JURNAL TRANSPORT DALAM FLOEM

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STUDI KLASIK TRANSPOR FLOEM

TRANSLOKASI DALAM FLOEM

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• Marcello Malpighi in 1686, ahli anatomi italia (1686) : kulit dilepaskan dari pohon dalam bentuk cincin mengelilingi batang (girdling) • T. G. Mason and E. J. Maskell (1928): • girdling tidak mempengaruhi transpirasi, water moves in the xylem, bagian dalam to the bark • ransport gula pada batang terhambat pada saat kulit dilepaskan • Gula terakumulasi pada bagian atas girdle 1

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JALUR TRANSLOKASI

Web Figure 10.1.A Tree trunk immediately after girdling (left) and later (right). Girdling is the removal of the bark of a tree in a ring around the trunk. At right, materials translocated from the leaves have accumulated in the region above the girdle and caused it to swell. (Click image to enlarge.)

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Floem: mentranslokasi hasil fs (asimilat ) dari daun dewasa ke bagian tumbuh dan penyimpan, al. akar. Td: • Sel-sel pengantar gula dan molekul organik lain = sieve element: sieve tube element (angiosperm) atau sieve cell (gimnosperm) = unsur tapis, buluh tapis • Companion cell = sel pengiring • Parenchyma cell Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Unsur-unsur tapis (sieve element) dewasa merupakan sel-sel hidup Berbeda dg xylem: mati, tanpa membran plasma, penebalan lignin

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Web Figure 10.1.B Photosynthate from the leaves appears in the sieve elements of phloem in the stem. 14CO2 was supplied to a source leaf of morning glory (Ipomea nil). 14C was incorporated into sugars synthesized in the photosynthetic process, which were then transported to other parts of the plant. The location of the label is revealed in the tissue cross sections by the presence of dark grains on the film. (A) shows a low magnification of the cross section of the stem (50×), revealing dark spots resulting from the silver grains in the film, shown in higher magnification (325×) in (B). The label is confined almost entirely to Siti FatonahBahan Ajar Fisiologi 8 the sieve elements of the phloem. (Courtesy of D. Fisher.) Tumbuhan 2014

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Sieve area Sieve area: terdapat pori pada dinding sel yang menghubungkan sel-sel Papan tapis mempunyai pori lebih besar: sebagai terusan untuk transport antar sel (gb. 10.5)

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Sel pengiring Buluh tapis berhubungan dengan sel pengiring (gb. 10.3; 10.4; 10.5) Plasmodesmata menembus dinding antara buluh tapis dan sel pengiring: perpindahan solute.

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Peran sel pengiring: • Transpor asimilat dari sel penghasil ke unsur tapis pada tulang daun minor. • Fungsi metabolik penting: sintesis protein • Adanya mitokondria: untuk suplai energi (ATP) ke unsur tapis

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3 tipe sel pengiring: 1.Ordinary companion cell: dinding sel dg permukaan dalam halus 2.Transfer cell: dinding sel yg jauh dari unsur tapis tjd ingrowth: meningkatkan pot. transfer solute mll membran 3.Intermediary cell: memungkinkan pengambilan solute mll hub sitoplasma, banyak plasmodesmata-bundle sheath cell

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POLA TRANSLOKASI: DARI SOURCE KE SINK

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Multiple sources and sinks

Source

Developing apex

Hasil fs = asimilat, fotosintat Source: organ pengekspor fotosintat, penghasil fotosintat : daun dewasa Sink: organ penerima fotosintat, non-fs: akar, umbi, buah, daun muda

Sink Source Translocation

Source

Sink Sink Sink Sink Sink Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Hubungan pembuluh Source mensuplay sink yang berhubungan langsung Arah vertikal daun = orthostich Jumlah internodus diantara daun-daun dg orthostich yang sama bervariasi Hubungan antara source dan sink terjadi karena adanya hubungan antar pembuluh = anastomose (gb. 10.8A)

Arah source-sink secara anatomi dan pola perkembangan Jarak terdekat Daun dewasa atas  tunas ujung, daun muda Daun bagian bawah  akar Tahap perkembangan Fase Vegetatif  akar dan tunas apikal Fase reproduktif  bunga, buah Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Modifikasi jalur translokasi Pelukaan atau pemangkasan dapat mengubah jalur translokasi

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Senyawa yg ditranslokasi: sukrosa, asam aino, hormon, ion inorganik

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LAJU PERPINDAHAN Velocity: jarak perjalanan senyawa tiap satuan waktu : cm/jam: 0,3-1,5m/jam Laju perpindahan masa: banyaknya senyawa yang melewati floem per sat waktu : g/jam.cm2 : 1-15 g/jam.cm2 Diukur dg perunut radioaktif

