Annathurai Gnanasambandam

Efficient developmental mis-targeting by the sporamin NTPP vacuolar signal to plastids in young leaves of sugarcane and Arabidopsis

Plant vacuoles are multi-functional, developmentally varied and can occupy up to 90% of plant cells. The N-terminal propeptide (NTPP) of sweet potato sporamin and the C-terminal propeptide (CTPP) of tobacco chitinase have been developed as models to target some heterologous proteins to vacuoles but so far tested on only a few plant species, vacuole types and “payload” proteins. Most studies have focused on lytic and protein-storage vacuoles, which may differ substantially from the sugar-storage vacuoles in crops like sugarcane. Our results extend the evidence that NTPP of sporamin can direct heterologous proteins to vacuoles in diverse plant species and indicate that sugarcane sucrose-storage vacuoles (like the lytic vacuoles in other plant species) are hostile to heterologous proteins. A low level of cytosolic NTPP-GFP (green fluorescent protein) was detectable in most cell types in sugarcane and Arabidopsis, but only Arabidopsis mature leaf mesophyll cells accumulated NTPP-GFP to detectable levels in vacuoles. Unexpectedly, efficient developmental mis-trafficking of NTPP-GFP to chloroplasts was found in young leaf mesophyll cells of both species. Vacuolar targeting by tobacco chitinase CTPP was inefficient in sugarcane, leaving substantial cytoplasmic activity of rat lysosomal β-glucuronidase (GUS) [ER (endoplasmic reticulum)-RGUS-CTPP]. Sporamin NTPP is a promising targeting signal for studies of vacuolar function and for metabolic engineering. Such applications must take account of the efficient developmental mis-targeting by the signal and the instability of most introduced proteins, even in “storage vacuoles”.

Early Exposure to Ethylene Modifies Shoot Development and Increases Sucrose Accumulation Rate in Sugarcane

AbstractSugarcane varieties (Saccharum spp. hybrids) that accumulate high levels of sucrose at the start of the harvest season are of considerable commercial interest. Our understanding of the factors that contribute to early sucrose accumulation in these varieties is limited. In this study we used the plant hormone ethylene to investigate the relationship between growth and early sucrose accumulation in sugarcane. The sugarcane variety KQ228 was exposed to a low concentration of the ethylene-forming compound 2-chloroethylphosphonic acid (CEPA) for a prolonged duration commencing from shoot emergence. The changes in sucrose accumulation and plant growth were investigated. Results from two glasshouse experiments revealed that the CEPA-treated plants accumulated a significantly higher amount of sucrose in their primary culm 2 and 3½ months post-germination. The treated plants had taller primary culms with many smaller internodes, smaller leaves, and a higher photosynthetic rate. Despite producing smaller internodes, treated culms were comparable in fresh weight and volume to the controls due to the compensating effect of faster internode formation. We identified three factors that may have contributed to the early accumulation of more sucrose in the treated culm: (1) the specific leaf area of young leaves was greater indicating efficient diversion of photoassimilate to sink tissue, (2) internode formation was initiated earlier, and (3) internodes continued to form at a faster rate. Consequently, a greater proportion of the internodes in the treated sugarcane matured earlier and began filling with sucrose sooner. The higher reducing sugar level in the apical region of the culm probably contributed to faster internode development. This coincided with elevated vacuolar and cell wall acid invertase gene expression that increased sucrose turnover in the vacuole and increased apoplastic uptake of reducing sugars. These findings extend our understanding of how some sugarcane varieties can naturally accumulate a high level of sucrose early in the season.

Efficient targeting of polyhydroxybutyrate biosynthetic enzymes to plant peroxisomes requires more than three amino acids in the carboxyl-terminal signal

Metabolic engineering of plant peroxisomes for biotechnological purposes typically requires efficient peroxisomal targeting of heterologous proteins. Type I peroxisomal targeting signals (PTS1) consist of three uncleaved amino acids (SKL or a conserved variant) at the carboxyl terminus and direct nuclear-encoded proteins into the peroxisomes of eukaryotic cells. PTS1 fusion with a heterologous protein results in peroxisomal targeting of that protein, but the minimal length of PTS1 required for efficient targeting in plants is vague. Here, we determine short effective PTS1 sequences derived from plant peroxisomal proteins to target four heterologous proteins, namely the green fluorescent protein (GFP) and the three enzymes required for polyhydroxybutyrate (PHB) production, PhaA, PhaB and PhaC, each fused to the C-terminus of GFP. Transient expression analysis in leaf cells of Saccharum sp. (sugarcane interspecific hybrids) indicated that a three amino acid (ARL) PTS1 effectively targeted only GFP and PhaB to peroxisomes. The same signal was not sufficient to target PhaA and only inefficiently targeted PhaC. An alternative, prototypic three amino acid (SKL) PTS1 was also insufficient to target PhaA and inefficient in targeting PhaC, whilst a six amino acid (RAVARL) PTS1 efficiently targeted both of these enzymes. This study highlights the need for more than a three amino acid PTS1 to target some heterologous proteins to plant peroxisomes.

