www.plntroot.org 48 Originl reserch rticle Compenstory growth of lterl roots responding to excision of seminl root tip in rice Tsus Kwi 1, Misuzu Nosk-Tkhshi 2, Akir Ymuchi 1, Yoshiki Inuki 2,3 1 Grdute School of Biogriculturl Sciences, Ngoy University, Ngoy, Aichi 464-861, Jpn 2 Interntionl Coopertion Center for Agriculturl Eduction, Ngoy University, Ngoy, Aichi 464-861, Jpn 3 PREST, JST, Kwguchi, Sitm 332-12, Jpn Corresponding uthor: Y. Inuki, E-mil: inukiy@gr.ngoy-u.c.jp, Phone: +81-52-789-73, Fx: +81-52-789-4222 Received on Jnury 3, 17; Accepted on My 11, 17 Astrct: Development of lterl roots (LRs) is promoted when prentl root elongtion is suppressed. Promotion of LR growth compenstes for prentl root growth, which contriutes to the mintennce of totl root length. However, the developmentl processes underlying the compenstory growth of LRs re still uncler. In this study, we induced LR compenstory growth in rice y surgicl excision of prent (seminl) root tip, nd nlyzed the morphologicl nd ntomicl chnges in LRs. Our nlysis reveled tht seminl roottip excision incresed the dimeter of first-order LRs y incresing in the numer of ground-tissue lyers nd the stele dimeter nd lso promoted elongtion, with higher order LRs emerging in the remining proximl portions. We lso found tht LR primordi in erly developmentl stges incresed in dimeter mrkedly nd then produced higher order LRs pronouncedly in response to seminl-root cutting. Furthermore, the degree of chnge in LR morphology differed depending on the distnce from the cut site, with chnges in morphologies wekening further from the cut site. Tken together, the morphologicl nd ntomicl chnges of LRs tht re induced y the root-tip excision of the prent vry continuously, nd re dependent on oth the developmentl stge of LR primordi t the site of root-cutting nd the distnce from the cut site. Keywords: compenstory growth, lterl root, lterl root primordi, rice (Oryz stiv L.), root tip excision Arevitions: CRs, crown roots; LR, lterl root; LRP, lterl root primordi; SR, seminl root Introduction The root systems of rice re composed of severl different types of roots. A seminl root is produced emryoniclly, wheres crown roots develop postemryoniclly from the stem. Seminl nd crown roots (prent roots) produce lterl roots (LRs), which re clssified into S-types nd L-types ccording to distinct morphologicl nd ntomicl chrcteristics (Kwt nd Shiym 1965, Kono et l. 1972, Ymuchi et l. 1987); S-type LRs re short nd thin, nd never produce higher order LRs, wheres L-type LRs re long nd thick, nd often produce higher order LRs. L-type LRs re more similr ntomiclly to prent roots for exmple in developed vsculr structure, nd the presence of sclerenchym nd multiple cortex lyers thn re S-types. These two different types of LRs vry in developmentl chrcteristics nd genetic control of their development (Kwt nd Shiym 1965, Sski et l. 1984, Wng et l. 6, Niones et l. 15). In ddition, production of L-type LRs is importnt for expnding the root system ecuse of their ility to produce higher order LRs. Development of LRs is promoted when elongtion of the prent root is inhiited; one such fctor inhiiting prent root elongtion is soil compction, s compct soils, which re heterogeneously distriuted, cuses mechnicl impednce of prent root growth. This mechnicl impednce of the prent root triggers LR development in vrious plnt species (Goss 1977, Shierlw nd Alston 1984, Atwell 199, Montgu et l. 1). Even in compct soil, LRs cn elongte into the nrrow pore spces nd crcks in the soil (Binghm nd Bengough 3, White nd Kwi T, Nosk-Tkhshi M, Ymuchi A, Inuki Y 17 Compenstory growth of lterl roots responding to excision of seminl root tip in rice. Plnt Root 11:48-57. doi:1.3117/plntroot.11.48 Copyrights 17, Plnt Root (JSRR), www.plntroot.org