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MEKANISME TRANSLOKASI DALAM FLOEM: Model aliran tekanan Aliran solute (aliran masa) dikendalikan oleh gradien tekanan antara source dan sink (∆Ψp) sbg akibat dari pemuatan dan pembongkaran floem. Ψw= Ψs+ Ψp; Ψp = Ψw – Ψs Pada jaringan source: Akumulasi gula di unsur tapis  - ∆Ψs , Ψw↓ ∆ Ψw  air masuk ke unsur tapis  Ψp↑  aliran masa Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Penerimaan akhir dari jalur translokasi: pembongkaran floem  gula di unsur tapis ↓  pada unsur tapis sink: Ψs lebih positif Ψw floem ↑ (dibanding xilem) air keluar dari floem: Ψp ↓ pada unsur tapis sink. ~ Selama pemuatan dan pembongkaran floem: perbedaan tekanan terpelihara, air menuju ke source dan keluar dari sink, fotosintat mengikuti aliran masa Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Bukti : Energi tidak dibutuhkan dalam mengendalikan translokasi, meskipun dibutuhkan utk pemeliharaan struktur tapis: perlakuan yang membatasi persediaan ATP : suhu rendah, anoxia, inhibitor tidak menghentikan translokasi Pengukuran tekanan turgor antara source dan sink: turgor source lebih tinggi dari sink

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PEMUATAN FLOEM Pemuatan floem: perpindahan fotosintat dari kloroplas mesofil ke unsur tapis source 1. Triosa fosfat di kloroplas  sitosol  sukrosa. Pati tersimpan  keluar dari kloroplas  glukosa  sukrosa 2. Sukrosa keluar dari mesofil  sekitar unsur tapis (dalam tulang daun terkecil): transport jarak-dekat 3. Pemuatan floem: Gula ditransport ke unsur tapis dan sel pengiring Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Perpindahan fotosintat dari sel mesofil ke unsur tapis melalui apoplast atau symplast

Jalur Apoplast: Pada tulang daun minor: sel pengiring: ordinary atau sel transfer

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Penyerapan sukrosa secara apoplast membutuhkan energi metabolik Pada source, sukrosa terakumulasi di unsur tapis dan sel pengiring > sel mesofil - pot osmotik (Ψs) di unsur tapis lebih negatif Sukrosa ditransport melawan gradien pot kimia Bukti: pemberian inhibitor respirasi pada jaringan source: ATP menurun, pemuatan gula terhambat Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Pemuatan melibatkan Sucrose-H+ symporter Energi didapatkan dari perpindahan proton menuju ke sel digunakan untuk penyerapan sukrosa pH tinggi (Konsentrasi ion H rendah) di apoplas menghambat akumulasi sukrosa ke unsur tapis

Symport: transpor yg menggunakan E dari pemompaan proton. Penyerapan sukrosa membutuhkan ATP dan pemompaan proton sebagai sucroseH+symporter

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Faktor2 yg mempengaruhi pengaturan pemuatan floem apoplas: • Potensial, turgor unsur tapis: penurunan turgor meningkatkan pemuatan • Konsentrasi sukrosa di apoplast: konsentrasi sukrosa yg tinggi di apoplas meningkatkan pemuatan • Keberadaan protein simport • Peningkatan ion K Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Pemuatan floem secara simplas Untuk transpor sukrosa dan gula lain: raffinose dan stachyose, pada tulang daun dg sel pengiring: intermediary cell. Butuh pembukaan plasmodesmata dintara sel yang dilalui

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Pemuatan apoplas terjadi pada tanaman yang mpy banyak sel pengiring ordinary atau sel transfer pada tulang daun minor. Tanaman yg mpy banyak plasmodesmata diantara floem dan sel skitarnya : pohon, semak,liana : di daerah tropis dan sub tropis. Tanaman dg sedikit plasmodesmata : herba : subtropis, kering. Tanaman dg lebih dari 2 tipe sel pengiring: simpas dan apolas: Coleus Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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PEMBONGKARAN FLOEM Import gula ke sink: 1. Pembongkaran unsur tapis: impor gula ke unsur tapis pada jaringan sink. 2. Transpor jarak pendek: gula ditranspor ke sel-sel sink. 3. Penyimpanan dan metabolisme : disimpan atau dimetabolisme

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Pembongkaran secara simplas atau apoplas Sink sangat bervariasi : jenis, fase tumbuh Jalur symplast: pada daun muda, ujung akar Cukup plasmodesmata

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Apoplast: Tipe 1: transpor melewati kompleks unsur tapis-sel pengiring secara apoplast, kemudian melalui symplast Tipe 2: paling umum terjadi: pembongkaran melalui unsur tapis secara symplast, diikuti apoplast 2A: diakhiri jalur symplast 2B : diakhiri jalur apoplast

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Transport Sugar May Be Hydrolyzed in the Apoplast

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Transport ke sink butuh E metabolik