Synthesis of Short-Chain-Length/Medium-Chain Length Polyhydroxyalkanoate (PHA) Copolymers in Peroxisomes of Transgenic Sugarcane Plants

Metabolic engineering of crops is a potential route to economically viable production of polyhydroxyalkanoates (PHAs), biodegradable and renewable alternatives to conventional plastics. In particular, short-chain-length (SCL)/medium-chain-length (MCL) PHA copolymers have attracted commercial interest for their wide range of potential applications. To date, examples of SCL/MCL PHA copolymer production in plant peroxisomes have involved single transgene approaches in transgenic Arabidopsis. We attempted to produce SCL/MCL PHA copolymers using a multigene strategy in peroxisomes of the high biomass food and industrial crop, sugarcane (Saccharum hybrids). Our approach involved peroxisomal targeting of a 3-ketothiolase, acetoacetyl-CoA reductase, enoyl-CoA hydratase and PHA synthase, as well as plastid targeting of a acyl-ACP thioesterase and 3-ketoacyl-ACP synthase to increase peroxisomal β-oxidation flux. Of 143 transgenic sugarcane lines generated by co-bombardment with the six transgenes, six were identified with PHA copolymers at up to 0.015% leaf dry mass, consisting mainly of saturated C4–C16 3-hydroxyalkanoic acids. One line with high acetoacetyl-CoA reductase and low 3-ketothiolase transcript levels had increased 3-hydroxybutyrate content, and acyl-ACP thioesterase and 3-ketoacyl-ACP synthase expression were associated with altered MCL monomer profiles. SCL/MCL PHA copolymer from the highest-yielding line showed a weight-average molecular weight of 111 KDa and polydispersity index of 1.2. Transmission electron microscopy of leaf sections from this line indicated the presence of PHA granules in peroxisomes. This work demonstrates SCL/MCL PHA copolymer biosynthesis in sugarcane peroxisomes and provides a basis for further development of mechanisms for controlling PHA composition in transgenic crop plants.

The N-terminal presequence from F1-ATPase β-subunit of Nicotiana plumbaginifolia efficiently targets green fluorescent fusion protein to the mitochondria in diverse commercial crops

Approximately 10-15% of plant nuclear genes appear to encode mitochondrial proteins that are directed to mitochondria by specific targeting signals. Reports on the heterologous function of these targeting signals are generally limited to one or a few species, with an emphasis on model plants such as tobacco and Arabidopsis. Given their sequence diversity and their insufficient testing in commercially important crops (including monocotyledonous crops), the extent to which these signals can be relied on for biotechnological purposes across species remains to be established. This study provides the experimental verification of a mitochondrial signal that is functional across diverse crop species, including five monocots (sugarcane, wheat, corn, sorghum and onion) and seven dicots (cucumber, cauliflower, tomato, capsicum, pumpkin, coriander and sunflower). In all 12 crops, transient assays following microprojectile bombardment showed that the N-terminal mitochondrial presequence from F1-ATPase β-subunit (ATPase-β) of Nicotiana plumbaginifolia Viv. targeted green fluorescent fusion protein to the mitochondria. The transient assay results in sugarcane were confirmed in stably transformed root cells. The ATPase-β signal should be a useful metabolic engineering tool for directing recombinant proteins to the mitochondrial matrix in diverse plant species of commercial interest.

Heterologous C-terminal signals effectively target fluorescent fusion proteins to leaf peroxisomes in diverse plant species

Peroxisomes are functionally diverse organelles that are wholly dependent on import of nuclear-encoded proteins. The signals that direct proteins into these organelles are either found at the C-terminus (type 1 peroxisomal targeting signal; PTS1) or N-terminus (type 2 peroxisomal targeting signal; PTS2) of the protein. Based on a limited number of tests in heterologous systems, PTS1 signals appear to be conserved across species. To further test the generality of this conclusion and to establish the extent to which the PTS1 signals can be relied on for biotechnological purposes across species, we tested two PTS1 signals for their ability to target fluorescent proteins in diverse plant species. Transient assays following microprojectile bombardment showed that the six amino acid PTS1 sequence (RAVARL) from spinach glycolate oxidase effectively targets green fluorescent fusion protein to the leaf peroxisomes in all 20 crops tested, including four monocots (sugarcane, wheat, corn and onion) and 16 dicots (carrot, cucumber, broccoli, tomato, lettuce, turnip, radish, cauliflower, cabbage, capsicum, celery, tobacco, petunia, beetroot, eggplant and coriander). Similarly, results indicated that the 10 amino acid PTS1 sequence (IHHPRELSRL) from pumpkin malate synthase effectively targets red fluorescent fusion protein to the leaf peroxisomes in all four crops tested including monocot (sugarcane) and dicot (cabbage, celery and pumpkin) species. These signal sequences should be useful metabolic engineering tools to direct recombinant proteins to the leaf peroxisomes in diverse plant species of biotechnological interest.