing leaf blades and elongating uppermost internodes,with SUS1 and 2 being the isoforms predominantly expressed within these tissues. In contrast to this, SUS transcript levels in the mature leaf blades and leaf sheaths were low. The transcript levels of each SUS gene were plotted individually to examine speciﬁc expression patterns (Fig. 2B). The mRNA levels of SUS1 were highest in elongating tissues com- pared to the other tissues examined, whilst SUS2 transcript levels were higher in root and internode tissue than other tissues examined. The transcripts of SUS3 or SUS4 were most abundant in either the panicles at the grain ﬁlling stage of development or dry seeds respectively, and were absent from or expressed only at very low levels in the other tissues examined. The mRNA levels of both SUS5 and 6were more highly expressed in sink leaf blades followed by internodes and roots compared to the other tissues examined. However, even in the sink leaf blades their expression levels were less than 10% of those for RUBIQ1,and were considerably lower than the expression levels of other four SUS genes.
2. Gene expression and activity of the Sus in the developing caryopsis. To examine the potential role(s) of SUS during grain ﬁlling in rice, the differential expression of the six SUS genes and Sus activity was determined in a deve- lopmental series of de-hulled rice caryopsis samples. Sus activity, grain weight and water content were measured in the series of developing caryopsis samples (Fig. 5A). The gain in dry weight of the caryopsis became prominent from 5 DAF corresponding with the start of a decrease in water content. The Sus activity in the caryopsis was around 50 U gFW1 during the early stage of the grain ﬁlling at 1–3 DAF, and increased two-fold by 10 DAF before steadily decreasing to one-third of the maximal level at 20 DAF.Transcript levels are shown in (Fig. 5B). SUS1mRNAwas most abundantly expressed in the early stage of the grain ﬁlling, at 1–3 DAF, and gradually decreased thereafter. The transcript level of SUS2 was at similar levels to those of SUS1 at 1 and 2 DAF,then increased and reached maximal levels during 5–10 DAF,and decreased thereafter. The expression level of the SUS3 was very low at 1 and 2 DAF, and increased rapidly to a peak at 5 DAF before decreasing thereafter. A distinctive pattern wasobserved for SUS4 expression level, which was very low from 1 to 3 DAF, and then increased to reach a maximal level at 10 DAF at which it remained constant for the remainder of the time-course studied. Compared to the other members of the rice Sus family, the mRNA levels of SUS5 and SUS6 were very low during the whole period examined. During the time-course studied, the total transcript levels of the six SUS genes were found to correlate strongly with the Sus activities (Fig. 5C).
3.The effect of submergence on Sus gene expression levels in germinating seedlings. It has been reported that sugar metabolism of rice seedlings is affected by hypoxia typically caused by submergence, and that Sus activity is suggested to be one of the key factors for the altered sugar metabolism in rice plants under submerged condition. To investigate the response of the SUS genes to submergence, their transcript levels were determined in the shoots of seedlings, germinated under submerged conditions,during a developmental time-course from 3 to 7 days after imbibition (DAI). The overall level of Sus activity remained relatively stable irrespective of germination conditions, but was higher in shoots of seedlings germinated under submerged condition compared to those germinated in the air at all the timepoints examined (Fig. 6A). The SUS1 transcript level initially increased from 3 to 4 DAI, and thereafter gradually decreased over the remainder of the time-course in the control seedlings. A similar trend was observed for SUS1 transcript in submerged seedlings; however, the transcript level was signiﬁcantly higher than that of the control at 3 and 4 DAI, and signiﬁcantly lower at 6 and 7 DAI (Fig. 6B). SUS2 transcript was present at comparable levels in both the control and submerged samples at 3 DAI, after which its level gradually declined in the control seedlings and signiﬁcantly increased in submerged seedlings throughout the remainder of the time course studied (Fig. 6C). The SUS3 transcript was not detectable and mRNA level of SUS4 remained unchanged with an abundance of less than 10% of RUBIQ1 in both control and submerged samples (data not shown). The mRNA levels of SUS5 and SUS6 decreased in both the control and submerged samples during the time-course, and were signiﬁcantly suppressed by submergence at 6 and 7 DAI, although the absolute transcript level of the two genes in the samples was only about 4% of RUBIQ1 at their highest (Fig. 6D and E).
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Labs working on this gene
a National Agricultural Research Center, 1-2-1 Inada, Joetsu, Niigata 943-0193, Japan.
b CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia.
Tatsuro Hirose, Graham N. Scofield, Tomio Terao, An expression analysis profile for the entire sucrose synthase gene family in rice, Plant Science, Volume 174, Issue 5, May 2008, Pages 534-543, ISSN 0168-9452, http://dx.doi.org/10.1016/j.plantsci.2008.02.009. (http://www.sciencedirect.com/science/article/pii/S0168945208000472)