Web Figure 10.9.A The possible fates of sucrose unloaded apoplastically in sink tissues. (1) Sucrose that enters the apoplast can be split into glucose and fructose by a wall invertase before entering a cell from a sink tissue, or (2) sucrose can be taken up into the cell unaltered. (3) Once in the symplast of the cell from the sink tissue, sucrose can be split into glucose and fructose by a cytoplasmic invertase, or (4) sucrose can enter the vacuole unaltered. (5) Once in the vacuole, sucrose can be split into glucose and fructose by a vacuolar invertase, or it can remain unaltered. Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

Jalur apoplast diperlukan pada biji berkembang karena tak ada hub symplast antara jar induk dan jaringan embrio. Gula keluar dari unsur tapis (pembongkaran) melalui symplast, ditransfer dari symplast menuju ke apoplast (dari kompleks unsur tapissel pengiring): tipe 2 Gula sebagiandapat dimetabolisme di apoplast atau tanpa diubah. Sukrosa dapat dihidrolisa mjd glukosa dan fruktosa dalam apoplast oleh enzym kmd menuju ke sink

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Pembongkaran symplast mll plasmodesmata : pasif: gula ditranspor dari konsentrasi tinggi (unsur tapis) ke konsentrasi rendah (sink) Pembongkaran apoplast: mll 2membran sel(sel unsur tapis dan sel jar sink): butuh energi

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Transisi daun dari sink ke sorce Daun yg mulai berkembang: sink Perkembangan selanjutnya : 25% pembesaran: transisi dari sink ke source Setelah 40-50% berkembang: source penuh

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ALOKASI DAN PARTISI FOTOSINTAT Pengaturan pengalihan syw carbon ke berbagai jalur metabolik = alokasi Distribusi fotosintat = partisi

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Alokasi Karbon yg difiksasi dalam sel source digunakan utk cadangan, metabolisme, transport: Sintesis syw cadangan: pati disintesis dan disimpan dalam kloroplas Kegunaan metabolik: syw karbon digunakan pd berbagai kompartment utk E atau kerangka C Sintesis syw transpor: membentuk gula transport diekspor ke sink, sebagian di vakuola Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Partisi Kemampuan yg lbh besar dr sink utk menyimpan atau memetabolisme gula transport  kemapuan lebih besar utk berkompetisi Vegetatif: sink utama : daun muda, batang Reproduktif: bunga, buah, biji

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Sink bersaing Jika jumlah sink berkurang proporsi fotosintat ke masing-masing sink lebih besar Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Partisi diantara sink yang bersaing tgtg pada: 1. Jarak antara source dan sink 2. Kekuatan sink Kekuatan sink: ukuran kapasitas sink thd akumulasi metabolit Kekuatan sink: ukuran sink x aktivitas sink Ukuran sink: total biomasa sink Aktivitas sink: laju pengambilan fotosintat Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Aktivitas dalam jaringan sink mempengaruhi penyerapan oleh sink Aktivitas sinK: Metabolisme dalam dinding sel Penyerapan dari apoplas Proses metabolik  pertumbuhan atau disimpan Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Faktor-faktor yg mempengaruhi kekuatan sink: Kekuatan sink berhub dg produktivitas dan hasil : • Pengisian biji pada sereal • Pertumbuhan buah

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• 4 Factors yang mempengaruhi translokasi fotosintat Faktor internal: (1) sucrose : S↑，export↑. (2) ATP↑ 、Pi↑ ， ATP↑ ， export ↑. (3) hormon: Auksin, giberelin, sitokini, asam absisat (ABA) import ↑ (4) size in sink , sink ↑， import ↑。 (5) Tipe sink (6) Banyaknya sink Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

Faktor luar: 1.Water: not enough, water potential↓ Pn↓，S↓，hambatan stomata ↑， transport↓. 2. Light: light↓，Pn↓，S↓，export↓ Transpor selama siang hari > malam hari

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3. Temperature： optimum 20-30℃。 T↓，transport↓ because of respiration↓，energy↓ ，Pn↓ photoassimilate↓， High T， transport↓, respiration ↑，Pn↓， photoassimilate↓

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In symplastic phloem unloading, transport sugars such as sucrose move through the plasmodesmata to the sink cells. In the sink cells, sucrose can be metabolized in the cytosol or the vacuole before being stored or entering metabolic pathways associated with growth of the tissue. When phloem unloading is apoplastic, however, there is an additional opportunity for metabolic change. The transport sugar can be partly metabolized in the apoplast, or it can cross the apoplast unchanged (Web Figure 10.9.A). For example, sucrose can be hydrolyzed into glucose and fructose in the apoplast by invertase, and glucose and/or fructose would then enter the sink cells. The fact that many monosaccharide transporters have been localized mainly in sink tissues supports the possible existence of this pathway.