www.plntroot.org 49 Kirkegrd 1, Hling et l. 11). Compenstory growth of LRs fcilittes the uptke of wter nd nutrients y mintining sufficient totl root length, which in turn contriutes to shoot growth mintennce (Montgu et l. 1). However, the developmentl processes underlying compenstory LR growth remin lrgely unknown. Compenstory growth of LRs cn e lso induced y surgicl excision of the root tip, which disrupts prent root elongtion nd stimultes the development of L-type LRs in the remining proximl portions of the root (Sski et l. 1984). The ility of plnt to produce higher order LRs in response to the root-tip excision is defined y the developmentl stge of the lterl root primordi (LRP); however, morphologicl nd ntomicl chnges induced y the root-tip excision nd their reltion with the developmentl stge of LRP hve yet to e determined. Here, we exmined the morphologicl nd ntomicl chnges in LRs induced y seminl-root cutting, focusing on the developmentl stge of LRP t the loction of the root cut nd the LR position reltive to the cut site. Morphologicl chnges in LRs induced y seminl-root cutting vried continuously ccording to the developmentl stge of LRP t the root-cutting site nd the LR position reltive to the cut site. We lso found tht the ntomicl chrcteristics of LRs were different depending on the developmentl stge of LRP t the root-cutting site. Moreover, nlyses of the reltionships etween severl LR chrcteristics reveled tht lrger dimeter LRs grow longer nd produce second-order LRs more frequently thn do smller dimeter LRs. Mterils nd Methods Plnt growth conditions nd root-cutting tretment Seedlings of rice (Oryz stiv L. vr. Tichung 65) were used in ll experiments. Rice seeds were sterilized y soking in Benlte solution dissolved in tp wter (2.5 g L 1 ) t 28 C for 3 d, then rinsed three times with tp wter nd trnsferred onto plstic nets floting on nutrient-free filtered wter in lck plstic ox. The wter ws erted with pump (OX-3, Tetr-Jpn, Jpn) eginning 2 d fter seedling trnsference nd kept t constnt level through the ddition of filtered wter. Seedlings were grown in growth chmer (MLR- 351, Snyo, Jpn) t 28 C under continuous light. Seminl root length nd the distnce from the se to the LR emergence site, where the youngest LR ppers from the outer surfce of the seminl root, were mesured 4 d fter trnsference (Fig. 1A). Seedlings hving more thn 1 mm distnce from the se to the LR emergence site s like Fig. 3A were used for the following experiments. The seminl roots were cut into vrious root-cutting rtios using cutting knife (Fig. 2A). Root-cutting rtio (%) ws clculted s: Distnce ehind root tip (mm) 1 Distnce from root tip to LR emergence site (mm) After root cutting, seedlings were put ck onto the floting nets nd llowed to continue to grow. Morphologicl nd ntomicl chrcteriztion Mesurement of emerged LR chrcteristics: Seminl roots were smpled nd fixed in FAA (5% formldehyde, 5% cetic cid, 63% ethnol [v v 1 ]) 1 d following root cutting (14 d fter trnsplnting). Dimeters of first-order LRs were mesured t the sl region using microscope (IX71, Olympus, Jpn) nd micrometer. For the mesurement of LR length, LRs, long with the seminl root, were fully extended on n gr plte, nd imges were tken with digitl cmer (D9, Nikon, Jpn) t 3 dpi. Length of the first-order LR ws mesured using imge-nlysis softwre (LIA for Win32, https:// www.gr.ngoy-u.c.jp/~shinkn/lia32). For evluting the effect of root cutting in the different root-cutting rtios, morphology of the LRs tht emerged in the region etween the excision site nd 1% higher rtio were nlyzed (Fig. 3A). For nlyzing the reltionship etween morphologicl chnges in LR induced y the root-tip excision nd distnce from the cut site to ech LR, morphology of the LRs tht emerged in the 1% root-cutting rtio were nlyzed every 5 mm up to distnce of 25 mm from the cut site (Fig. 5A). Oservtion of LRP y rpid trnsprency method: Seminl roots of 4-d-old seedlings were fixed nd clered with TOMEI-I (Hsegw et l. 16). Imges of LRP in the clered seminl roots were cptured using differentil interference microscope (BX6, Olympus, Jpn), nd distnce from root tip to the LR emergence site nd distnce from root tip to the LRP were mesured. LRP position ws defined s reltive distnce from root tip, which is distnce from root tip to LRP sed on distnce from root tip to the LR emergence site s 1% (Fig. 1A). Width of the meristem region, excluding the Tsche (which origintes from the endodermis of the prent root; Kwt nd Shiym, 1965) nd distnce from the se to the tip were mesured s LRP dimeter nd length using imging softwre (LIA for Win32), respectively (Fig. 1B).
