Abstract
Arabidopsis thaliana SAG12, a senescence-specific gene encoding a cysteine protease, is widely used as a molecular marker for the study of leaf senescence. To date, its potential orthologues have been isolated from several plant species such as Brassica napus and Nicotiana tabacum. However, little information is available in rubber tree (Hevea brasiliensis), a rubber-producing plant of the Euphorbiaceae family. This study presents the identification of SAG12-like genes from the rubber tree genome. Results showed that an unexpected high number of 17 rubber orthologues with a single intron were found, contrasting the single copy with two introns in Arabidopsis. The gene expansion was also observed in another two Euphorbiaceae plants, castor bean (Ricinus communis) and physic nut (Jatropha curcas), both of which contain 8 orthologues. In accordance with no occurrence of recent whole-genome duplication (WGD) events, most duplicates in castor and physic nut were resulted from tandem duplications. In contrast, the duplicated HbSAG12H genes were derived from tandem duplications as well as the recent WGD. Expression analysis showed that most HbSAG12H genes were lowly expressed in examined tissues except for root and male flower. Furthermore, HbSAG12H1 exhibits a strictly senescence-associated expression pattern in rubber tree leaves, and thus can be used as a marker gene for the study of senescence mechanism in Hevea.
Highlights
Leaf senescence, the last stage of leaf development, is a complex but highly regulated developmental process that is controlled by an array of internal and external factors [1,2]
The initiative BLAST search resulted in 54 loci putatively encoding Arabidopsis SAG12 homologues from the rubber tree genome, and 17 out of them were proved to be true orthologues by the reciprocal BLASTP
These orthologues were denoted as HbSAG12H1–17, which were found to be distributed across 9 out of 7,453 scaffolds [38], i.e., scaffold0048 (5), scaffold0247 (2), scaffold0583 (2), scaffold0696 (2), scaffold1445 (1), scaffold2360 (2), scaffold0683 (1), scaffold0420 (1) and scaffold1086 (1) (Table 1)
Summary
The last stage of leaf development, is a complex but highly regulated developmental process that is controlled by an array of internal and external factors [1,2]. Internal factors include age and levels of plant hormones, whereas external factors include shade/darkness, desiccation, drought, heat, nutrient stresses, pathogen infection and various hormones such as cytokinin (CK), auxin, ethylene (ET), jasmonic acid (JA), salicylic acid (SA) and abscisic acid (ABA) [3,4]. These factors trigger a series of coordinated events such as shifts in gene expression, loss of chlorophyll, reduction of photosynthesis, degradation of macromolecules, relocation of nutrients, the breakdown of organelles, and cell death [5,6]. The potential orthologues of AtSAG12 were isolated from several species, e.g., BnSAG12-1 and BnSAG12-2 in Brassica napus [18], SPG31 in Ipomoea batatas [19], ccyp in Gossypium hirsutum [20], NtCP1 and NtSAG12 in Nicotiana tabacum [21,22]
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