Mesophyll conductance: a third player in the photosynthesis game Jaume Flexas The China - EU Workshop on Phenotypic Profiling and Technology Transfer on Crop Breeding, Barcelona
Photosynthesis traditionally viewed as a sugar factory with only TWO limiting steps: CO2 supply (STOMATA) Demand function Supply function Photosynthetic machinery functioning SUGAR
Photosynthesis as a diffusion process A = gs (Ca Ci) = gm (Ci Cc) Traditionally considered LARGE and CONSTANT But: IT IS NOT SO LARGE...
But CO2 is used in photosynthesis INSIDE chloroplasts There is long way accross!!!! CO2 enters leaves through stomata
SUB-STOMATAL CAVITY: Where CO2 initially ARRIVES CHLOROPLASTS: Where photosynthesis OCCURS How to REACH photosynthetic sites?
The CO2 path A Ca gb B Cs GAS PHASE gs gias gw LIPIDLIQUID PHASE gliq Cc Ci Ci s gm cw Cc st pm ce pg 50 mm 0.2 mm Flexas et al. 2008. Plant Cell Environ. 31, 602-621
Evans et al. (2009) J. Exp. Bot. 60, 2235-2248
0,5 0,4 a ab 0,3 A A abc A A bc 0,2 A c c A 0,1 d Se H er m i-d bs ec id uo us D ec id uo us Ev er gr ee Li n ve C rw on or ife ts rs /H or nw or ts se s 0,0 G ra s gs or gm (mol CO2 m-2 s-1) Stomatal conductance Mesophyll conductance Group Flexas et al. 2012. Plant Science 196, 70-84
CO2 assimilation Restricted gm = photosynthetsis inefficiency [CO2] (proxy Ci)
STRUCTURAL CANDIDATES FOR SETTING gm RESISTANCES ALONG THE LIQUID PHASE Evans et al. (2009) J. Exp. Bot. 60, 2235-2248
Tomàs et al. (submitted)
However gm is only partly dependend of leaf anatomy 0.4-2 -1-1 gm (mol CO2 m s bar ) 0.5 0.3? 0.2 0.1 0.0 0 50 100 150 200 250-2 LMA (g m ) Flexas et al. 2008. Plant Cell Environ. 31, 602-621
... AND IT IS NOT SO CONSTANT! - Salinity (Bongi & Loreto 1989 Plant Physiol. 90, 1408-1416) - Leaf ontogeny (Loreto et al. 1994 Photosynth. Res. 41, 397-403) - Altitude (Kogami et al. 2001 Plant Cell Environ. 24, 529-538) - Shade (Hanba et al. 2002 Plant Cell Environ. 25, 1021-1030) - Drought (Flexas et al. 2002 Funct. Plant Biol. 29, 461-471) - Temperature (Bernacchi et al. 2002 Plant Physiol. 130, 1992-1998) - CO2 (Singsaas et al. 2003 Plant Cell Environ. 27, 41-50) - O3 (Eichelmann et al. 2004 Plant Cell Environ. 27, 479-495)
High CO2 applied 0.6 0.6 0.5 0.5 0.4 0.4 0.3 Ambient CO2 restored 0.3 0.2 0.2 0.1 0.1 0.0 0.0 0 2000 4000-2 -1 gm (mol CO2 m s ) -2-1 gs (mol CO2 m s ) B 6000 Time (seconds) Flexas et al. (2007) Plant Cell Environ. 30, 1284-1298
METABOLIC CANDIDATES FOR RAPID CHANGES IN gm - Carbonic anhydrase (Gillon and Yakir 2000 Plant Physiol. 123, 201213) - Aquaporins (Flexas et al. 2006 Plant J. 48, 427-439) - Others (Flexas et al. 2008 Plant Cell Environ. In press)
AQUAPORINS NtAQP1 transformed plants Anti-sense (AS) Control Antisense (CAS) Over-express (O) Control Overexpress (CO)
NtAQP1 affects mesophyll conductance to CO2 GENOTYPE gm (mol CO2 m-2 s-1 bar-1) AS 0.176 0.012 a CAS 0.242 0.021 b CO 0.322 0.020 c O 0.401 0.045 d Flexas et al. (2006) Plant J. 48, 427-439
Aquaporing activity (GATING) under water stress Miyazawa et al. 2008. Funct. Plant Biol. 35, 553-564.
OVERALL: gm limits photosynthesis as much as stomata and more than biochemistry Species Factor Total photosynthesis limitation (%) SL (%) MCL (%) BL (%) Reference Arabidopsis thaliana Senescent leaves 41 13 28 0 Flexas et al. 2007 Beta maritima Severe water stress 97 21 37 39 Galmés et al. 2007 Lavatera maritima Severe water stress 87 70 7 10 Galmés et al. 2007 Phlomis italica Severe water stress 96 22 52 22 Galmés et al. 2007 Vitis berlandieri x rupestris Moderate water stress 30 15 15 0 Flexas et al. 2009 Vitis berlandieri x rupestris Severe water stress 79 26 48 5 Flexas et al. 2009 Nicotiana sylvestris Rewatering High Light 35 15 20 0 Gallé et al. 2009 Nicotiana sylvestris Rewatering Mid Light 26 19 5 2 Gallé et al. 2009 Nicotiana sylvestris Rewatering Low Light 30 30 0 0 Gallé et al. 2009 Olea europaea Water stress High VPD 78 54 22 2 Pérez-Martín et al. 2009 Olea europaea Water stress Low VPD 82 34 44 4 Pérez-Martín et al. 2009 Rhamnus ludovici-salvatoris Shade leaves 62 24 24 14 Mattos et al. unpub. Beta vulgaris Excess Zinc (300 µm) 48 42 5 0 Sagardoy et al. submit. AVERAGE STRESS 61 30 24 7
gm to improve photosynthesis and yield
gm to improve photosynthesis and yield
gm to improve photosynthesis and yield Barbour et al. 2010. Plant Cell Environ 33, 1176-1185
gm to improve photosynthesis and yield Low gs & High gm High gs & gm Low gs & gm Barbour et al. 2010. Plant Cell Environ 33, 1176-1185
GLOBAL ANALYSIS SHOWS HOW TO IMPROVE WATER USE EFFICIENCY AN / gs ( mol CO2 mol-1) 200 150 100 50 0 0 1 2 3 4-1 gm / gs (mol CO2 mol ) Flexas et al., unpublished 5
Take-home messages Mesophyll conductance (gm) is as FINITE and RAPIDLY VARIABLE as stomatal conductance gm is a really important LIMITATION to photosynthesis having a GENETIC BASIS (structural + metabolic) Hence improving gm is a TARGET to genetically improve photosynthesis In addition, it is a perfect target to improve WUE as well, specially under drought-prone conditions