www.plntroot.org 5 Histologicl nlysis of LRs: The thickest LR mong the LRs tht emerged 4 d following 1%, 5%, 6%, 8% nd 1% root-cutting in ech plnt were smpled. The smpled LRs were plced in FAA solution, nd de-erted with vcuum for 3 min. The FAA solution ws then renewed nd smples were kept for 24 h t 4 C. Fixed smples were dehydrted in grded ethnol series nd emedded in Technovit 71 (Hereus Kulzer, Germny); emedded smples were then sectioned into 6-μm-thick slices using rotry microtome (RM2125 RTS, Leic, Germny) nd oserved under microscope (IX71, Olympus, Jpn). Stele dimeters nd thicknesses of the ground tissues (exodermis, sclerenchym, cortex, nd endodermis) were mesured using imging softwre (LIA for Win32), nd the numer of ground-tissue lyers were counted (Fig. 4A). Dt nlysis The morphologicl nd ntomicl chrcteristics of the LRs of the different groups were compred using ANOVA nd multiple comprison Tukey s test in R commnder ver. 2.3-1 (Fox 5). The reltionships etween reltive distnce from root tip nd LRP size ws nlyzed using regression nlysis, nd correltion coefficients were clculted in Microsoft Excel 16. Results Reltionship etween developmentl stge of LRP nd responsiveness to root-tip excision For determining the reltionship etween LRP developmentl stge nd responsiveness to the roottip excision, we first nlyzed the LRP developmentl stge in the intct seminl roots of 4-d-old seedlings. As result, positive correltions were detected etween LRP size (dimeter or length) nd reltive distnce from root tip, which were stronger thn the correltions etween LRP size (dimeter or length) nd the distnce from root tip to LRP (r =.68 nd.64 respectively; dt not shown), suggesting tht LRP locted t the sme reltive distnce from root tip within root tip to the LR emergence site re t the sme developmentl stges in the seminl roots of 4-d-old seedlings (Fig. 1A D; see Mterils nd Methods). Bsed on the reveled reltionship etween LRP developmentl stge nd reltive distnce from root A B Tip Bse Tsche Endodermis Pericycle C LRP dimeter (μm) 1 1 1 8 6 r =.81* 1 1 r =.85* 1 8 6 6 8 1 6 8 1 Reltive distnce from root tip (%) D LRP length (μm) Reltive distnce from root tip (%) Fig. 1. Reltionship etween position nd size of LRP on intct seminl roots. (A) Schemtic digrm of 4-d-old seminl root. LRP position ws defined s the reltive distnce from root tip sed on distnce from root tip to the LR emergence site, where the youngest LR ppers from the outer surfce of the seminl root, s 1%. (B) Schemtic digrm of LRP in seminl root; LRP dimeter nd length were defined s the width of the meristem region () nd the distnce from the se to tip (), respectively. (C, D) Reltionships etween reltive distnce from root tip to primordi within root tip to the LR emergence site nd dimeter (C) or length (D) of LRP in seminl roots. Asterisk indictes significnt correltion etween reltive distnce from root tip nd LRP dimeter or length (P <.1, n = 44 primordi from 5 plnts).