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A recent study with potato plants has shown that apoplastic unloading predominated in elongating stolons (Viola et al. 2001). Stolons are underground lateral shoots that grow from the main stem of the potato plant, characterized by elongated internodes and hooked apical tips, that form tubers at their apices in response to environmental signals. When tuberization started in stolons, phloem unloading shifted from apoplastic to symplastic transport. Histochemical analysis of potato lines transformed with the promoter of an apoplastic invertase gene (invGE) linked to a reporter gene showed invertase activity in the elongating stolon, associated with apoplastic unloading (Web Figure 10.9.B). In the developing tuber, apoplastic loading and invertase activity was observed in a small apical region, which was the apical area of the stolon progressively engulfed by the swelling subapical regions during tuberization. Most of the tuber showed symplastic unloading and lacked expression of the invertase gene. Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Web Figure 10.9.B Expression of an apoplastic invertase (invGE) revealed by GUS staining. (A) GUS staining is restricted to the apical hook region of an elongating stolon (arrow). (B) Developing tuber showing GUS staining associated with the apical bud region (arrow). Bar in (A) = 1 mm; bar in (B) = 500 µm. (From Viola et al. 2001.)

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Energy Requirements for Unloading in Developing Seeds and Storage Organs Developing seeds have proven to be a most interesting system in which to study unloading processes. In legumes such as soybean, the embryo can be removed from the seed coat. In this way, unloading from the seed coat into the apoplast can be studied without the influence of the embryo, and uptake into the embryo can also be investigated separately. Studies with legumes have shown that both entry of sucrose into the apoplast and uptake into the embryo are mediated by transporters and are active. In cereals like wheat, only uptake into the embryo is active; the loss of sucrose (sucrose efflux) from the maternal tissues is passive (down the concentration gradient), because the subsequent active step keeps the sucrose concentration in the apoplast low. In corn, the cell wall invertase helps maintain a low apoplastic sucrose concentration by splitting the disaccharide into monosaccharides. In general, sugar–proton symport mechanisms appear to function in the uptake of sugars from the apoplast, as in sucrose uptake into the soybean embryo. Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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Storage organs often accumulate sugars to high concentrations, for example, in sugar beet taproot and sugarcane stem. This sugar accumulation requires active membrane transport, since energy is required to move sugars into storage compartments against a concentration gradient. Sugar transport into the vacuoles of storage cells such as those of sugar beet is thought to be accomplished by a sucrose– proton antiport (see textbook Chapter 6). In this case, a vacuolar H+-ATPase pumps protons into the vacuole; the antiport carrier then moves sucrose into the vacuole in exchange for protons, which exit the vacuole down their electrochemical-potential gradient (see textbook Figure 6.11).

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Possible Mechanisms Linking Sink Demand and Photosynthetic Rate in Starch Storers Photosynthetic rate and sink demand are linked processes, particularly in species which store starch. Increased sink demand is correlated with increased photosynthesis, and vice versa. An accumulation of photosynthate (starch, sucrose, or hexoses), in the source leaf could be one of the signals coupling photosynthetic rate with sink demand. Possible mechanisms include:

1. Inhibition by starch. When sink demand is low, high starch levels in the source could physically disrupt the chloroplasts, interfere with CO2 diffusion, or block light absorption. Little evidence supports this hypothesis.

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2. Phosphate availability. When sink demand is low, photosynthesis could be restricted by a lack of free orthophosphate in the chloroplast (Du et al. 2000). Under conditions of low demand by the sink, sucrose synthesis is usually reduced, and less phosphate is thus available for exchange with triose phosphate from the chloroplast (via the phosphate translocator). If starch synthesis, which releases orthophosphate in the chloroplast, could not release phosphate fast enough, a deficiency in phosphate would ensue. ATP synthesis and thus CO2 fixation would decline. A study with potato plants transformed with antisense DNA to the phosphate translocator provides support for this hypothesis (Riesmeier et al. 1993). The transformed plants, which displayed reduced phosphate translocator activity, allocated proportionately more carbon into starch and less into sucrose. These effects were accompanied by a reduction in the light- and CO2-saturated rates of photosynthesis in young plants. Siti Fatonah- Bahan Ajar Fisiologi Tumbuhan 2014

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3. Regulation by sugars. High sugar levels decrease the transcription rate and expression of genes for many photosynthetic enzymes (Koch 1996). The changes in gene expression occur over the same time frame as the source adjustments already described. For example, in source leaves of spinach (Spinacia oleracea), mRNA for several photosynthetic enzymes decreased when soluble carbohydrates accumulated as a result of inhibition of export from the leaf (Krapp and Stitt 1995). Although transcript levels began to decline almost immediately, changes in photosynthetic enzyme activity were apparent only after several days. In this species at least, photosynthesis appeared to be inhibited because of changes in gene expression, not because of phosphate limitation, as discussed earlier.

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