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Apoplast phloem loading is determined by the cellular location and transport function of membrane-bound proteins. The following statements describe these proteins:

A. SWEETs are the sugar transporter proteins and are capable of transporting only sucrose and not glucose.

B. Double mutants of Arabidopsis SWEET11 and SWEET12 result in carbohydrate accumulation in the source leaves and slower export of photoassimilates.

C. H⁺-symport mechanism loads sucrose or polyols into Sieve Element (SE)/Companion Cell (CC) complexes.

D. Several clades of sucrose/H⁺ symporters (SUTs or SUCs) are localized to plasma membranes of minor vein SE/CCs and participate in apoplastic loading.

Which one of the following combinations is correct?

Correct option is C

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Explanation:

Statement A: Incorrect

• SWEET transporters facilitate the movement of both sucrose and glucose.
• SWEET proteins function as passive uniporters, meaning they allow sugars to move along their concentration gradient.
• Since SWEETs transport both sucrose and glucose, the statement is incorrect.

Statement B: Correct

• In Arabidopsis, SWEET11 and SWEET12 function as sucrose efflux transporters that move sucrose out of mesophyll cells into the apoplast.
• Double mutants of SWEET11 and SWEET12 cause carbohydrate accumulation in source leaves, leading to reduced sucrose export.
• This proves their essential role in phloem loading.

Statement C: Correct

• Sucrose and polyols enter SE/CC complexes via a proton-symport mechanism.
• H⁺-ATPase pumps protons into the apoplast, generating a proton gradient.
• Sucrose/H⁺ symporters (SUTs or SUCs) use this gradient to actively transport sucrose into SE/CCs.

Statement D: Correct

• Sucrose/H⁺ symporters (SUTs or SUCs) are located in the plasma membranes of minor vein sieve elements (SE) and companion cells (CCs).
• These transporters participate in apoplastic phloem loading by actively bringing sucrose into SE/CC complexes.