www.plntroot.org 51 tip, seminl roots were cut into vrious root-cutting rtios for determining the reltionship etween LRP developmentl stge nd responsiveness to the roottip excision (Fig. 2A; see Mterils nd Methods). No LRs with higher order LRs emerged from intct seminl roots. For root-cutting rtios of less thn 6%, mximum dimeter of first-order LR (hereinfter dimeter of first-order LR is referred to simply s LR dimeter), which is dimeter of the thickest LR mong the LRs tht emerged fter root cutting, ws greter thn 16 μm (control with intct roots: 14. ± 4.6 μm, n = 3 plnts), nd lmost ll of the thickest LRs produced second-order LRs (Fig. 2B). For root-cutting rtios of more thn 6%, increses in the mximum LR dimeter grdully slowed long with incresing root-cutting rtio, nd few LRs produced second-order LRs. For more quntittive evlution of the reltionship etween LRP developmentl stge nd responsiveness to the root-tip excision, seminl roots were cut into 1%, 5%, 6%, 8%, nd 1% root-cutting rtios, nd morphology of the LRs emerged fter root cutting were nlyzed (Fig. 3A; see Mterils nd Methods). Mximum LR dimeters were virtully identicl in the 1% nd 5% root-cutting rtios, nd LRs with second-order LRs emerged eqully in oth (Fig. 3B, C, G, H). In the 6% root-cutting rtio, the increse in the mximum LR dimeter induced y root cutting ws smller thn those of the 1% nd 5% root-cutting rtios, nd few LRs with second-order LRs emerged (Fig. 3D, G, H). The mximum dimeter of LRs ws slightly higher in the 8% root-cutting rtio thn tht of control (Fig. 3E, G). Notly, some LRs with second-order LRs tht sometimes emerged in the 8% root-cutting rtio hd much smller dimeter (out 1 μm) thn those tht emerged in the 1% nd 5% root-cutting rtios. In the 1% rootcutting rtio, LR dimeter ws unffected y root cutting, nd no LRs produced higher order LRs (Fig. 3F, G, H). The dimeter of LRs tht emerged in the region etween the excision site nd 1% higher rtio ws distriuted successively in the 1%, 5%, 6%, nd 8% root-cutting rtios (Fig. 3I), nd LRs with second-order LRs were more often of lrger dimeter in ech root-cutting rtio. Antomicl chnges of LR induced y seminl-root cutting in the different developmentl stges of LRP We nlyzed the ntomicl chrcteristics of LRs of vrious dimeters emerged in the different root- A Reltive distnce from root tip % 1% 1% root-cutting rtio 5% root-cutting rtio B Mximum LR dimeter (μm) 2 2 18 16 1 1 1 + 2nd order LRs 2nd order LRs 6 8 1 Root-cutting rtio(%) Root tip (%) LR emergence site (1%) Fig. 2. Reltionship etween root-cutting rtio nd LR morphology. (A) Schemtic digrm showing the definition of the rootcutting rtio. (B) Seminl roots were cut into vrious root-cutting rtios. Dimeter of the thickest LR mong the LRs tht emerged fter root cutting in ech plnt nd whether second-order LRs were produced re shown (n = 44 plnts).
www.plntroot.org 52 G H Mximum LR dimeter (μm) 25 15 1 5 c c cut 1% 5% 6% 8% 1% Numer of LRs with 2nd order LRs 6 5 4 3 2 1 cut 1% 5% 6% 8% 1% I 1 Reltive frequency (%) 9 8 7 6 5 3 1 1% Root-cutting rtio Root-cutting rtio 5% 6% 8% 1% <6 <8 <1 <1 <1 <16 <18 < <2 <2 <6 <8 <1 <1 <1 <16 <18 < <2 <2 <6 <8 <1 <1 <1 <16 <18 < <2 <2 <6 <8 <1 <1 <1 <16 <18 < <2 <2 <6 <8 <1 <1 <1 <16 <18 < <2 <2 LR dimeter (μm) cut 2nd order LRs 2nd order LRs + 2nd order LRs Fig. 3. Morphologicl chrcteristics of LR tht emerged in the different root-cutting rtios. Seminl roots were cut into 1%, 5%, 6%, 8%, nd 1% root-cutting rtios, nd the morphology of the LRs tht emerged in the region etween excision site nd 1% higher rtio were nlyzed. Morphologies of LRs emerged in the regions corresponding to those of root cuttings in intct seminl roots were nlyzed for the control ( cut). (A) Imge of seminl root efore the root cutting (r = 5 mm). Arrow heds indicte the cut site of ech root-cutting rtio. Squres indicte the regions where morphologicl chrcteristics of emerged LRs were mesured for ech root-cutting rtio. SR, Seminl root; CRs, Crown roots. (B F) Imges of seminl nd LRs ner the cut site 1 d fter root cutting (rs = 5 mm). (G, H) Mximum LR dimeter (G) nd the numer of LRs with second-order LRs (H). Different letters indicte significnt differences etween groups (P <.5); vlues represent mens ± SDs (n = 1 plnts for ech root-cutting rtio). (I) Distriutions of LR dimeters with or without second-order LRs.
www.plntroot.org 53 cutting rtios (Fig. 4A; see Mterils nd Methods). Antomicl chrcteristics were virtully identicl in the 1% nd 5% root-cutting rtios (Fig. 4B D). In the 6% root-cutting rtios, stele dimeter, ground-tissue thickness nd the numer of groundtissue lyers were smller thn those of the 1% nd 5% root-cutting rtios, ut still significntly lrger thn those of LRs emerged in intct seminl roots. In the 8% root-cutting rtio, stele dimeter of LRs ws significntly lrger thn tht of control, wheres ground-tissue thickness nd the numer of groundtissue lyers were not. Reltionship etween morphologicl chnges in LR induced y root-tip excision nd distnce from cut site to ech LR We exmined how fr the effects of the root-tip excision extended in the remining proximl portions of the 1% root-cutting rtio y nlyzing LR morphology through the 1 6% rtios from root tip, in which LRP hd the sme potentil to respond to the root-tip excision s shown in Fig. 2B (Fig. 5A; see Mterils nd Methods). Mximum LR dimeter incresed in distnces up to 15 mm from the cut site (Fig. 5B), wheres significnt increses in the mximum length of the first-order LR nd emergence of LRs with second-order LRs were limited to distnces of 5 mm from the cut site (Fig. 5C, D). The degree of chnge wekened with distnce from the cut site for ll LR morphologicl chrcteristics (Fig. 5B D). We next nlyzed the reltionships etween LR morphologicl chrcteristics using the three individuls hving different distriutions of LR dimeters nd lengths of first-order LRs (Fig. 5E). In the region where oth dimeter nd length of first-order LRs were incresed to some extent ( 1 mm from the cut site; Fig. 5B, C), lrger dimeter LRs tended to ecome longer nd produced secondorder LRs more frequently regrdless of distnce from the cut site to the LRP in ll individuls. A B Stele dimeter (μm) 5 3 1 c c cut 1% 5% 6% 8% 1% Root-cutting rtio C D Thickness of ground tissue (μm) 7 6 5 3 1 c c cut 1% 5% 6% 8% 1% The numer of ground-tissue lyers 7 6 5 4 3 2 1 c c cut 1% 5% 6% 8% 1% Root-cutting rtio Root-cutting rtio Fig. 4. Reltionship etween developmentl stge of LRP nd ntomicl chnges of LR induced y seminl-root cutting. The thickest LR mong the LRs emerged in ech root-cutting rtio were cross-sectioned nd their ntomy nlyzed. Antomicl chrcteristics of LRs emerged in the regions corresponding to those of root cuttings in intct seminl roots were nlyzed for the control ( cut). (A) Imge of cross-section of LR indicting stele () nd ground tissue () (r = 1 μm). (B D) Stele dimeter (B), thickness of ground tissue (C), nd the numer of ground-tissue lyers (D) of LRs emerged in different root-cutting rtios. Different letters indicte significnt differences etween groups (P <.5); vlues represent mens ± SDs (n = 5 plnts for ech root-cutting rtio).
www.plntroot.org 54 A 1% B C Reltive distnce from root tip 6% 1% % 5 1 15 25 Distnce from cut site (mm) Distnce from cut site (mm) D 25-25 15 15-1 15 1-15 5 1 5-1 5-5 cut c c d e 1 3 Mximum LR dimeter (μm) Distnce from cut site (mm) 25-25 15 15-1 15 1-15 5 1 5-1 5-5 cut Distnce from cut site (mm) 25-25 15 15-1 15 1-15 5 1 5-1 5-5 cut 6 8 Mximum length of 1st order LR (mm) 2 4 6 8 Numer of LRs with 2nd order LRs E Length of 1st order LR (mm) 5 3 1 Individul 1 Individul 2 Individul 3 5 15 25 5 15 25 5 15 25 LR dimeter (μm) 5 mm 2nd order LRs 5 1 mm 2nd order LRs 5 mm + 2nd order LRs 5 1 mm + 2nd order LRs Fig. 5. Reltionship etween distnce from the cut site nd LR morphology. LRs in the 1% root-cutting rtio were nlyzed every 5 mm up to distnce of 25 mm from the cut site. Morphologies of LRs emerged in the regions corresponding to those of the root cutting in intct seminl roots were nlyzed for the control ( cut). (A) Schemtic digrm showing the region nlyzed. (B D) Mens of mximum LR dimeter (B), mximum length of first-order LR (C), nd the numer of LRs with second-order LRs (D) of ech region. Different letters indicte significnt differences etween groups (P <.5); vlues represent mens ± SDs (n = 1 plnts). (E) Reltionships etween LR dimeter, length of first-order LR, nd production of second-order LRs t distnces of 5 mm nd 5 1 mm from the cut site. Dt from three different individuls, which hve different distriutions of LR dimeters nd lengths of first-order LRs, were shown.
www.plntroot.org 55 Discussion The morphology of LRs tht emerged following the root-tip excision vried continuously depending on the LRP developmentl stge t the site of root cutting (Fig. 6A). Reltionship etween LRP developmentl stge nd reltive distnce from root tip to LRP ws reveled (Fig. 1C, D), which is consistent with oservtions for crown roots (Sski et l. 1984), nd enled us to exmine the reltionship etween LRP developmentl stge nd responsiveness to seminl-root cutting y using the vrious root-cutting rtios. In less thn 6% rootcutting rtio, LR dimeter incresed to over 16 μm, with the thick LRs producing second-order LRs (Fig. 2B, Fig. 3B, C, G, H, Fig. 6A). The LRP t the 6% rtio from root tip hd dimeter of 8 μm nd length of 7 μm (Fig. 1C, D), demonstrting tht the dimeters of LRP t erly developmentl stges increse mrkedly nd produce second-order LRs in response to the root-tip excision. Moreover, increses in LR dimeter grdully declined with incresing root-cutting rtio, nd few LRs produced second-order LRs in more thn 6% root-cutting rtio (Fig. 2B, Fig. 3D H, Fig. 6A). In ddition, smll increses in LR dimeters were lso induced in LRP t lter developmentl stges. We found tht there ws reltionship etween LRP developmentl stge nd emergence of LRs with higher order LRs in seminl roots, which is consistent with oservtion for crown roots (Sski et l. 1984), nd furthermore, tht the dimeter of LRs tht emerged fter the root-tip excision vries continuously depending on the LRP developmentl stge t the site of the root cutting. Seminl-root cutting induces the ntomicl chnges in LRs nd the ntomicl chrcteristics of LRs re different depending on the developmentl stge of LRP t the root-cutting site. LRP t erly developmentl stge mrkedly incresed oth the numer of ground-tissue lyers (nd hence increse in thickness of ground tissue) nd stele dimeter responding to seminl-root cutting (Fig. 4B D). The increse in the numer of ground-tissue lyers ws driven y the ctivtion of periclinl division of the initil cells in the LRP (Kwt nd Shiym 1965); it my e tht seminl-root cutting ctivtes periclinl division of the initil cell in LRP. In lter LRP developmentl stge, the increse in groundtissue lyers decresed s proceeding of LRP development, however, stele dimeter ws still incresed to some extent even in LRP t 8% rtio from root tip (Fig. 4B, D). Few LRs produced second-order LRs for root-cutting rtios of more thn 6%, ut LRs with comprtively smll dimeters t times produced second-order LRs in the 8% root-cutting rtio (Fig. 3I). Thus, the increse in LR dimeter induced y the increse in the numer of ground-tissue lyers is not necessry for production of second-order LRs, ut developed stele my ply n importnt role in the production of higher order LRs. This hypothesis is supported y the finding tht thicker roots with second-order LRs tend to hve lrger stele with thicker lte metxylem, nd greter undnces of proto-xylem nd protophloem (Sski et l. 1981). The degree of chnge in LR morphology induced y the root-tip excision is dependent on the distnce from the cut site (Fig. 6B). Mximum dimeter nd length of first-order LRs were reltively lrger, nd LRs with second-order LRs emerged more frequently the closer to the cut site (Fig. 5B D, Fig. 6B). The region where increse in LR dimeter occurred ws further distnt from the cut site thn those of increse in length of first-order LRs nd production of second-order LRs. Reltive LR dimeter mong the LRs tht emerged in the region where oth dimeter nd length of first-order LRs were incresed to some extent y the root-tip excision determines the length of the first-order LRs nd whether or not the LR produces second-order LRs (Fig. 6B). Reltively lrge dimeter LRs were longer thn smllerdimeter LRs nd produced second-order LRs in the 1% root-cutting rtio regrdless of distnce from the cut site (Fig. 5E, Fig. 6B), indicting tht reltive LR dimeter mong the LRs tht emerged from the sme prent root fter the root-tip excision might determine the length of the first-order LR nd whether the LR produces second-order LRs. Root picl dimeter ws positively correlted with root elongtion rte (Chn et l. 1989, Pges 1995, Lecompte nd Pges 6, Wu et l. 16), nd is regrded s n indictor of the sink strength of root (Thler nd Pges 1999). Thus, incresed ssimilte supply to first-order LRs of lrger dimeter nd sink strength s result of excision of seminl root tip induces preferentil elongtion nd production of second-order LRs. Bsed on these findings, we re now plnning to undertke severl experiments to further our understnding of the moleculr mechnisms underlying compenstory LR growth. Acknowledgements We thnk Ms. Kimiyo Inuki, Ms. Eiko Murkmi, nd Ms. Ski Nishiuchi for their vlule technicl supports. This work ws supported y PRESTO JST, the Grnt-in-Aid from the Jpn Society for the Promotion of Science (No. 15H4435), nd JST/JICA,
www.plntroot.org 56 A B Fig. 6. (A) Schemtic model illustrting the reltionship etween the morphologicl chnges in LRs induced y the root-tip excision nd LRP developmentl stge. (Left) Prior to root cutting, the LRP developmentl stge proceeded long the seminl root, which positively correlted with reltive distnce from root tip within root tip to the LR emergence site. (Middle) LRP nd LR dimeters incresed s result of root cutting, with the degree of chnge in dimeter depending on the LRP developmentl stge t the site of the root cutting. For root-cutting rtios of less thn 6%, dimeters incresed y more thn 16 μm, ut grdully declined for root-cutting rtios of more thn 6%. (Right) Lrge-dimeter LRs often produced second-order LRs in the region for root-cutting rtios of less thn 6%, ut second-order LRs rrely emerged from first-order LRs for root-cutting rtios of more thn 6%. (B) Schemtic model showing the reltionship etween morphologicl chnges in LR nd distnce from the cut site. The region where increse in LR dimeter occurred ws further distnt from the cut site thn those of increses in length of first-order LR nd production of second-order LRs. Thicker LRs grew longer nd produced second-order LRs t greter frequency regrdless of distnce from root tip.
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